Reduction of the two-photon temporal distinguishability for measurement-device-independent quantum key distribution
Haobo Ge, Akihisa Tomita, Atsushi Okamoto, Kazuhisa Ogawa
Optics Letters, 49, 4, 822, 822, Optica Publishing Group, 05 Feb. 2024
Scientific journal, Measurement-device-independent quantum key distribution (MDI-QKD) has been proven to protect legitimate users from attacks against measurement devices. The MDI-QKD requires that the two photons arriving at the instrument be indistinguishable. Precise time control is required to eliminate the distinguishability due to differences in photon arrival times. In the conventional methods, the time difference between photons is measured at a measuring instrument (Charlie), and a control signal is transmitted to the users (Alice and Bob). However, this method requires a long feedback loop, and the control may become unstable for long-distance transmission. This article proposes a method in which the photon arrival time difference is detected and controlled at Charlie. The reference signal for the time control is generated by an optical frequency comb in synchronization with the quantum signal. Therefore, the quantum signal photons can also be synchronized by synchronizing the reference signal pulses. A proof-of-principle experiment confirmed that the time synchronization accuracy required for protocol execution could be obtained. This proposal simplifies the implementation of the MDI-QKD.

Single-shot digital holography separating the real and imaginary parts of a signal beam in the rhombic region of the Fourier plane
Nobuhiro Yamagishi, Atsushi Okamoto, Akihisa Tomita
Optical Review, 30, 2, 217, 231, Springer Science and Business Media LLC, 28 Mar. 2023
Scientific journal

Analysis of the Effects of the Two-Photon Temporal Distinguishability on Measurement-Device-Independent Quantum Key Distribution
Haobo Ge, Akihisa Tomita, Atsushi Okamoto, Kazuhisa Ogawa
IEEE Transactions on Quantum Engineering, 4, 1, 8, Institute of Electrical and Electronics Engineers (IEEE), 2023
Scientific journal

Spatial mode compensation technique using progressive phase conjugation
Zeyu Shen, Atsushi Okamoto, Shuanglu Zhang, Akihisa Tomita
Optical Review, Springer Science and Business Media LLC, 12 Sep. 2022, [Peer-reviewed]
English, Scientific journal

Spatial-light-modulator-based optical-fiber joint switch for few-mode multicore fibers
Shuanglu Zhang, Atsushi Okamoto, Yuta Abe, Ryo Watanabe, Akihisa Tomita, Daiki Soma, Yuta Wakayama, Takehiro Tsuritani
Optics Express, 29, 24, 39096, 39106, Optica Publishing Group, 22 Nov. 2021, [Peer-reviewed]
English, Scientific journal, To realize simplified cost-efficient optical networks with routing flexibility and scaling potential, a spatial-light-modulator-based optical-fiber joint switch for few-mode multicore fibers is proposed herein, which can route all spatial channels together as a unit. Numerical simulations and experiments were performed, and the results show that the signal paths for a 6-mode 19-core fiber can be simultaneously switched to the target output ports using the proposed method, and the mode-field patterns of the diffracted light can be maintained after joint switching. Further, the maximum port crosstalk can be reduced considerably from -11.6 to -25.1 dB by changing the position of the output port in the proposed method.

Complex counterpart of variance in quantum measurements for pre- and postselected systems
Kazuhisa Ogawa, Natsuki Abe, Hirokazu Kobayashi, Akihisa Tomita
Physical Review Research, 3, 3, American Physical Society (APS), 21 Jul. 2021, [Peer-reviewed]
English, Scientific journal

Direct measurement of ultrafast temporal wavefunctions
Kazuhisa Ogawa, Takumi Okazaki, Hirokazu Kobayashi, Toshihiro Nakanishi, Akihisa Tomita
Optics Express, 29, 13, 19403, 19403, The Optical Society, 21 Jun. 2021, [Peer-reviewed]
English, Scientific journal, The large capacity and robustness of information encoding in the temporal mode of photons is important in quantum information processing, in which characterizing temporal quantum states with high usability and time resolution is essential. We propose and demonstrate a direct measurement method of temporal complex wavefunctions for weak light at a single-photon level with subpicosecond time resolution. Our direct measurement is realized by ultrafast metrology of the interference between the light under test and self-generated monochromatic reference light; no external reference light or complicated post-processing algorithms are required. Hence, this method is versatile and potentially widely applicable for temporal state characterization.

Spatial mode exchange technique using volume holograms with a random optical diffuser for reduction of crosstalk
Shuanglu Zhang, Atsushi Okamoto, Taijun Shiba, Hotaka Hayashi, Kazuhisa Ogawa, Akihisa Tomita, Taketoshi Takahata, Satoshi Shinada, Yuta Goto, Naoya Wada
Optical Review, 28, 2, 181, 189, Springer Science and Business Media LLC, Apr. 2021, [Peer-reviewed]
English, Scientific journal, In mode-division multiplexing systems, the transmission quality is restricted by the differential mode delay (DMD). We previously proposed a spatial mode exchange technique using volume holograms (VHET) to reduce the DMD. VHET can be used for the exchange of multiple spatial modes with different transmission speeds using a single device. However, in VHET, incidents of cross-talk (XT) caused by non-target holograms severely degrade the exchange performance. To address this issue, we propose a spatial mode exchange technique that uses volume holograms in combination with a random optical diffuser. By scattering the intensity distribution of the signal beam uniformly using a random optical diffuser, high exchange performance can be attained due to the reduction in the influence of non-targeted holograms. We confirmed the basic operation of the proposed scheme by considering the crosstalk component coupled to the few-mode fiber in a mode-division multiplexing system. The simulation results show that the maximum XTs are significantly suppressed to - 15.8 dB.

Operational formulation of weak values without probe systems
Kazuhisa Ogawa, Hirokazu Kobayashi, Akihisa Tomita
Physical Review A, 101, 4, American Physical Society (APS), 17 Apr. 2020, [Peer-reviewed]
English, Scientific journal

State preparation robust to modulation signal degradation by use of a dual parallel modulator for high-speed BB84 quantum key distribution systems
Weiyang Zhang, Yu Kadosawa, Akihisa Tomita, Kazuhisa Ogawa, Atsushi Okamoto
OPTICS EXPRESS, 28, 9, 13965, 13977, OPTICAL SOC AMER, Apr. 2020, [Peer-reviewed], [Corresponding author]
English, Scientific journal, Security certification of quantum key distribution systems with a practical device is essential for their social deployment. Considering the transmitter, we investigate quantum state generation affected by degraded electrical signals from practical bandwidth-limited devices on high-speed phase-encoding BB84 quantum key distribution systems. The state preparation flaw caused by this degradation undesirably enhances the distinguishability between the two bases for the BB84 protocol and decreases the key generation rate. We propose the state preparation with a dual parallel modulator for increasing the robustness to signal degradation. To verify the effectiveness of the dual parallel modulator, we characterize the generated states using state tomography and estimate the key generation rate based on the Gottesman-Lo-Liitkenhaus-Preskill theory with fidelity derived from the estimated density matrices. Simulation results show that the key generation rate remains unaffected by modulation voltage shifts up to 20%. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Mode-selective switch for ROADM using volume holograms and spatial light modulator
Hotaka Hayashi, Atsushi Okamoto, Tomohiro Maeda, Taijun Shiba, Kazuhisa Ogawa, Akihisa Tomita, Taketoshi Takahata, Satoshi Shinada, Yuta Goto, Naoya Wada
Next-Generation Optical Communication: Components, Sub-Systems, and Systems IX, SPIE, 31 Jan. 2020
International conference proceedings

Three-dimensional tomographic imaging and quantitative phase measurement using virtual phase conjugation
Satoshi Kawashima, Atsushi Okamoto, Tomohiro Maeda, Kazuhisa Ogawa, Akihisa Tomita, Ryusuke Osaki, Naoki Hayashi
Progress in Biomedical Optics and Imaging - Proceedings of SPIE, 11245, SPIE, 2020
English, International conference proceedings, We have proposed virtual-phase-conjugation-based optical tomography (VPC-OT) using a virtual phase conjugation technology for single-shot and three-dimensional optical tomography. In VPC-OT, a random-spatial-phase-modulated probe beam is irradiated to the sample to be measured, and the complex amplitude of the signal composed of a superimposition of light reflected from each layer of the sample is measured. A three-dimensional tomogram of intensity and phase is obtained by reproducing the measured complex amplitude using a phase conjugate wave in a virtual optical system built in a computer. At this time, by changing the parameters of the virtual optical system, it becomes possible to obtain information of various tomographic planes from the data obtained with a single measurement. In the ideal virtual phase conjugate reproduction process, free space propagation can be assumed
however, in the actual measurement, due to the distortion of the waves and the surroundings of the sample to be measured, a mismatch will occur in modulation and demodulation, and the separation accuracy between different tomographic planes would be degraded. We perform an experiment to clarify the characteristics of VPC-OT in this situation. In this experiment, three-dimensional optical tomography is performed using an etching glass having a periodic structure of 30 μm as a sample, and the phase distribution is measured quantitatively. Furthermore, by placing a cover glass in front of the object and performing the same measurement, we discuss the characteristics and performance of VPC-OT when there is an optical distortion around the sample to be measured.

Collective measurement of DMD in 6-mode 19-core fiber using low-coherence digital holography
Yuta Abe, Atsushi Okamoto, Kazuhisa Ogawa, Akihisa Tomita, Daiki Soma, Yuta Wakayama, Takehiro Tsuritani
Proceedings of SPIE - The International Society for Optical Engineering, 11309, SPIE, 2020
English, International conference proceedings, In the space division multiplexing system, differential mode delay (DMD) is an essential parameter for transmission quality and signal processing. In this paper, we proposed the collective measurement method for the DMD of each core in fewmode multi-core fiber (FM-MCF) using low-coherence digital holography (LCDH). In the conventional method, given that the DMD of each core needs to be measured individually, the number of measurements increases in proportion to the number of cores, which causes fluctuation of the measurement conditions. In contrast, the proposed method reduces the measurement time and realizes the accurate measurement under the same conditions for all cores. We experimentally demonstrate the proposed method by using 6-mode 19-core fiber as a fiber under test. As a result, DMD values were successfully measured for all 19 cores.

Spatial mode exchange technique using volume holograms with a random optical diffuser to reduce modal cross-talks
Shuanglu Zhang, Atsushi Okamoto, Taijun Shiba, Hotaka Hayashi, Kazuhisa Ogawa, Akihisa Tomita, Taketoshi Takahata, Satoshi Shinada, Yuta Goto, Naoya Wada
NEXT-GENERATION OPTICAL COMMUNICATION: COMPONENTS, SUB-SYSTEMS, AND SYSTEMS IX, 11309, SPIE-INT SOC OPTICAL ENGINEERING, 2020
English, International conference proceedings, In mode-division multiplexing (MDM) systems, transmission quality is restricted by differential mode delay (DMD). We have proposed and developed a spatial-mode exchange technique using volume holograms (VHET) as a leading technology to reduce DMD. VHET can equilibrate the transmission time of each spatial mode which has different transmission speeds in a few mode fiber using a volume hologram. This technology enables low signal distortion and high spectral efficiency, which are indispensable to the long-haul transmission for the MDM system. However, in this technology, modal cross-talks (MXTs), which are caused by inter-page cross- talks in the volume hologram, seriously degrade the performance of VHET. In this study, we proposed a method of combining a volume hologram and a random optical diffuser to reduce the MXTs. In our method, the intensity distribution of the input spatial mode is diffused uniformly by a random optical diffuser. The high exchange performance will be attained because the non- targeted holograms included in the multiplexed holograms do not affect most of the spatial mode. Moreover, our method can be applied to the communication wavelength bands using the dual wavelength method. We confirmed the basic operation of the proposed scheme using a linearly polarized mode group comprising LP0,1, LP1,1, and LP2,1. Compared with the conventional VHET, the simulation results show that the maximum MXTs are significantly suppressed from 0.4 to -15.8 dB.

Wavefront superposition method for accurate and efficient mode conversion　　　　　　　　　　　　　　　
Tomohiro Maeda, Atsushi Okamoto, Kazuhisa Ogawa, Akihisa Tomita
Applied Optics, 58, 25, 6899, 6905, Aug. 2019, [Peer-reviewed]
English, Scientific journal

Highly Accurate Mode Conversion Using Iterative Spatial Cross Modulation.　　　　　　　　　　　　　　　
Kazuhisa OgawaMaeda, T, Okamoto, A, Ogawa, K, Tomita, A, Wakayama, Y, Tsuritani, T
2019 24th OptoElectronics and Communications Conference (OECC) and 2019 International Conference on Photonics in Switching and Computing (PSC), 1, 3, Jul. 2019, [Peer-reviewed]
English, International conference proceedings

Implementation Security Certification of Decoy-BB84 Quantum Key Distribution Systems
Akihisa Tomita
ADVANCED QUANTUM TECHNOLOGIES, 2, 5-6, WILEY, Jun. 2019, [Peer-reviewed], [Lead author]
English, Scientific journal, One of the most important obstacles for the social deployment of Quantum Key Distribution (QKD) systems is the lack of standards for security certification. Proper selection of application will circumvent other obstacles, such as cost, key generation distance, and rate. Nevertheless, security certification of real systems, or "implementation security certification" is indispensable. Such certification should be clear and tractable for non-experts and should be performed with standardized characterization methods. Otherwise, the potential users cannot adopt the QKD system with confidence, because the highest level of security is what QKD appeals. This report discusses the process of the implementation security certification and the requirements for device characterization. It also presents several case studies on the requirements and experimental characterization of QKD devices for the security certification., 34191611

A framework for measuring weak values without weak interactions and its diagrammatic representation
K. Ogawa, O Yasuhiko, H. Kobayashi, T. Nakanishi, A. Tomita
New Journal of Physics, 21, 4, 043013, 043013, IOP Publishing, Apr. 2019, [Peer-reviewed]
English, Scientific journal

Diagrammatic representation of direct measurement systems of quantum states, processes, and measurements　　　　　　　　　　　　　　　
Kazuhisa Ogawa, Hirokazu Kobayashi, Akihisa Tomita
Optics InfoBase Conference Papers, 2019, OSA - The Optical Society, 2019
English, International conference proceedings, A diagrammatic representation that intuitively shows the relation between measurement systems using qubit probes and their outcomes is introduced. This representation enables us to design direct measurement systems of quantum states, processes, and measurements systematically.

Mode conversion based on dual-phase modulation utilizing interference of two-phase-modulated beams　　　　　　　　　　　　　　　
Tomohiro Maeda, Atsushi Okamoto, Kazuhisa Ogawa, Akihisa Tomita, Yuta Wakayama, Takehiro Tsuritani
Optical Review, 25, 6, 734, 742, Oct. 2018, [Peer-reviewed]
English, Scientific journal

Experimental implementation of multiplexing/demultiplexing in digital images using virtual phase conjugation for holographic data storage
Goto Yuta, Okamoto Atsushi, Takabayashi Masanori, Ogawa Kazuhisa, Tomita Akihisa
OPTICAL REVIEW, 25, 5, 549, 554, Oct. 2018, [Peer-reviewed]

Tracking quantum error correction
Fukui Kosuke, Tomita Akihisa, Okamoto Atsushi
PHYSICAL REVIEW A, 98, 2, 22 Aug. 2018, [Peer-reviewed]

High-Threshold Fault-Tolerant Quantum Computation with Analog Quantum Error Correction
Kosuke Fukui, Akihisa Tomita, Atsushi Okamoto, Keisuke Fujii
Physical Review X, 8, 2, American Physical Society, 25 May 2018, [Peer-reviewed]
English, Scientific journal, To implement fault-tolerant quantum computation with continuous variables, the Gottesman-Kitaev-Preskill (GKP) qubit has been recognized as an important technological element. However, it is still challenging to experimentally generate the GKP qubit with the required squeezing level, 14.8 dB, of the existing fault-tolerant quantum computation. To reduce this requirement, we propose a high-threshold fault-tolerant quantum computation with GKP qubits using topologically protected measurement-based quantum computation with the surface code. By harnessing analog information contained in the GKP qubits, we apply analog quantum error correction to the surface code. Furthermore, we develop a method to prevent the squeezing level from decreasing during the construction of the large-scale cluster states for the topologically protected, measurement-based, quantum computation. We numerically show that the required squeezing level can be relaxed to less than 10 dB, which is within the reach of the current experimental technology. Hence, this work can considerably alleviate this experimental requirement and take a step closer to the realization of large-scale quantum computation.

Virtual phase conjugation based optical tomography for single-shot three-dimensional imaging
Yuta Goto, Atsushi Okamoto, Atsushi Shibukawa, Kazuhisa Ogawa, Akihisa Tomita
Optics Express, 26, 4, 3779, 3790, OSA - The Optical Society, 19 Feb. 2018, [Peer-reviewed]
English, Scientific journal, We propose a virtual phase conjugation (VPC) based optical tomography (VPC-OT) for realizing single-shot optical tomographic imaging systems. Using a computer-based numerical beam propagation, the VPC combines pre-modulation and post-demodulation of the probe beam’s wavefront, which provides an optical sectioning capability for resolving the depth coordinates. In VPC-OT, the physical optical microscope system and VPC are coupled using digital holography. Therefore, in contrast to conventional optical tomographic imaging (OTI) systems, this method does not require additional elements such as low-coherence light sources or confocal pinholes. It is challenging to obtain single-shot three-dimensional (3D) tomographic images using a conventional OTI system
however, this can be achieved using VPC-OT, which employs both digital holography and computer based numerical beam propagation. In addition, taking into account that VPC-OT is based on a complex amplitude detection using digital holography, this method allows us to simultaneously obtain quantitative phase contrast images. Using an objective lens with a numerical aperture (NA) of 0.8, we demonstrate a single-shot 3D imaging of frog blood cells with a depth resolution of 0.94 µm.

Classical reconstruction of interference patterns of position-wave-vector-entangled photon pairs by the time-reversal method
Kazuhisa Ogawa, Hirokazu Kobayashi, Akihisa Tomita
Physical Review A, 97, 2, American Physical Society, 13 Feb. 2018, [Peer-reviewed]
English, Scientific journal, The quantum interference of entangled photons forms a key phenomenon underlying various quantum-optical technologies. It is known that the quantum interference patterns of entangled photon pairs can be reconstructed classically by the time-reversal method
however, the time-reversal method has been applied only to time-frequency-entangled two-photon systems in previous experiments. Here, we apply the time-reversal method to the position-wave-vector-entangled two-photon systems: the two-photon Young interferometer and the two-photon beam focusing system. We experimentally demonstrate that the time-reversed systems classically reconstruct the same interference patterns as the position-wave-vector-entangled two-photon systems.

Volume holographic spatial mode demultiplexer with a dual-wavelength method
Shimpei Shimizu, Atsushi Okamoto, Fumiya Mizukawa, Kazuhisa Ogawa, Akihisa Tomita, Taketoshi Takahata, Satoshi Shinada, Naoya Wada
Applied Optics, 57, 2, 146, 153, OSA - The Optical Society, 10 Jan. 2018, [Peer-reviewed]
English, Scientific journal, Volume holographic demultiplexers (VHDMs) provide spatial mode demultiplexing using simple optical systems. However, applying VHDM to practical optical communication systems is difficult, as typical holographic media have no sensitivity in the infrared region, which includes optical transmission bands. In this paper, we propose a VHDM scheme combined with a dual-wavelength method (DWM). Using the DWM, VHDMs are able to perform mode demultiplexing in the optical transmission bands. We experimentally demonstrated the basic operation of our proposal using experiments performed at an 850-nm wavelength. In addition, we performed numerical simulations to investigate the application of VHDM to the C-band.

Fundamental demonstration of mode-group demultiplexing technique based on volume holographic demultiplexer
Shimpei Shimizu, Atsushi Okamoto, Fumiya Mizukawa, Kazuhisa Ogawa, Akihisa Tomita, Taketoshi Takahata, Satoshi Shinada, Naoya Wada
22nd Microoptics Conference, MOC 2017, 2017-, 260, 261, Institute of Electrical and Electronics Engineers Inc., 02 Jan. 2018, [Peer-reviewed]
English, International conference proceedings, We demonstrated the mode-group demultiplexing technique based on a volume holographic demultiplexer (VHDM) for mode-group division multiplexing transmission using multi-mode fibers. In the experiment, the VHDM achieved the separating of three mode-group channels with a single holographic recording medium. Thus, the modal cross-talk was obtained as -14.7 dB.

Compensation of optical aberration for improvement of image quality in virtual-phase-conjugation based optical tomography
Yuta Goto, Atsushi Okamoto, Kazuhisa Ogawa, Akihisa Tomita
22nd Microoptics Conference, MOC 2017, 2017-, 358, 359, Institute of Electrical and Electronics Engineers Inc., 02 Jan. 2018, [Peer-reviewed]
English, International conference proceedings, We propose virtual-phase-conjugation based optical tomography (VPC-OT) incorporated with the compensation of the optical aberration for improving the imaging quality. The compensation scheme is implemented by optically measuring the transmission function of the beam propagation. Then, we confirmed that this scheme greatly improves the signal level of VPC-OT.

Selective mode conversion using dual-phase modulation
Tomohiro Maeda, Atsushi Okamoto, Kazuhisa Ogawa, Akihisa Tomita, Yuta Wakayama, Takehiro Tsuritani
22nd Microoptics Conference, MOC 2017, 2017-, 68, 69, Institute of Electrical and Electronics Engineers Inc., 02 Jan. 2018, [Peer-reviewed]
English, International conference proceedings, We propose a selective mode conversion method using dual-phase modulation which performs the full-complex amplitude modulation by use of two phase-type spatial light modulators. In the experiment, the crosstalk to undesired spatial mode was supressed less than -21.2 dB for the 4-LP modes of a few-mode fiber.

Measurement of differential mode delay using reference-free low-coherence digital holography
Shogo Hoshino, Atsushi Okamoto, Yuta Goto, Kazuhisa Ogawa, Akihisa Tomita, Yuta Wakayama, Takehiro Tsuritani
Proc. SPIE, 10561, 105610D, SPIE, Jan. 2018, [Peer-reviewed]
English, International conference proceedings

Selective spatial mode attenuator using phase-intensity-phase modulation toward mode-division multiplexing transmission
Zihan Zhou, Atsushi Okamoto, Shimpei Shimizu, Akihisa Tomita, Kazuhisa Ogawa, Yuta Wakayama, Takehiro Tsuritani
Proc. SPIE, 10561, 105610H, SPIE-INT SOC OPTICAL ENGINEERING, Jan. 2018, [Peer-reviewed]
English, International conference proceedings, In a mode division multiplexing (MDM) transmission using few-mode fibers (FMFs), differential modal gain (DMG) in amplifiers and mode dependent loss (MDL) cause deterioration in signal quality. Therefore, the techniques which selectively attenuate multiplexed modes and equalize optical powers among mode channels are required for a long-haul MDM transmission. In this paper, we propose a selective mode attenuator using phase-intensity-phase (PIP) modulation. The PIP modulation, consisting of a cascade of two phase and one intensity modulation masks connected by optical Fourier transforms (OFTs), makes it possible to selectively attenuate multiplexed mode channels with high accuracy. In the proposed method, the intensity distributions of spatial modes are converted by the phase modulation and OFT, before doing attenuation by intensity mask located between the two phase masks. Due to the action of a pair of phase masks having phase conjugate relations, accurately selective mode attenuation can be performed even if the number of modes is increased over three. To confirm the basic operation of the proposed method, we perform a numerical simulation for power equalization among six spatial modes (LP01, LP11a, LP11b, LP21a, LP21b, and LP02) having different powers. The phase and intensity masks are designed by using simulated annealing. Moreover, we also evaluate modal crosstalk (MXT) characteristics and the wavelength dependence of the equalization in C-band. The results show that the optical powers of all modes are successfully equalized for any wavelengths and the MXT smaller than -25 dB were achieved between all modes.

Quantum key distribution with an efficient countermeasure against correlated intensity fluctuations in optical pulses
Ken-ichiro Yoshino, Mikio Fujiwara, Kensuke Nakata, Tatsuya Sumiya, Toshihiko Sasaki, Masahiro Takeoka, Masahide Sasaki, Akio Tajima, Masato Koashi, Akihisa Tomita
npj Quantum Information, 4, 1, Springer Nature, Jan. 2018, [Peer-reviewed]
English, Scientific journal, Quantum key distribution (QKD) allows two distant parties to share secret keys with the proven security even in the presence of an eavesdropper with unbounded computational power. Recently, GHz-clock decoy QKD systems have been realized by employing ultrafast optical communication devices. However, security loopholes of high-speed systems have not been fully explored yet. Here we point out a security loophole at the transmitter of the GHz-clock QKD, which is a common problem in high-speed QKD systems using practical band-width limited devices. We experimentally observe the inter-pulse intensity correlation and modulation pattern-dependent intensity deviation in a practical high-speed QKD system. Such correlation violates the assumption of most security theories. We also provide its countermeasure which does not require significant changes of hardware and can generate keys secure over 100 km fiber transmission. Our countermeasure is simple, effective and applicable to wide range of high-speed QKD systems, and thus paves the way to realize ultrafast and security-certified commercial QKD systems.

Analog Quantum Error Correction with Encoding a Qubit into an Oscillator
Kosuke Fukui, Akihisa Tomita, Atsushi Okamoto
PHYSICAL REVIEW LETTERS, 119, 18, AMER PHYSICAL SOC, Nov. 2017, [Peer-reviewed], [International Magazine]
English, Scientific journal, To implement fault-tolerant quantum computation with continuous variables, Gottesman-Kitaev-Preskill (GKP) qubits have been recognized as an important technological element. However, the analog outcome of GKP qubits, which includes beneficial information to improve the error tolerance, has been wasted, because the GKP qubits have been treated as only discrete variables. In this Letter, we propose a hybrid quantum error correction approach that combines digital information with the analog information of the GKP qubits using a maximum-likelihood method. As an example, we demonstrate that the three-qubit bit-flip code can correct double errors, whereas the conventional method based on majority voting on the binary measurement outcome can correct only a single error. As another example, we show that a concatenated code known as Knill's C-4/C-6 code can achieve the hashing bound for the quantum capacity of the Gaussian quantum channel (GQC). To the best of our knowledge, this approach is the first attempt to draw both digital and analog information to improve quantum error correction performance and achieve the hashing bound for the quantum capacity of the GQC.

Digital image multiplexing/demultiplexing method using spatial spectral diffusion and virtual phase conjugation technique for reduction of dynamic range consumption in holographic medium
Yuta Goto, Atsushi Okamoto, Masanori Takabayashi, Kazuhisa Ogawa, Akihisa Tomita
JAPANESE JOURNAL OF APPLIED PHYSICS, 56, 9, IOP PUBLISHING LTD, Sep. 2017, [Peer-reviewed]
English, Scientific journal, The recording density in holographic data storage (HDS) systems is determined by the dynamic range of the recording medium. If a hologram is recorded with a small exposure, the consumption amount of the dynamic range per hologram is reduced, and the recording density can be improved. In this report, we propose a digital image multiplexing/demultiplexing method using spatial spectral diffusion and a virtual phase conjugation technique to reduce the consumption amount of the dynamic range in a recording medium. In our method, multiple signals are simultaneously recorded in a medium by multiplexing those signals using a beam combiner before recording, unlike the general multiplexing method of HDS. Then, each original signal can be reproduced independently by utilizing the property of a random diffuser and a phase-conjugated light. In the simulation, we confirmed that our method can improve the recording density because the consumption of the dynamic range is reduced. (C) 2017 The Japan Society of Applied Physics

Spatial mode demultiplexing technique using angularly multiplexed volume holograms with a phase plate
Shimpei Shimizu, Atsushi Okamoto, Fumiya Mizukawa, Kazuhisa Ogawa, Akihisa Tomita, Taketoshi Takahata, Satoshi Shinada, Naoya Wada
Japanese Journal of Applied Physics, 56, 9S, 09NA05-1, 09NA05-5, Institute of Physics, Aug. 2017, [Peer-reviewed]
English, Scientific journal, In this paper, we propose a spatial mode demultiplexing technique that uses a volume holographic demultiplexer (VHDM) with a phase plate. The VHDM can separate the multiplexed spatial modes with a single device by using angularly multiplexed volume holograms. In the VHDM, modal cross-talks, which are called inter-page cross-talks in the holographic data storages, are generated, and hence the separation performance of the VHDM is degraded by the intensity overlap between spatial modes. Therefore, we propose a novel scheme wherein phase modulation with a phase plate is added to the conventional scheme. The proposed scheme can achieve high separation performance by modulating the phase of spatial modes to reduce the intensity overlap. In this study, we demonstrated the basic operation of the proposed method for a specific linearly polarized mode group. The separation performance of the VHDM was observed to be markedly improved by the proposed method.

Quantum key distribution network for multiple applications　　　　　　　　　　　　　　　
A. Tajima, T. Kondoh, T. Ochi, M. Fujiwara, K. Yoshino, H. Iizuka, T. Sakamoto, A. Tomita, E. Shimamura, S. Asami, M. Sasaki
Quantum Science and Technology, 2, 3, 034003, Jul. 2017, [Peer-reviewed]
English, Scientific journal

Generation of phase-squeezed optical pulses with large coherent amplitudes by post-selection of single photon and weak cross-Kerr non-linearity
Matsuoka Fumiaki, Tomita Akihisa, Shikano Yutaka
Quantum Studies: Mathematics and Foundations, 4, 2, 159, 169, Springer Science and Business Media LLC, Jun. 2017, [Peer-reviewed]
English, Scientific journal

Intensity fluctuation of a gain-switched semiconductor laser for quantum key distribution systems
Kensuke Nakata, Akihisa Tomita, Mikio Fujiwara, Ken-ichiro Yoshino, Akio Tajima, Atsushi Okamoto, Kazuhisa Ogawa
OPTICS EXPRESS, 25, 2, 622, 634, OPTICAL SOC AMER, Jan. 2017, [Peer-reviewed]
English, Scientific journal, Security certification of quantum key distribution (QKD) systems under practical conditions is necessary for social deployment. This article focused on the transmitter, and, in particular, investigated the intensity fluctuation of the optical pulses emitted by a gain-switched semiconductor laser used in QKD systems implementing decoy-BB84 protocol. A large intensity fluctuation was observed for low excitation, showing strong negative correlation between the adjacent pulses, which would affect the final key rate. The fluctuation decreased and the correlation disappeared as excitation increased. Simulation with rate equations successfully reproduced the experimental results and revealed that the large fluctuation originates from an intrinsic instability of gain-switched lasers driven periodically at a rate comparable to the inverse of carrier lifetime, as in GHz-clock QKD systems. Methods for further reduction of the intensity fluctuation were also discussed. (C) 2017 Optical Society of America

Reference-free holographic diversity interferometry via iterative measurements for high accuracy phase detection
Yuta Goto, Atsushi Okamoto, Yuta Wakayama, Kazuhisa Ogawa, Jin Nozawa, Akihisa Tomita, Takehiro Tsuritani
OPTICS EXPRESS, 24, 21, 24739, 24749, OPTICAL SOC AMER, Oct. 2016, [Peer-reviewed]
English, Scientific journal, To obtain a phase distribution without the use of an optical path besides an object beam, a reference-free holographic diversity interferometry (RF-HDI) has been proposed. Although the RF-HDI can generate an internal reference beam from the object beam, the method has a problem of measurement accuracy due to insufficient power of the internal reference beam. To solve the problem, we newly propose a RF-HDI via iterative measurements. Our method improves the measurement accuracy by utilizing iterative measurements and feedback of each obtained phase image to the measurement system. In the experiment, the phase image, which has a random pattern, can be measured as an object beam with a higher accuracy than in the conventional RF-HDI. To support this result, we also evaluated the wavefront accuracy and optical power efficiency of an internal reference beam in this method. As a result, we verified that our method enables us to generate an internal reference beam that has the wavefront of a near single plane wave and a higher power efficiency than the conventional RF-HDI. In addition, our method can be applied to measurement for the modal content in an optical fiber, atmosphere turbulence, etc., where it is difficult to prepare an external reference beam with a high coherency. (C) 2016 Optical Society of America

Virtual interferogram-generation algorithm for robust complex amplitude measurement using two interferograms
Jin Nozawa, Atsushi Okamoto, Kazuhisa Ogawa, Akihisa Tomita
OPTICS EXPRESS, 24, 21, 24002, 24017, OPTICAL SOC AMER, Oct. 2016, [Peer-reviewed]
English, Scientific journal, We propose a virtual interferogram-generation algorithm using two interferograms. This algorithm can measure a complex amplitude of a signal beam with high accuracy even when its intensity is greater than the intensity of a reference beam. Unlike the conventional algorithm that uses two interferograms, our algorithm can compute measurements when the phase shift of interferograms in not equal to pi/2. Our method generates two phase-shifted holograms in a computer by capturing the intensities of two signal beams, two reference beams, and two interferograms. The complex amplitude of a signal beam is calculated by four interference patterns, two holograms, and two interferograms. The proposed algorithm can drastically suppress the calculation error caused by the smaller value between the intensity of the reference beam and can choose the most suitable phase shift. (C) 2016 Optical Society of America

Digital confocal microscopy using a virtual 4f-system based on numerical beam propagation for depth measurement without mechanical scanning
Yuta Goto, Atsushi Okamoto, Masataka Toda, Yasuyuki Kuno, Jin Nozawa, Kazuhisa Ogawa, Akihisa Tomita
Japanese Journal of Applied Physics, 55, 8S3, 08RE04, 08RE04, IOP Publishing, Jul. 2016, [Peer-reviewed]
English, Scientific journal

Entanglement generation by communication using phase-squeezed light with photon loss
Fumiaki Matsuoka, Akihisa Tomita, Atsushi Okamoto
PHYSICAL REVIEW A, 93, 3, 033308, AMER PHYSICAL SOC, Mar. 2016, [Peer-reviewed]
English, Scientific journal, To implement fault-tolerant quantum computation, entanglement generation with low error probability and high success probability is required. In a previous paper, we proposed the use of squeezed coherent light as a probe to generate entanglement between two atoms by communication and showed that the error probability is reduced well below the threshold of fault-tolerant quantum computation [F. Matsuoka et al., Phys. Rev. A 88, 022313 (2013)]. In this paper, we investigate the effect of photon loss mainly due to finite coupling efficiency to the cavity. The error probability with photon loss is calculated using a beam-splitter model for homodyne measurement of probe light. We examine the optimum conditions of the amplitude of the probe light and the degree of squeezing to minimize the error probability. We show that the phase -squeezed probe light yields lower error probability than a coherent -light probe, even with photon losses. A fault-tolerant quantum computation algorithm can be implemented under 87% transmittance by concatenating a seven-qubit error correction code for the phase-flip error.

Virtual interferogram-generation algorithm for phase measurement using two interferograms
Jin Nozawa, Atsushi Okamoto, Masataka Toda, Yasuyuki Kuno, Akihisa Tomita
PRACTICAL HOLOGRAPHY XXX: MATERIALS AND APPLICATIONS, 9771, SPIE-INT SOC OPTICAL ENGINEERING, 2016, [Peer-reviewed]
English, International conference proceedings, Phase-shifting digital holography is a technique for phase measurement with high spatial resolution and applied to many fields. This technique typically requires four phase-shifted interferograms between a signal beam and a reference beam. We focused on the two-step phase shifting algorithm, which needs only two phase-shifted interferograms and an intensity distribution of the reference beam to reduce the number of required interferograms. However, in this algorithm, the intensity of the reference beam must be much greater than that of the signal beam because this algorithm uses the quadratic formula and the inside of square root must be positive. This leads to the saturation of the dynamic range of the image sensor and the degradation of accuracy. In this paper, we propose a virtual interferogram-generation algorithm (VIGA) to improve the performance of phase-shifting digital holography only using two interferograms. This algorithm virtually generates a pi phase-shifted interferogram by the intensity distribution of the signal beam and that of the reference beam with an observed interferogram. Therefore, capturing two real interferograms and generating two virtual interferograms, the four-step phase shifting algorithm can be used for this method. Comparing to the conventional algorithm, the VIGA has no limitation in terms of the magnitude of the intensity. This means that the intensity of the reference beam and that of the signal beam can be equalized and the dynamic range saturation of the image sensors can be prevented. Therefore, the VIGA makes highly accurate phase measurement possible owing to the effective utilization of the dynamic range of the image sensors.

Highly accurate spatial mode generation using spatial cross modulation method for mode division multiplexing
Hiroki Sakuma, Atsushi Okamoto, Atsushi Shibukawa, Yuta Goto, Akihisa Tomita
NEXT-GENERATION OPTICAL COMMUNICATION: COMPONENTS, SUB-SYSTEMS, AND SYSTEMS V, 9774, SPIE-INT SOC OPTICAL ENGINEERING, 2016, [Peer-reviewed]
English, International conference proceedings, We propose a spatial mode generation technology using spatial cross modulation (SCM) for mode division multiplexing (MDM). The most well-known method for generating arbitrary complex amplitude fields is to display an off-axis computer-generated hologram (CGH) on a spatial light modulator (SLM). However, in this method, a desired complex amplitude field is obtained with first order diffraction light. This critically lowers the light utilization efficiency. On the other hand, in the SCM, the desired complex field is provided with zeroth order diffraction light. For this reason, our technology can generate spatial modes with large light utilization efficiency in addition to high accuracy. In this study, first, a numerical simulation was performed to verify that the SCM is applicable for spatial mode generation. Next, we made a comparison from two view points of the coupling efficiency and the light utilization between our technology and the technology using an off-axis amplitude hologram as a representative complex amplitude generation method. The simulation results showed that our technology can achieve considerably high light utilization efficiency while maintaining the enough coupling efficiency comparable to the technology using an off-axis amplitude hologram. Finally, we performed an experiment on spatial modes generation using the SCM. Experimental results showed that our technology has the great potential to realize the spatial mode generation with high accuracy.

Spatial-Mode Conversion using Random Diffuser and Spatial Light Modulator for Reduction of Modal Crosstalk
Koki Ishii, Atsushi Okamoto, Takehiro Tsuritani, Yuta Wakayama, Yuta Goto, Akihisa Tomita
NEXT-GENERATION OPTICAL COMMUNICATION: COMPONENTS, SUB-SYSTEMS, AND SYSTEMS V, 9774, SPIE-INT SOC OPTICAL ENGINEERING, 2016, [Peer-reviewed]
English, International conference proceedings, The mode-division multiplexing (MDM) technique enables the transmission of multiple signals within a multi-mode fiber (MMF) or a few-mode fiber (FMF). To construct an efficient and flexible MDM network in the same way as a wavelength-division multiplexing network, a mode conversion method with low modal crosstalk is required for switching between arbitrary spatial modes. However, in general, modal crosstalk is strongly dependent on the intensity pattern before mode conversion, and it is increased particularly for higher order modes. In order to reduce modal crosstalk, we propose a method using a random diffuser and a spatial light modulator (SLM). In the proposed method, firstly, the input spatial mode is dispersed uniformly by the random diffuser. Subsequently, the diffused phase distribution is canceled and converted into the desired spatial mode by the SLM, which displays phase difference between desired and diffused modes. Consequently, every spatial mode can be evenly converted into a desired mode. Here, we numerically simulate and confirm that the proposed method can reduce modal crosstalk compared to the conversion method without the random diffuser.

Two-channel algorithm for single-shot, high-resolution measurement of optical wavefronts using two image sensors
Jin Nozawa, Atsushi Okamoto, Atsushi Shibukawa, Masanori Takabayashi, Akihisa Tomita
APPLIED OPTICS, 54, 29, 8644, 8652, OPTICAL SOC AMER, Oct. 2015, [Peer-reviewed]
English, Scientific journal, We propose a two-channel holographic diversity interferometer (2ch-HDI) system for single-shot and highly accurate measurements of complex amplitude fields with a simple optical setup. In this method, two phase-shifted interference patterns are generated, without requiring a phase-shifting device, by entering a circularly polarized reference beam into a polarizing beam splitter, and the resulting patterns are captured simultaneously using two image sensors. However, differences in the intensity distributions of the two image sensors may lead to serious measurement errors. Thus, we also develop a two-channel algorithm optimized for the 2ch-HDI to compensate for these differences. Simulation results show that this algorithm can compensate for such differences in the intensity distributions in the two image sensors. Experimental results confirm that the combination of the 2ch-HDI and the calculation algorithm significantly enhances measurement accuracy. (c) 2015 Optical Society of America

Two-channel algorithm for single-shot, high-resolution measurement of optical wavefronts using two image sensors.
Nozawa J, Okamoto A, Shibukawa A, Takabayashi M, Tomita A
Applied optics, 54, 29, 8644, 8652, Oct. 2015, [Peer-reviewed]

23pAN-6 Generation of Phase-Squeezed Optical Pulses with Large Coherent Amplitudes by Post-Selection of Single Photons and Weak Cross-Kerr Nonlinearity
Matsuoka F., Tomita A., Shikano Y.
Meeting Abstracts of the Physical Society of Japan, 70, 716, 716, The Physical Society of Japan (JPS), 2015
Japanese

Optical tomography using a random diffuser and digital phase conjugation
Yuta Goto, Atsushi Okamoto, Atsushi Shibukawa, Akihisa Tomita, Kunihiro Sato
Progress in Biomedical Optics and Imaging - Proceedings of SPIE, 9319, SPIE, 2015, [Peer-reviewed]
English, International conference proceedings, We proposed a new technology for tomographic imaging based on beam diffusion and wavefront reconstruction through digital phase conjugation (DPC). The principle of this technology is highly unique and completely different from that of well-known optical coherence tomography (OCT) because it does not utilize the coherence property of light. In our experiment, it was shown that the depth resolution of smaller than 51/4m is obtained when using the objective lens with NA of 0.42. In addition, we achieved the extraction of the information from a specific specimen among several specimens arranged along optical axis.

VIRTUAL INTERFEROGRAM-GENERATION ALGORITHM FOR PHASE-SHIFTING DIGITAL HOLOGRAPHY
Jin Nozawa, Atsushi Okamoto, Masataka Toda, Yasuyuki Kuno, Akihisa Tomita
2015 20TH MICROOPTICS CONFERENCE (MOC), IEEE, 2015, [Peer-reviewed]
English, International conference proceedings, A virtual interferogram-generation algorithm is proposed to improve the performance of phase-shifting digital holography. Combining the intensity distribution of a signal and that of a reference beam with two observed interferograms, this algorithm generates four interferograms with different phases. Simulation results confirmed the basic operation of this algorithm.

OPTIMIZATION OF DIFFRACTION EFFICIENCY AND COUPLING EFFICIENCY IN SPATIAL MODE CONVERSION FOR PHOTONIC CROSS CONNECTER
Yanfeng Zhao, Atsushi Okamoto, Tomokazu Oda, Akihisa Tomita, Masatoshi Bunsen, Satoshi Honma
2015 20TH MICROOPTICS CONFERENCE (MOC), IEEE, 2015, [Peer-reviewed]
English, International conference proceedings, A photonic cross-connecter (PXC), which is capable of dynamic parallel conversions between different spatial modes, has been proposed. In this numerical simulation, to explore the potential of PXC, we investigated the optimal parameter settings at the hologram recording by implementing conversion from LP11 mode to LP21 mode.

HIGH-RESOLUTION AND SIMULTANEOUS MEASUREMENT ALONG THE DEPTH DIRECTION USING VIRTUAL PHASE CONJUGATION FOR OPTICAL TOMOGRAPHY
Yuta Goto, Atsushi Okamoto, Atsushi Shibukawa, Akihisa Tomita, Masanori Takabayashi
2015 20TH MICROOPTICS CONFERENCE (MOC), IEEE, 2015, [Peer-reviewed]
English, International conference proceedings, To improve the measurement speed of optical tomography based on the virtual phase conjugation, a virtual measurement position control technique is proposed. Tomographic images were observed with high depth-resolution and high acquisition rate in the numerical simulations.

Parallel and Simultaneous Spatial Mode Conversion using Photorefractive Crystal for Photonic Cross-Connect
Yanfeng Zhao, Atsushi Okamoto, Tomohiro Maeda, Yuki Hirasaki, Akihisa Tomita, Masatoshi Bunsen
NEXT-GENERATION OPTICAL COMMUNICATION: COMPONENTS, SUB-SYSTEMS, AND SYSTEMS IV, 9389, SPIE-INT SOC OPTICAL ENGINEERING, 2015, [Peer-reviewed]
English, International conference proceedings, In this paper, we present a novel technology for photonic cross-connect (PXC) in spatial mode domain for the realization of advanced and flexible optical transmission of spatial modes. The PXC is a kind of all -optical devices to switch highspeed optical signals for mode-division multiplexing (MDM) network and it is able to perform signal labeling in the spatial mode domain similar to current photonic switching in the wavelength domain. In addition, parallel and simultaneous mode conversion can be realized using multiplex holograms in a photorefractive crystal (PRC). In our experiment, during the recording process, a rewritable hologram is recorded in the PRC (LiNbO3) through the interference between the signal beam with certain input mode and the reference beam with the phase distribution of the desired output mode. Signal beams are generated by computer generated hologram (CGH) using a spatial light modulator (SLM) instead of an optical fiber emergent beam, and reference beams are generated by phase only modulation using another SLM. Subsequently, during the converting process, the input signal beam is converted into the desired output mode through the holographic diffract ion in the crystal and free-space propagation by an optical lens. By using phase code multiplexing method, parallel mode conversions can be realized. We performed an experiment on parallel mode conversions of several different conversion pairs. Signal beams and reference beams intersected in the PRC with an angle of 18.43 degree. The intensity distributions of converted modes were observed by CCD camera set on the Fourier plane. We confirmed that the two modes inter-conversion of LP11 with LP21 was successfully implemented.

Optical tomography using a random diffuser and digital phase conjugation
Yuta Goto, Atsushi Okamoto, Atsushi Shibukawa, Akihisa Tomita, Kunihiro Sato
OPTICAL TOMOGRAPHY AND SPECTROSCOPY OF TISSUE XI, 9319, SPIE-INT SOC OPTICAL ENGINEERING, 2015, [Peer-reviewed]
English, International conference proceedings, We proposed a new technology for tomographic imaging based on beam diffusion and wavefront reconstruction through digital phase conjugation (DPC). The principle of this technology is highly unique and completely different from that of well-known optical coherence tomography (OCT) because it does not utilize the coherence property of light. In our experiment, it was shown that the depth resolution of smaller than 5 mu m is obtained when using the objective lens with NA of 0.42. In addition, we achieved the extraction of the information from a specific specimen among several specimens arranged along optical axis.

IMPROVEMENT OF MEASUREMENT-SPEED BY VIRTUAL OPTICAL-SYSTEM FOR CONFOCAL LASER SCANNING MICROSCOPE
Yuta Goto, Atsushi Okamoto, Masataka Toda, Yasuyuki Kuno, Akihisa Tomita
2015 20TH MICROOPTICS CONFERENCE (MOC), IEEE, 2015, [Peer-reviewed]
English, International conference proceedings, We propose a virtual optical-system for the confocal laser scanning microscope, which enables to measure tomographic images with the high acquisition rate. In our technique, by utilizing the virtual optical-system based on complex-amplitude detection, multi-layered tomographic images can be measure collectively without mechanical scanning.

EXPANSION METHOD FOR DEPTH MEASUREMENT RANGE BASED ON NUMBER THEORY USING TWO WAVELENGTH LIGHT SOURCES
Jin Nozawa, Atsushi Okamoto, Masataka Toda, Yasuyuki Kuno, Akihisa Tomita
2015 20TH MICROOPTICS CONFERENCE (MOC), IEEE, 2015, [Peer-reviewed]
English, International conference proceedings, The expansion method for the depth measurement range is proposed. After capturing two wavelength phase images, the depth information is reconstructed by estimating the number of phase discontinuity in these images. The measurable depth-range can be expanded to hundreds times of wavelength.

Experiment on three-dimensional display using spatial cross modulation method
Yuta Kan, Atsushi Okamoto, Akihisa Tomita, Atsushi Shibukawa, Hisatoshi Funakoshi
PRACTICAL HOLOGRAPHY XXIX: MATERIALS AND APPLICATIONS, 9386, SPIE-INT SOC OPTICAL ENGINEERING, 2015, [Peer-reviewed]
English, International conference proceedings, We performed an experiment on a 3D display by employing spatial cross modulation ( SCM) using a random diffuser and phase-only spatial light modulator (SLM). In our experiment, a 3D object, which is constructed using four 2D images acquired from different spatial positions, is generated and the encoded 3D object is then displayed on the SLM. The different object images are observed when the image sensor is moved to different spatial positions. The experimental results revealed that the SCM method allows the generation of arbitrarily complex amplitude fields with high diffraction efficiency and with almost no speckle noise.

Evaluation of the phase randomness of a light source in quantum-key-distribution systems with an attenuated laser
Toshiya Kobayashi, Akihisa Tomita, Atsushi Okamoto
PHYSICAL REVIEW A, 90, 3, 032320, AMER PHYSICAL SOC, Sep. 2014, [Peer-reviewed]
English, Scientific journal, Phase-randomized light is one of the key assumptions in the security proof of the Bennett-Brassard 1984 (BB84) quantum-key-distribution (QKD) protocol using an attenuated laser. Though the assumption has been believed to be satisfied for conventional systems, it should be reexamined for current high-speed QKD systems. The phase correlation may be induced by the overlap of the optical pulses, the interval of which decreases as the clock frequency increases. The phase randomness was investigated experimentally by measuring the visibility of interference. An asymmetric Mach-Zehnder interferometer was used to observe the interference between adjacent pulses from a gain-switched distributed feedback laser diode driven at 10 GHz. Low visibility was observed when the minimum drive current was set far below the threshold, while interference emerged when the minimum drive current was close to the threshold. The theoretical evaluation on the impact of the imperfect phase randomization provides target values for the visibility to guarantee the phase randomness. The experimental and theoretical results show that secure implementation of decoy BB84 protocol is achievable even for the 10-GHz clock frequency by using the laser diode under proper operating conditions.

Background light reduction method with a double phase conjugate mirror and a phase plate for optical inter-satellite communications
Kaori Nishimaki, Atsushi Okamoto, Atsushi Shibukawa, Akihisa Tomita, Yoshihisa Takayama, Masatoshi Bunsen
JAPANESE JOURNAL OF APPLIED PHYSICS, 53, 8, IOP PUBLISHING LTD, Jul. 2014
English, Scientific journal, In optical intersatellite communication, the background light consisting mainly of sunlight decrease transmission rate with increasing bit error. A spatial filtering system using a double-phase-conjugate mirror (DPCM) and a phase plate is proposed to block the background light, which cannot be filtered out by using wavelength filters, polarization filters, or dousers. In this system, the effect of wavefront distortion caused by the phase plate and the compensation effect of the distortion by phase conjugate light from the DPCM are effectively combined for spatially separating signal light and background light. We demonstrated the reduction of the background light by the proposed system and optimized the phase plate for maximizing this reduction effect. As a result, it was clarified that the transmission rate can be drastically improved up to 11-fold. (C) 2014 The Japan Society of Applied Physics

Modified E91 protocol demonstration with hybrid entanglement photon source
Mikio Fujiwara, Ken-ichiro Yoshino, Yoshihiro Nambu, Taro Yamashita, Shigehito Miki, Hirotaka Terai, Zhen Wang, Morio Toyoshima, Akihisa Tomita, Masahide Sasaki
OPTICS EXPRESS, 22, 11, 13616, 13624, OPTICAL SOC AMER, Jun. 2014, [Peer-reviewed]
English, Scientific journal, We report on an experimental demonstration of the modified Ekert 91 protocol of quantum key distribution using a hybrid entanglement source with two different degrees of freedoms, a 1550 nm time-bin qubit and 810 nm polarization qubit. The violation of the Clauser-Horne-Shimony-Holt inequality could be demonstrated for the entanglement between the polarization qubit in free space and the time-bin qubit through 20 km fiber transmission. The secure key rate in our system is estimated 70-150 bps. (C) 2014 Optical Society of America

Dynamically Reconfigurable Characteristics of a Double Phase Conjugate Mirror Using Sn2P2S6 Crystals and Their Application to Optical Inter-Satellite Communication
Kaori Nishimaki, Atsushi Okamoto, Atsushi Shibukawa, Masanori Takabayashi, Akihisa Tomita, Yoshihisa Takayama
OPTICAL REVIEW, 21, 3, 415, 424, OPTICAL SOC JAPAN, May 2014, [Peer-reviewed]
English, Scientific journal, A double phase conjugate mirror (DPCM), created by two mutually incoherent beams entering photorefractive nonlinear materials, can generate a phase conjugate beam whose reflectivity may be greater than 100%. Even though the conditions of the incident beams are changed, the DPCM can be dynamically reconfigured by using a Sn2P2S6 crystal with a high response speed. These features of the DPCM are advantageous, particularly in an optical inter-satellite communication system. In particular, use of the phase conjugate beam from the DPCM offers wavefront compensation and amplification in satellite communication. In addition, the dynamically reconfigurable DPCM using a Sn2P2S6 crystal relaxes the acquisition accuracy of the signal beam in the system. In this study, the temporal and spatial operating characteristics of the DPCM using a Sn2P2S6 crystal were first clarified. Next, an inter-satellite system based on the DPCM was proposed, and it was demonstrated that our system significantly improves the tolerance of the acquisition accuracy and tracking time. (C) 2014 The Japan Society of Applied Physics

Digital phase conjugate mirror by parallel arrangement of two phase-only spatial light modulators
Atsushi Shibukawa, Atsushi Okamoto, Yuta Goto, Satoshi Honma, Akihisa Tomita
OPTICS EXPRESS, 22, 10, 11918, 11929, OPTICAL SOC AMER, May 2014, [Peer-reviewed]
English, Scientific journal, In a conventional digital phase conjugation system, only the phase of an input light is time-reversed. This deteriorates phase conjugation fidelity and restricts application fields to specific cases only when the input light has uniformly-distributed scattered wavefront. To overcome these difficulties, we present a digital phase conjugate mirror based on parallel alignment of two phase-only spatial light modulators (SLMs), in which both amplitude and phase of the input light can be time-reversed. Experimental result showed that, in the phase conjugation through a holographic diffuser with diffusion angle of 0.5 degree, background noises decrease to 65% by our digital phase conjugation mirror. (C)2014 Optical Society of America

Spatial cross modulation method using a random diffuser and phase-only spatial light modulator for constructing arbitrary complex fields
Atsushi Shibukawa, Atsushi Okamoto, Masanori Takabayashi, Akihisa Tomita
Optics Express, 22, 4, 3968, 3982, 24 Feb. 2014, [Peer-reviewed]
English, Scientific journal, We propose a spatial cross modulation method using a random diffuser and a phase-only spatial light modulator (SLM), by which arbitrary complex-amplitude fields can be generated with higher spatial resolution and diffraction efficiency than off-axis and double-phase computergenerated holograms. Our method encodes the original complex object as a phase-only diffusion image by scattering the complex object using a random diffuser. In addition, all incoming light to the SLM is consumed for a single diffraction order, making a diffraction efficiency of more than 90% possible. This method can be applied for holographic data storage, threedimensional displays, and other such applications. © 2014 Optical Society of America.

Progressive phase conjugation and its application in reconfigurable spatial-mode extraction and conversion
Atsushi Okamoto, Tomohiro Maeda, Yuki Hirasaki, Akihisa Tomita, Kunihiro Sato
MICRO-OPTICS 2014, 9130, SPIE-INT SOC OPTICAL ENGINEERING, 2014, [Peer-reviewed]
English, International conference proceedings, We develop a new technology, which is referred to as progressive phase conjugation (PPC), in which phase conjugation is electrically performed without requiring a coherent reference beam by fusion using a reference-free spatial phase detection and spatial phase modulation. This method enables remote setting of a phase detector from the signal transmitter without an additional transmission line for the reference beam. It also enables realization of high-speed and dynamic wavefront compensation owing to its open-loop architecture using the single-shot phase detection method. Therefore, the PPC is applicable to a wide range of optical communication technologies, including the reconfigurable spatial-mode extraction and conversion of mode transmission in a multi-mode fiber (MMF). In our experiment, spatial modes are generated by directing a laser beam into a MMF with a 50-micron core diameter. At the output side of the optical fiber, the phase distributions of the spatial modes are detected using the reference-free phase detector constructed by combining a spatial filtering method with holographic diversity interferometry using two CCD imagers. Then, the phase conjugate distribution of the detected phase pattern is displayed on a LCOS-type SLM. We confirm that the PPC system can extract a specific mode pattern with a considerably low crosstalk of less than 1% by displaying the corresponding phase-conjugation pattern on the SLM. In addition, we demonstrated a reconfigurable spatial-mode conversion by the phase control technology using the SLM. By applying the spatial phase modulation to an optical beam incident on the SLM, the spatial mode of the output beam is flexibly changed.

High - speed bridge circuit for InGaAs avalanche photodiode single - photon detector
Hirofumi Hashimoto, Akihisa Tomita, Atsushi Okamoto
ADVANCES IN PHOTONICS OF QUANTUM COMPUTING, MEMORY, AND COMMUNICATION VII, 8997, SPIE-INT SOC OPTICAL ENGINEERING, 2014, [Peer-reviewed]
English, International conference proceedings, Because of low power consumption and small footprint, avalanche photodiodes (APD) have been commonly applied to photon detection. Recently, high speed quantum communication has been demonstrated for high bit-rate quantum key distribution. For the high speed quantum communication, photon detectors should operate at GHz-clock frequencies. We propose balanced detection circuits for GHz-clock operation of InGaAs-APD photon detectors. The balanced single photon detector operates with sinusoidal wave gating. The sinusoidal wave appearing in the output is removed by the subtraction from APD signal without sharp band-elimination filters. Omission of the sharp filters removes the constraint on the operating frequency of the single photon detector. We present two designs, one works with two identical APDs, the other with one APD and a low-pass filter. The sinusoidal gating enables to eliminate the gating noise even with the simple configuration of the latter design. We demonstrated the balanced single photon detector operating with 1.020GHz clock at 233 K, 193 K, and 186.5 K. The dark count probability was 4.0 x 10(-4) counts/pulse with the quantum efficiency of 10% at 233K, and 1.6 x 10(-4) counts/pulse at 186.5 K. These results were obtained with easily available APDs (NR8300FP-C. C, RENESASS) originally developed for optical time-domain reflectmeters.

Reconfigurable Spatial Mode Conversion using a Phase-Type Spatial Light Modulator
Tomohiro Maeda, Atsushi Okamoto, Yuki Hirasaki, Atsushi Shibukawa, Akihisa Tomita
2014 OPTOELECTRONICS AND COMMUNICATIONS CONFERENCE AND AUSTRALIAN CONFERENCE ON OPTICAL FIBRE TECHNOLOGY (OECC/ACOFT 2014), 517, 518, IEEE, 2014, [Peer-reviewed]
English, International conference proceedings, We propose the reconfigurable spatial mode conversion method using a phase-type spatial light modulator, and implement the experiment for interconversion among LP01, LP11, and higher-order spatial modes.

Adaptive Compensation for Extraction of Spatial Modes using Progressive Phase Conjugation
Yuki Hirasaki, Atsushi Okamoto, Tomohiro Maeda, Akihisa Tomita, Kunihiro Sato
2014 OPTOELECTRONICS AND COMMUNICATIONS CONFERENCE AND AUSTRALIAN CONFERENCE ON OPTICAL FIBRE TECHNOLOGY (OECC/ACOFT 2014), 230, +, IEEE, 2014, [Peer-reviewed]
English, International conference proceedings, We experimentally perform the adaptive compensation of distorted spatial modes after passing through a multi-mode fiber and the simultaneous extraction of multiple spatial modes by using progressive phase conjugation.

Evaluation of the phase correlation between the optical pulses for transmission in quantum key distribution
Toshiya Kobayashi, Akihisa Tomita, Atsushi Okamoto
ADVANCES IN PHOTONICS OF QUANTUM COMPUTING, MEMORY, AND COMMUNICATION VII, 8997, SPIE-INT SOC OPTICAL ENGINEERING, 2014, [Peer-reviewed]
English, International conference proceedings, One of the key assumptions in the security proof of BB84 quantum key distribution (QKD) protocol with a weak coherent light source is that the phase of each pulse emitted from the source is random. If the phases correlated, an efficient eavesdropping method exists, and more sacrifice bits are necessary to guarantee the security of the protocol. The phase correlation between the adjacent pulses was examined experimentally, for the first time, by measuring the visibility of interference. An asymmetric Mach-Zehnder interferometer was used to observe the interference between adjacent pulses from a distributed feedback (DFB) laser diode driven by 10 GHz clock signal. Clear interference was observed when the DC bias current exceeds the laser threshold. This high visibility implies phase correlation between pulses. In contrast, only a low visibility, i.e. very weak phase correlation, was observed when the DC bias current was set far below the threshold. The phase correlation became stronger as the DC bias current increased. The experimental results imply that a QKD equipment implementing BB84 protocol with a coherent light source is secure as long as the laser is operated in gain-switch even for 10 GHz clock frequency.

Effectiveness of double phase conjugate mirror for optical inter-satellite communication with large shift tolerance
Kaori Nishimaki, Atsushi Okamoto, Akihisa Tomita, Yoshihisa Takayama, Masatoshi Bunsen
Technical Digest of the 18th Microoptics Conference, MOC 2013, 01 Dec. 2013
We propose an innovative optical inter-satellite communication system with a double phase conjugate mirror. The proposed system improves the acquisition, tracking and background light reduction functions by utilization of phase conjugation. This paper reports on the temporal and spatial characteristics of double phase conjugate mirror to consider large shift tolerance. © 2013 The Japan Society of Applied Physics.

Digital Image Diffusion Technique for Suppressing Interpage Crosstalk in Holographic Data Storage
Atsushi Shibukawa, Atsushi Okamoto, Yuta Wakayama, Akihisa Tomita, Hisatoshi Funakoshi, Kunihiro Sato
JAPANESE JOURNAL OF APPLIED PHYSICS, 52, 9, IOP PUBLISHING LTD, Sep. 2013, [Peer-reviewed]
English, Scientific journal, To suppress interpage crosstalk that strongly restricts holographic data storage capacity, a digital image diffusion technique was proposed. In this technique, signal and crosstalk components are separated from the propagation angle difference produced by using a random diffuser and the nature of phase conjugate lights. The most attractive feature of the technique is that crosstalk suppression can be conducted while mostly keeping the conventional holographic data storage system unchanged by integrating optical processing and digital processing effectively. The connection between the two types of processing is realized by using two spatial light modulators for complex amplitude generation and phase detection methods for complex amplitude measurement. The simulation results showed that the proposed technique could increase the recording density 5.5-fold and reduce the shift pitch by a factor of 5 in comparison with conventional multiplexing methods when the diffusion angle of the diffuser is 10 degrees. (c) 2013 The Japan Society of Applied Physics

Double-Referential Holography and Spatial Quadrature Amplitude Modulation
Keisuke Zukeran, Atsushi Okamoto, Masanori Takabayashi, Atsushi Shibukawa, Kunihiro Sato, Akihisa Tomita
JAPANESE JOURNAL OF APPLIED PHYSICS, 52, 9, IOP PUBLISHING LTD, Sep. 2013, [Peer-reviewed]
English, Scientific journal, We proposed a double-referential holography (DRH) that allows phase-detection without external additional beams. In the DRH, phantom beams, prepared in the same optical path as signal beams and preliminary multiplexed in a recording medium along with the signal, are used to produce interference fringes on an imager for converting a phase into an intensity distribution. The DRH enables stable and high-accuracy phase detection independent of the fluctuations and vibrations of the optical system owing to medium shift and temperature variation. Besides, the collinear arrangement of the signal and phantom beams leads to the compactness of the optical data storage system. We conducted an experiment using binary phase modulation signals for verifying the DRH operation. In addition, 38-level spatial quadrature amplitude modulation signals were successfully reproduced with the DRH by numerical simulation. Furthermore, we verified that the distributed phase-shifting method moderates the dynamic range consumption for the exposure of phantom beams. (c) 2013 The Japan Society of Applied Physics

Entanglement generation by communication using squeezed states
Fumiaki Matsuoka, Akihisa Tomita, Atsushi Okamoto
Physical Review A - Atomic, Molecular, and Optical Physics, 88, 2, 12 Aug. 2013, [Peer-reviewed]
English, Scientific journal, In order to improve the error probability of generating entanglement by communication for quantum computation, we propose the use of squeezed light. When generating entanglement between two atoms by communication, the error probability can be reduced by increasing the distance between quantum states of probe light in phase space. The phase rotation of light depends on the atom-photon coupling strength and the light amplitude, which are limited in practice. A large error probability has been expected for coherent probe light. If we assume typical values of light amplitude and phase rotation, α=100 and θ=0.01, the error probability is estimated to be Pcoh(min)=0.14 and Pcoh(hom)=0.23 for minimum error discrimination and homodyne measurements. The error probability can be reduced to Psqu(min)=1.73×10-8 and Psqu(hom)=4.09×10-5 using squeezed coherent light, where the same values of the mean photon number and the phase rotation angle are assumed for the coherent light probe. These values satisfy the requirements for scalable quantum computation. © 2013 American Physical Society.

Mode demultiplexer using angularly multiplexed volume holograms
Yuta Wakayama, Atsushi Okamoto, Kento Kawabata, Akihisa Tomita, Kunihiro Sato
OPTICS EXPRESS, 21, 10, 12920, 12933, OPTICAL SOC AMER, May 2013, [Peer-reviewed]
English, Scientific journal, This study proposes a volume holographic demultiplexer (VHDM) for extracting the spatial modes excited in a multimode fiber. A unique feature of the demultiplexer is that it can separate a number of multiplexed modes output from a fiber in different directions by using multi-recorded holograms without beam splitters, which results in a simple configuration as compared with that using phase plates instead of holograms. In this study, an experiment is conducted to demonstrate the basic operations for three LP mode groups to confirm the performance of the proposed VHDM and to estimate the signal-to-crosstalk noise ratio (SNR). As a result, an SNR of greater than 20 dB is obtained. (C) 2013 Optical Society of America

Selective multimode excitation using volume holographic mode multiplexer
Kohei Aoki, Atsushi Okamoto, Yuta Wakayama, Akihisa Tomita, Satoshi Honma
Optics Letters, 38, 5, 769, 771, 01 Mar. 2013, [Peer-reviewed]
English, Scientific journal, We propose a mode multiplexer based on volume holograms to realize a simple and efficient mode-division-multiplexed transmission system that supports a large number of modes. Selective multiexcitation of three spatial modes into a conventional multimode fiber is experimentally demonstrated. This device could potentially multiplex 10 or more modes. Future perspectives of the mode multiplexer for application in mode-division multiplexing are also discussed. © 2013 Optical Society of America.

Conversion and extraction of spatial modes from a multimode fiber by reference-free holographic-diversity interferometry　　　　　　　　　　　　　　　
Yuki Hirasaki, Atsushi Okamoto, Tomohiro Maeda, Akihisa Tomita, Yuta Wakayama
2013 18th OptoElectronics and Communications Conference Held Jointly with 2013 International Conference on Photonics in Switching, OECC/PS 2013, 2013
English, International conference proceedings, The wave fronts of speckle fields exiting a multimode fiber are controlled by using a spatial light modulator in cooperation with reference-free holographic diversity interferometry for the high performance adaptive compensation and multiplex transmission. © 2013 IEICE.

Spatial mode excitation and separation using spatial phase control technology　　　　　　　　　　　　　　　
Atsushi Okamoto, Akihisa Tomita, Kento Kawabata, Yuta Wakayama
2013 18th OptoElectronics and Communications Conference Held Jointly with 2013 International Conference on Photonics in Switching, OECC/PS 2013, 2013
English, International conference proceedings, Computer-generated holography is actively used to generate the spatial mode distribution required for mode excitation and hologram writing. In addition, we propose a new transmission scheme using the phase conjugation technology. © 2013 IEICE.

Holographic-diversity interferometry for reference-free phase detection
Tomohiro Maeda, Atsushi Okamoto, Akihisa Tomita, Yuki Hirasaki, Yuta Wakayama, Masatoshi Bunsen
Pacific Rim Conference on Lasers and Electro-Optics, CLEO - Technical Digest, 2013, [Peer-reviewed]
English, International conference proceedings, Reference-free phase detection is demonstrated using advanced holographic diversity interferometry in which a virtual light source is locally produced from an object light passed through testing samples to simultaneously generate two phase-shifted interferograms. © 2013 IEEE.

Long-term Field Demonstration of WDM Quantum Key Distribution System with Stabilization Control
K. Yoshino, T. Ochi, M. Fujiwara, A. Tomita, M. Sasaki, A. Tajima
2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS PACIFIC RIM (CLEO-PR), IEEE, 2013, [Peer-reviewed]
English, International conference proceedings, A wavelength-division multiplexing quantum key distribution system with stabilization control was demonstrated through a 22-km field fiber in five-day continuous operation. Quantum bit error rates were kept below 3% in all key blocks.

Separation of LP modes using volume holographic demultiplexer with a dual-wavelength method for mode division multiplexing
Kento Kawabata, Atsushi Okamoto, Satoshi Honma, Yuta Wakayama, Kunihiro Sato, Akihisa Tomita
NEXT-GENERATION OPTICAL COMMUNICATION: COMPONENTS, SUB-SYSTEMS, AND SYSTEMS II, 8647, SPIE-INT SOC OPTICAL ENGINEERING, 2013, [Peer-reviewed]
English, International conference proceedings, We propose a volume holographic mode demultiplexer incorporating a dual-wavelength method, which enables us to easily and flexibly set up the receiving system in mode division multiplexing for a large number of mode multiplexing. This demultiplexer can separate a lot of multiplexed modes with different wavelengths through angularly hologram multiplexing by appropriate angular difference between the two holographic writing beams. Thereby, the proposed method can be applicable to wavelength division multiplexing systems. In this study, we demonstrated the mode separation using the proposed demultiplexer with dual-wavelength. The results showed that the separation ratios of three LP modes reached around 90%.

High-Speed Quantum Key Distribution System for 1-Mbps Real-Time Key Generation
Akihiro Tanaka, Mikio Fujiwara, Ken-ichiro Yoshino, Seigo Takahashi, Yoshihiro Nambu, Akihisa Tomita, Shigehito Miki, Taro Yamashita, Zhen Wang, Masahide Sasaki, Akio Tajima
IEEE JOURNAL OF QUANTUM ELECTRONICS, 48, 4, 542, 550, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Apr. 2012, [Peer-reviewed]
English, Scientific journal, A high-speed quantum key distribution (QKD) system has been developed with the goal of a 1-Mbps final secure key generation rate under 10-dB transmission loss, which corresponds to 50 km of standard single mode fiber. For the purpose of speeding-up all processes in QKD sequence, we apply a wavelength-division-multiplexing (WDM) technique using the colorless interferometric technique and a key distillation hardware (HW) engine. We establish a novel WDM scheme, sharing interferometers and their temperature regulators over multiple channels, which enables us to increase the number of channels with a small impact on system cost and size. To generate a secure key while satisfying both high speed and high security, we develop a key distillation HW engine which enables us to execute key distillation with 1-Mbit code length in real time. We have experimentally evaluated the performance of the developed system through installed fiber. By operating three wavelength channels, a new, world leading key generation rate of greater than 200 kbps over a 14.5-dB transmission loss has been achieved.

High-speed wavelength-division multiplexing quantum key distribution system
Ken-ichiro Yoshino, Mikio Fujiwara, Akihiro Tanaka, Seigo Takahashi, Yoshihiro Nambu, Akihisa Tomita, Shigehito Miki, Taro Yamashita, Zhen Wang, Masahide Sasaki, Akio Tajima
OPTICS LETTERS, 37, 2, 223, 225, OPTICAL SOC AMER, Jan. 2012, [Peer-reviewed]
English, Scientific journal, A high-speed quantum key distribution system was developed with the wavelength-division multiplexing (WDM) technique and dedicated key distillation hardware engines. Two interferometers for encoding and decoding are shared over eight wavelengths to reduce the system's size, cost, and control complexity. The key distillation engines can process a huge amount of data from the WDM channels by using a 1 Mbit block in real time. We demonstrated a three-channel WDM system that simultaneously uses avalanche photodiodes and superconducting single-photon detectors. We achieved 12 h continuous key generation with a secure key rate of 208 kilobits per second through a 45 km field fiber with 14.5 dB loss. (C) 2012 Optical Society of America

Sub-geiger mode single-photon detector using a low-darkcurrent InGaAs avalanche photodiode
Yoshito Miyamoto, Kenji Tsujino, Jun Kataoka, Akihisa Tomita
European Conference and Exhibition on Optical Communication, ECEOC 2012, 2012, [Peer-reviewed]
English, International conference proceedings, We have developed a single-photon detector that uses an InGaAs avalanche photodiode (APD) operating in sub-Geiger mode. Sub-Geiger mode operation is a technique in which an APD is operated at a bias voltage that is lower than the breakdown voltage. This mode considerably reduces afterpulse probability, and single photons that arrive randomly can be detected. In the present study, we reduced the dark count rate of the sub-Geiger mode single-photon detector by using a low-darkcurrent InGaAs APD. Consequently, we obtained a dark count rate (DCR) of 5.6 counts per second (cps) with single-photon detection efficiency (SPDE) of 0.2%. Our single-photon detector is comparable in DCR to that of superconducting nanowire single-photon detectors. © 2012 OSA.

Field demonstration of high-speed wavelength-division multiplexing quantum key distribution system and its stabilized operation
Ken-Ichiro Yoshino, Mikio Fujiwara, Akihiro Tanaka, Seigo Takahashi, Yoshihiro Nambu, Akihisa Tomita, Shigehito Miki, Taro Yamashita, Zhen Wang, Masahide Sasaki, Akio Tajima
Technical Digest - 2012 17th Opto-Electronics and Communications Conference, OECC 2012, 737, 738, 2012, [Peer-reviewed]
English, International conference proceedings, A high-speed quantum key distribution system with the wavelength-division multiplexing was demonstrated in 12hour continuous operation. We achieved a secure key rate of 208 kbps through a 45-km field fiber with 14.5-dB loss. © 2012 IEEE.

Tunable Holographic Wavelength Filter Using a Photorefractive Polymer for Ultra-Broad Reconfigurable Add-Drop Multiplexers
Yuta Wakayama, Atsushi Okamoto, Akihisa Tomita
2012 International Conference on Photonics in Switching (PS), IEEE, 2012, [Peer-reviewed]
English, International conference proceedings, Experimental result shows a broad 175-nm tuning range in a photorefractive polymer-based holographic filter for reconfigurable optical add-drop multiplexing. The tuning range of the filter can further be enhanced to 770 nm.

NOVEL PHOTON DETECTION TECHNOLOGIES FOR QUANTUM COMMUNICATIONS
Sasaki M, Tanaka A, Fujiwara M, Yoshino K, Takahashi S, Nambu Y, Tomita A, Tajima A, Miki S, Yamashita T, Terai H, Wang Z, Tsujino K, Fukuda D, Takeoka M
Advanced Photon Counting Techniques Vi, 8375, 2012, [Peer-reviewed]

Security in Photonic Networks: Threats and Security Enhancement
Ken-Ichi Kitayama, Masahide Sasaki, Soichiro Araki, Makoto Tsubokawa, Akihisa Tomita, Kyo Inoue, Katsuyoshi Harasawa, Yuki Nagasako, Atsushi Takada
JOURNAL OF LIGHTWAVE TECHNOLOGY, 29, 21, 3210, 3222, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Nov. 2011, [Peer-reviewed]
English, Scientific journal, We address emerging threats to the security of photonic networks as these networks become heterogeneous being opened to the upper layers, other operators, and end users. We review the potential threats, mainly loss of the confidentiality of user data transmitted through optical fibers and disturbances of network control, both of which could seriously damage the entire network. We then propose a novel conceptual model of a secure photonic network by introducing a quantum key distribution (QKD) network to its legacy structure. Secure keys generated by the QKD network are managed by key management agents (KMAs) and used to encrypt not only user data but also control signals. The KMAs cooperate with the generalized multiprotocol label-switching controller for secure path provisioning and drive photonic and modern crypto engines in appropriate combinations. Finally, we present a roadmap of a deployment scenario, starting from niche applications such as mission critical and business applications and the next. Digital cinema distribution through a photonic network is presented as an example of a niche application.

Efficient and low-noise single-photon avalanche photodiode for 1.244-GHz clocked quantum key distribution
Y. Nambu, S. Takahashi, K. Yoshino, A. Tanaka, M. Fujiwara, M. Sasaki, A. Tajima, S. Yorozu, A. Tomita
OPTICS EXPRESS, 19, 21, 20531, 20541, OPTICAL SOC AMER, Oct. 2011, [Peer-reviewed]
English, Scientific journal, An efficient and low-noise 1.244-GHz gating InGaAs single-photon avalanche photodiode (SAPD) was developed for a high-speed quantum key distribution (QKD) system. An afterpulsing probability of 0.61% and a dark count probability per gate of 0.71 x 10(-6) were obtained at a detection efficiency of 10.9% for 1.55-mu m photons. Furthermore, our SAPD successfully coped with high detection efficiency (<= 25%) and quite low afterpulsing noise (<= 3% for <= 25% efficiency) at the same time. Its potential was verified using the actual QKD setups installed over a metropolitan area network. (C) 2011 Optical Society of America

Afterpulse-like phenomenon of superconducting single photon detector in high speed quantum key distribution system
M. Fujiwara, A. Tanaka, S. Takahashi, K. Yoshino, Y. Nambu, A. Tajima, S. Miki, T. Yamashita, Z. Wang, A. Tomita, M. Sasaki
OPTICS EXPRESS, 19, 20, 19562, 19571, OPTICAL SOC AMER, Sep. 2011, [Peer-reviewed]
English, Scientific journal, We discuss our estimates of the performance of a superconducting single photon detector (SSPD) in a high speed quantum key distribution (QKD) system. We find that at high repetition operation reflections from the readout circuit at room temperature causes an afterpulse-like phenomenon, and drastically increases the quantum bit error rate (QBER). Such effects are not seen during low frequency operation. By using an amplifier with a small reflection coefficient S11, we succeed in reducing the afterpulse-like phenomenon and increasing a secure key rate. (C) 2011 Optical Society of America

Multilayer Collinear Holographic Memory with Movable Random Phase Mask
Atsushi Shibukawa, Atsushi Okamoto, Akihisa Tomita, Masanori Takabayashi, Kunihiro Sato
JAPANESE JOURNAL OF APPLIED PHYSICS, 50, 9, JAPAN SOC APPLIED PHYSICS, Sep. 2011, [Peer-reviewed]
English, Scientific journal, We proposed a new multilayer collinear holographic memory (MCHM) with a movable random phase mask that can act as an interlayer crosstalk reducer. First, to clarify the feasibility of our proposed device, we showed that it has an extremely sharp shift selectivity along the thickness direction of medium of nearly 5.0 mu m when the numerical aperture of the objective lens is 0.60. Next, we demonstrated that the utilization of the movable random phase mask can suppress interlayer crosstalk sufficiently, resulting in the improvement in the quality of reconstructed signals. Moreover, we revealed that the MCHM, in which the parallel use of a multilayered technique and collinear shift multiplexing is performed, brings out the medium potential sufficiently in terms of signal to noise ratio and medium dynamic range consumption. Finally, we verified that the MCHM can achieve a nearly 2 times larger data density at least compared with a conventional collinear holographic memory. (C) 2011 The Japan Society of Applied Physics

Symbol Error Characteristics of Hybrid-Modulated Holographic Data Storage by Intensity and Multi Phase Modulation
Masanori Takabayashi, Atsushi Okamoto, Akihisa Tomita, Masatoshi Bunsen
JAPANESE JOURNAL OF APPLIED PHYSICS, 50, 9, JAPAN SOC APPLIED PHYSICS, Sep. 2011, [Peer-reviewed]
English, Scientific journal, For the purpose of increasing the data density of holographic memories, we propose a novel signal modulation method named hybrid modulation method, which is a combination of multi level phase-modulated signals and intensity-modulated signals. As a first step, we show that the data density of holographic data storage is mainly determined by multiplexing numbers and by coding efficiency in input signals. In this study, we focus on increasing coding efficiency using the hybrid modulation method. From the results of recording and reading simulations based on the fast-Fourier transform beam propagation method, the hybrid modulation method achieves approximately 2 times higher data density than the conventional methods of two-level phase-only modulation and two-level intensity-only modulation. (C) 2011 The Japan Society of Applied Physics

Field demonstration of quantum key distribution in the Tokyo QKD Network
M. Fujiwara, H. Ishizuka, S. Miki, T. Yamashita, Z. Wang, A. Tanaka, K. Yoshino, Y. Nambu, S. Takahashi, A. Tajima, A. Tomita, T. Hasegawa, T. Tsurumaru, M. Matsui, T. Honjo, K. Tamaki, Y. Tokura, M. Sasaki
2011 International Quantum Electronics Conference (IQEC) and Conference on Lasers and Electro-Optics (CLEO) Pacific Rim incorporating the Australasian Conference on Optics, Lasers and Spectroscopy and the Australian Conference on Optical Fibre Technology, IEEE, Aug. 2011, [Peer-reviewed]
International conference proceedings

Holographic diversity interferometry for optical storage
Atsushi Okamoto, Keisuke Kunori, Masanori Takabayashi, Akihisa Tomita, Kunihiro Sato
OPTICS EXPRESS, 19, 14, 13436, 13444, OPTICAL SOC AMER, Jul. 2011, [Peer-reviewed]
English, Scientific journal, This study proposes holographic diversity interferometry (HDI), a system that combines information from spatially dispersed plural image sensors to reconstruct complex amplitude distributions of light signals. HDI can be used to generate four holographic interference fringes having different phases, thus enabling optical phase detection in a single measurement. Unlike conventional phase-shifting digital holography, this system does not require piezoelectric elements and phase shift arrays. In order to confirm the effectiveness of HDI, we generated optical signals having multilevel phases and amplitudes by using two SLMs and performed an experiment for detection and demodulation with HDI. (C) 2011 Optical Society of America

Field test of quantum key distribution in the Tokyo QKD Network
M. Sasaki, M. Fujiwara, H. Ishizuka, W. Klaus, K. Wakui, M. Takeoka, S. Miki, T. Yamashita, Z. Wang, A. Tanaka, K. Yoshino, Y. Nambu, S. Takahashi, A. Tajima, A. Tomita, T. Domeki, T. Hasegawa, Y. Sakai, H. Kobayashi, T. Asai, K. Shimizu, T. Tokura, T. Tsurumaru, M. Matsui, T. Honjo, K. Tamaki, H. Takesue, Y. Tokura, J. F. Dynes, A. R. Dixon, A. W. Sharpe, Z. L. Yuan, A. J. Shields, S. Uchikoga, M. Legre, S. Robyr, P. Trinkler, L. Monat, J. -B. Page, G. Ribordy, A. Poppe, A. Allacher, O. Maurhart, T. Laenger, M. Peev, A. Zeilinger
OPTICS EXPRESS, 19, 11, 10387, 10409, OPTICAL SOC AMER, May 2011, [Peer-reviewed]
English, Scientific journal, A secure communication network with quantum key distribution in a metropolitan area is reported. Six different QKD systems are integrated into a mesh-type network. GHz-clocked QKD links enable us to demonstrate the world-first secure TV conferencing over a distance of 45km. The network includes a commercial QKD product for long-term stable operation, and application interface to secure mobile phones. Detection of an eavesdropper, rerouting into a secure path, and key relay via trusted nodes are demonstrated in this network. (C) 2011 Optical Society of America

Evaluation of the performance in multilayer collinear holographic memory with movable random phase mask
Atsushi Shibukawa, Atsushi Okamoto, Akihisa Tomita, Masanori Takabayashi, Kunihiro Sato, Masatoshi Bunsen
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2011
English, International conference proceedings

Holographic diversity detection of spatial quadrature amplitude modulation signal for dual-stage holographic memory
Keisuke Kunori, Atsushi Okamoto, Akihisa Tomita, Masanori Takabayashi
Optics InfoBase Conference Papers, Optical Society of America (OSA), 2011
English, International conference proceedings, We propose a new method to detect multi-level phase signals by generating 4-pieces of digital hologram by single shot hologram reading without using the reference light of a resolution higher than the signal page data. © 2011 OSA.

Tokyo QKD network and the evolution to secure photonic network　　　　　　　　　　　　　　　
M. Sasaki, M. Fujiwra, H. Ishizuka, W. Klaus, K. Wakui, M. Takeoka, A. Tanaka, K. Yoshino, Y. Nambu, S. Takahashi, A. Tajima, A. Tomita, T. Domeki, T. Hasegawa, Y. Sakai, H. Kobayashi, T. Asai, K. Shimizu, T. Tokura, T. Tsurumaru, M. Matsui, T. Honjo, K. Tamaki, H. Takesue, Y. Tokura, J. F. Dynes, A. Dixon, A. W. Sharpe, Z. L. Yuan, A. J. Shields, S. Uchikoga, M. Legré, S. Robyr, P. Trinkler, L. Monat, J. B. Page, G. Ribordy, A. Poppe, A. Allacher, O. Maurhart, T. Länger, M. Peev, A. Zeilinger
Optics InfoBase Conference Papers, 2011
English, International conference proceedings, A novel secure communication network with quantum key distribution in a metropolitan area is reported. Different QKD schemes are integrated to demonstrate secure TV conferencing over a distance 45km, stable long-term operation, and application to secure mobile phones. © OSA/ CLEO 2011.

Tokyo QKD network and the evolution to secure photonic network　　　　　　　　　　　　　　　
M. Sasaki, M. Fujiwra, H. Ishizuka, W. Klaus, K. Wakui, M. Takeoka, A. Tanaka, K. Yoshino, Y. Nambu, S. Takahashi, A. Tajima, A. Tomita, T. Domeki, T. Hasegawa, Y. Sakai, H. Kobayashi, T. Asai, K. Shimizu, T. Tokura, T. Tsurumaru, M. Matsui, T. Honjo, K. Tamaki, H. Takesue, Y. Tokura, J. F. Dynes, A. Dixon, A. W. Sharpe, Z. L. Yuan, A. J. Shields, S. Uchikoga, M. Legré, S. Robyr, P. Trinkler, L. Monat, J. B. Page, G. Ribordy, A. Poppe, A. Allacher, O. Maurhart, T. Länger, M. Peev, A. Zeilinger
Optics InfoBase Conference Papers, 2011
English, International conference proceedings, A novel secure communication network with quantum key distribution in a metropolitan area is reported. Different QKD schemes are integrated to demonstrate secure TV conferencing over a distance 45km, stable long-term operation, and application to secure mobile phones. © OSA/ CLEO 2011.

Apodization along thickness direction of holographic transmission grating in Sb-doped Sn2P2S6
Y. Wakayama, A. Okamoto, A. Tomita, A. A. Grabar, K. Sato, H. Nihei
Proceedings of the International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD, 41, 42, 2011, [Peer-reviewed]
English, International conference proceedings, This report demonstrates a grating apodization method along the thickness direction in a real-time holographic medium for maximizing the diffraction efficiency. Dynamic photo-induced index modulation is examined by the finite-difference beam-propagation method. © 2011 IEEE.

High-photon-detection-efficiency silicon avalanche photodiode with charge-sensitive amplifier
Kenji Tsujino, Akihisa Tomita
2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011, 2011, [Peer-reviewed]
English, International conference proceedings, High-efficiency photon counters are critical in quantum optics applications and, in particular, in optical quantum information science, where imperfect photon detection can introduce significant errors in quantum communication or quantum processing. It was recently reported that superconducting transition edge sensors have a high photon detection efficiency of more than 95% [1,2]
however, these photon detectors have a few disadvantages, e.g., their small active area, which covers less than 20 × 20 7mu
m2, and the requirement for ultralow temperatures. By contrast, avalanche photodiodes (APDs) have active areas lager than 200 m across and can be conveniently operated between room temperature and liquid nitrogen temperature. However, the best photon detection efficiency reported for a silicon (Si) APD is still only 76.4% [3], with almost no progress seen since the 1990s. In this work, we developed a high-photon detection efficiency photon counter based on a Si APD with a low-noise charge-sensitive amplifier. © 2011 IEEE.

Hybrid entanglement photon pair source for fiber-space flexible QKD network
Mikio Fujiwara, Morio Toyoshima, Masahide Sasaki, Ken-Ichiro Yoshino, Yoshihiro Nambu, Akihisa Tomita
2011 International Conference on Space Optical Systems and Applications, ICSOS'11, 264, 268, 2011, [Peer-reviewed]
English, International conference proceedings, We report on the experimental demonstration of a source of hybrid entanglement pairs between two different degrees of freedom, a 1550 nm time-bin qubit and an 810 nm polarization qubit. The hybrid entanglement source is constructed of a non-degenerate photon pair source and a format transformer from time encoding to polarization encoding. This entanglement photon source will provide a fiber- space flexible QKD system. © 2011 IEEE.

All-optical demultiplexer based on dynamic multiple holograms for optical MIMO processing and mode division multiplexing
Takuya Oda, Atsushi Okamoto, Daiki Soma, Akihisa Tomita, Yuta Wakayama
BROADBAND ACCESS COMMUNICATION TECHNOLOGIES V, 7958, SPIE-INT SOC OPTICAL ENGINEERING, 2011, [Peer-reviewed]
English, International conference proceedings, We propose an all-optical demultiplexer based on dynamic multiple holograms using a photorefractive material for mode division multiplexing and optical MIMO processing systems. This demultiplexer can separate a specific spatial mode from multiplexed modes in a multi-mode fiber. In this study, we clarify a mode separation performance with various combinations of LP modes, and show the signal-to-noise ratio (SNR) can be improved by choosing appropriate mode combinations. In addition, to reveal the dependence of mode separation of the intensity ratio of the recording lights, we study the fluctuation of the SNR for LP0,1 mode by the combinations of multiplexed modes. Moreover, two additional recording techniques with a spatial light modulator (SLM) and phase conjugator are also proposed to improve the performance of the mode separation.

Autonomous and dynamic reconfigurable waveguide for optical interconnection with large shift-tolerance
Atsushi Okamoto, Kazutaka Hira, Alexander A. Grabar, Hisatoshi Funakoshi, Yuta Wakayama, Akihisa Tomita
OPTOELECTRONIC INTERCONNECTS AND COMPONENT INTEGRATION XI, 7944, SPIE-INT SOC OPTICAL ENGINEERING, 2011, [Peer-reviewed]
English, International conference proceedings, In order to realize a dynamic reconfiguration technique that automatically switches configurations and functions of an optical device, we need a technique to control freely the connections of light inside and between devices without needing submicron-level alignments. In this study, we investigate the behavior of dynamic index gratings with nanoscale reversible self-organization in Sn2P2S6 crystals that we have newly developed so as to realize an autonomous and dynamic reconfigurable optical waveguide by externally controlling its motions with light and examine its basic properties. Experimental results showed autonomous and dynamic reconfigurations of the optical waveguide formed in a Sb doped Sn2P2S6 crystal with a 4 mm thickness for variations of an incident light position. We have successfully reconfigured the waveguide by a self-organization based on a photorefractive effect without cutting time series signals flowing through the waveguide, for variations of an incident light position long as approximately 2000 mu m. Furthermore, we have recognized tolerance up to around 0.2 degrees for incidence angles in the experiment. This technique allows us to connect light freely without needing spatial adjustments in a nanostructured optical waveguide seen in photonic crystal fibers. Moreover, it is a technique that can be applied to dynamic connections between optical fibers and integrated waveguides accompanied with time variations of spatial modes. We also verified a possibility of removable and replaceable optical connection by utilizing large shift-tolerance of the autonomous and dynamic reconfigurable waveguide.

Tokyo QKD Network and the evolution to Secure Photonic Network
M. Sasaki, M. Fujiwra, H. Ishizuka, W. Klaus, K. Wakui, M. Takeoka, A. Tanaka, K. Yoshino, Y. Nambu, S. Takahashi, A. Tajima, A. Tomita, T. Domeki, T. Hasegawa, Y. Sakai, H. Kobayashi, T. Asai, K. Shimizu, T. Tokura, T. Tsurumaru, M. Matsui, T. Honjo, K. Tamaki, H. Takesue, Y. Tokura, J. F. Dynes, A. Dixon, A. W. Sharpe, Z. L. Yuan, A. J. Shields, S. Uchikoga, M. Legre, S. Robyr, P. Trinkler, L. Monat, J. -B. Page, G. Ribordy, A. Poppe, A. Allacher, O. Maurhart, T. Laenger, M. Peev, A. Zeilinger
2011 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO), IEEE, 2011, [Peer-reviewed]
English, International conference proceedings, A novel secure communication network with quantum key distribution in a metropolitan area is reported. Different QKD schemes are integrated to demonstrate secure TV conferencing over a distance 45km, stable long-term operation, and application to secure mobile phones.

A Scalable Full Quantum Key Distribution System based on Colourless Interferometric Technique and Hardware Key Distillation
A. Tanaka, M. Fujiwara, K. Yoshino, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, A. Tajima
2011 37TH EUROPEAN CONFERENCE AND EXHIBITION ON OPTICAL COMMUNICATIONS (ECOC 2011), IEEE, 2011, [Peer-reviewed]
English, International conference proceedings, A scalable full QKD system using WDM and a key distillation HW engine was developed. The three-channel multiplexed system achieved the world summit level key generation rate of higher than 200 kbps with 14.5-dB transmission loss.

A Novel Conceptual Model of Secure Photonic Networks
Ken-ichi Kitayama, Masahide Sasaki, Soichiro Araki, Makoto Tsubokawa, Akihisa Tomita, Kyo Inoue, Katsuyoshi Harasawa, Yuki Nagasako, Atsushi Takada
2011 13TH INTERNATIONAL CONFERENCE ON TRANSPARENT OPTICAL NETWORKS (ICTON), IEEE, 2011, [Peer-reviewed]
English, International conference proceedings, We address emerging threats to the security of photonic networks as these networks become heterogeneous being opened to the upper layers, multi-operator, and end users. We review the potential threats, mainly loss of the confidentiality of user data transmitted through optical fibres and disturbances of network control, both of which could seriously damage the entire network. We then propose a novel conceptual model of a secure photonic network by introducing a quantum key distribution (QKD) network to its legacy structure. Secure keys generated by the QKD network are managed by key management agents (KMAs) and used to encrypt not only user data but also control signals. The KMAs cooperate with the Generalized Multi-Protocol Label Switching (GMPLS) controller for secure path provisioning and drive photonic and modern crypto engines in appropriate combinations. Finally, we present a roadmap of a deployment scenario, starting from niche applications such as mission critical and business applications.

Ultra-wide tuning range of reconfigurable optical add-drop multiplexer using photorefractive polymer
Yuta Wakayama, Atsushi Okamoto, Akihisa Tomita, Kunihiro Sato
BROADBAND ACCESS COMMUNICATION TECHNOLOGIES V, 7958, SPIE-INT SOC OPTICAL ENGINEERING, 2011, [Peer-reviewed]
English, International conference proceedings, We propose a reconfigurable optical add-drop multiplexer with ultra-broadband tuning range over 770 nm using a photorefractive polymer. This add-drop multiplexer can optically switch the assignment of the waveband used for communications by only changing the incident angles of two beams writing a hologram. In this work, we clarified the condition in which the channel tuning can be carried out without the location change in the port array. We simulated on the add-drop operation, and showed the ultra-wide tuning range from 780 to 1550 nm. Moreover, we suggested an apodization method for the grating distribution by controlling the intensity ratio of the writing beams to enhance the diffraction efficiency in add-drop operation. As a result, we showed that the apodization method for the grating distribution can improve the diffraction efficiency.

Experimental Demonstration Of Single-Photon Detection Using InGaAs Avalanche Photodiode Operated In Sub-Geiger Mode
Kenji Tsujino, Yoshito Miyamoto, Jun Kataoka, Akihisa Tomita
QUANTUM COMMUNICATION, MEASUREMENT AND COMPUTING (QCMC): THE TENTH INTERNATIONAL CONFERENCE, 1363, AMER INST PHYSICS, 2011, [Peer-reviewed]
English, International conference proceedings, The avalanche photodiode (APD) is a promising device for single-photon detection. In this study, we operated an InGaAs APD in the sub-Geiger mode. In this mode, a gate pulse is not needed; therefore, we can detect photons with asynchronous timing. The detection efficiency and the dark count rate were 2.2% and 7900 counts per second (cps), respectively.

Optical Inter-Satellite Communication with Dynamically Reconfigurable Optical Device Using Sn2P2S6 Crystal
Kaori Nishimaki, Atsushi Okamoto, Tomohiro Fujita, Alexander A. Grabar, Masanori Takabayashi, Jun Uozumi, Akihisa Tomita, Yoshihisa Takayama
FREE-SPACE LASER COMMUNICATION TECHNOLOGIES XXIII, 7923, SPIE-INT SOC OPTICAL ENGINEERING, 2011, [Peer-reviewed]
English, International conference proceedings, Since the optical inter-satellite communication has attractive advantages such as high-speed transmission with high confidence, almost no electronic magnetic interference, and low power consumption, it has been activity investigated. However, directivity control of the laser beams requires a bulky and complicated system in satellite mobile communications. A more flexible and high accurate system with small and simple mechanism has been desired. In this study, we propose a new method of optical inter-satellite communication with a dynamically reconfigurable optical directional device in which diffraction gratings are automatically rewritten and reorganized in response of incident conditions by moving satellites. For realizing such a device, we have developed Sn2P2S6 crystals which have a high sensitive photorefractivity and dynamic reconfigurable property. Furthermore, this crystal has hundreds times faster response than conventional photorefractive materials such as BaTiO3. These features are extremely advantageous to construct a high-speed and flexible communication system with a large tolerance to displacement of moving satellites. To investigate the possibility of the dynamically reconfigurable optical inter-satellite communication system, we experimentally evaluate the temporal and spatial characteristics of Sn2P2S6 crystals for the variation of the beam incident angle. Moreover, the diffraction beam from the crystal has phase conjugate wavefronts of the beam entering from the counter direction. We try to utilize this behavior to suppress the beam spread and to reduce the background light such as sunlight with a spatial filtering technique that has sensitivity in wavefront differences of the signal and background light. (241words)

Special Section on Leading-Edge Developments in Quantum Cryptographic Systems FOREWORD
Akihisa Tomita
IEICE TRANSACTIONS ON FUNDAMENTALS OF ELECTRONICS COMMUNICATIONS AND COMPUTER SCIENCES, E93A, 5, 871, 871, IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG, May 2010, [Peer-reviewed]
English

High speed quantum key distribution system
Akihisa Tomita, Ken-ichiro Yoshino, Yoshihiro Nambu, Akio Tajima, Akihiro Tanaka, Seigo Takahashi, Wakako Maeda, Shigehito Miki, Zhen Wang, Mikio Fujiwara, Masahide Sasaki
OPTICAL FIBER TECHNOLOGY, 16, 1, 55, 62, ELSEVIER SCIENCE INC, Jan. 2010, [Peer-reviewed], [Invited]
English, Scientific journal, Quantum key distribution (QKD) systems can generate unconditionally secure common key between remote users. Improvement of QKD performance, particularly on key generation rate, has been required to meet current network traffic. The present paper considers system requirement to improve key generation rate by increasing photon detection rate and reducing error rate. A high-speed QKD system should be equipped with low loss receivers with high visibility, highly efficient photon detectors with small dark count probability, and a stable clock synchronization system with low stray light to the quantum signals. A solution for these issues are given by employing planar lightwave circuit interferometers, single photon detection circuits and modules, and clock synchronization based on wavelength division multiplexing (WDM) technique. A QKD system has been developed by combining these techniques. The system working with 625 MHz clock generated sift key at the rate of 2.4 kbps through a 97-km single installed fiber core. (c) 2009 Elsevier Inc. All rights reserved.

Implementation of a High-Speed Quantum Key Distribution System for Metropolitan Networks-
Akihisa Tomita, Akihiro Tanaka, Ken-ichiro Yoshino, Seigo Takahashi, Yoshihiro Nambu, Akio Tajima
2010 CONFERENCE ON OPTICAL FIBER COMMUNICATION OFC COLLOCATED NATIONAL FIBER OPTIC ENGINEERS CONFERENCE OFC-NFOEC, IEEE, 2010, [Peer-reviewed]
English, International conference proceedings, Improvement of QKD performance, particularly on key generation rate, has been required to meet current network traffic. A high-speed QKD system should be equipped with low loss receivers with high visibility, highly efficient photon detectors with small dark count probability, and a stable clock synchronization system with low stray light to the quantum signals, because the key generation rate is determined by photon detection rate and error rate. High speed post processing is also required to improve key generation through-put. A solution for these issues will be discussed. (C) 2010 Optical Society of America

Transmission Experiment of Quantum Keys over 50 km Using High-Performance Quantum-Dot Single-Photon Source at 1.5 mu m Wavelength
Kazuya Takemoto, Yoshihiro Nambu, Toshiyuki Miyazawa, Kentaro Wakui, Shinichi Hirose, Tatsuya Usuki, Motomu Takatsu, Naoki Yokoyama, Ken'ichiro Yoshino, Akihisa Tomita, Shinichi Yorozu, Yoshiki Sakuma, Yasuhiko Arakawa
APPLIED PHYSICS EXPRESS, 3, 9, JAPAN SOC APPLIED PHYSICS, 2010, [Peer-reviewed]
English, Scientific journal, We have developed a high-performance single-photon source (SPS) operating at 1.5 mu m wavelength. The source is an InAs/InP quantum dot with a horn-shaped nanostructure. A resonant excitation to the p-shell state helps achieve a single-photon efficiency of 5.8% after coupling into a single-mode fiber with a second-order correlation value of g((2))(0) similar to 0.055. The performance of the source has been assessed by integrating it into a conventional quantum key distribution system. We have successfully transmitted secure keys over a 50 km commercial fiber, exceeding the previously reported range for an SPS operating below 1.3 mu m. (c) 2010 The Japan Society of Applied Physics

Performance of hybrid entanglement photon pair source for quantum key distribution
M. Fujiwara, M. Toyoshima, M. Sasaki, K. Yoshino, Y. Nambu, A. Tomita
APPLIED PHYSICS LETTERS, 95, 26, 261103, AMER INST PHYSICS, Dec. 2009, [Peer-reviewed]
English, Scientific journal, We report on the experimental demonstration of a source of hybrid entanglement pairs between two different degrees of freedom, a 1550 nm time-bin qubit and an 810 nm polarization qubit. The polarization qubit at 810 nm is transformed by an asymmetric Mach-Zehnder interferometer consisted of a Glan laser prism and a polarization-maintaining fiber. We obtained visibilities of 95.8% and 88% with tolerance +/- 0.2% and +/- 1% along Z-Z and X-X axes on the Poincare sphere, respectively, with a coincidence count rate of more than 800 c/s after entanglement format transformation. These values are well above the threshold of 70.7% needed to violate a Bell inequality and allow distilling a secure key in the quantum key distribution.

Ensuring Quality of Shared Keys Through Quantum Key Distribution for Practical Application
Akihiro Tanaka, Wakako Maeda, Seigo Takahashi, Akio Tajima, Akihisa Tomita
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 15, 6, 1622, 1629, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Nov. 2009, [Peer-reviewed]
English, Scientific journal, We investigated the characteristics of shared keys obtained through quantum key distribution (QKD) from the viewpoint of application in cipher communication. We demonstrated that the shared keys at each stage in QKD satisfy the criteria determined by a standardized randomness test by physically compensating for randomness degrading factors. We also examined the increase in the error rate of the shared keys produced by privacy amplification, and succeeded in suppressing the error rate increase by applying a new privacy amplification scheme, which yielded a sufficiently low error rate for the final keys. These investigations showed that the final keys obtained using quantum cryptosystems are available as crypto keys in cipher communication.

Technologies for Quantum Key Distribution Networks Integrated With Optical Communication Networks
Wakako Maeda, Akihiro Tanaka, Seigo Takahashi, Akio Tajima, Akihisa Tomita
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 15, 6, 1591, 1601, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Nov. 2009, [Peer-reviewed], [Invited]
English, Scientific journal, Secure networks composed of optical networks and quantum key distribution (QKD) are considered in this paper. A network layer structure is defined to focus on issues important with respect to the QKD network layer that controls QKD links and supplies a secure key for message transmission. A multiple-user QKD network layer can be constructed with timing synchronization, secure key sharing between indirectly connected nodes with switched QKD systems and key relays, and key management to control the volume of key generation and supply in response to the demand from optical network nodes.

Updating Quantum Cryptography Report ver. 1
Donna Dodson, Mikio Fujiwara, Philippe Grangier, Masahito Hayashi, Kentaro Imafuku, Ken-ichi Kitayama, Prem Kumar, Christian Kurtsiefer, Gaby Lenhart, Norbert Luetkenhaus, Tsutomu Matsumoto, William J. Munro, Tsuyoshi Nishioka, Momtchil Peev, Masahide Sasaki, Yutaka Sata, Atsushi Takada, Masahiro Takeoka, Kiyoshi Tamaki, Hidema Tanaka, Yasuhiro Tokura, Akihisa Tomita, Morio Toyoshima, Rodney van Meter, Atsuhiro Yamagishi, Yoshihisa Yamamoto, Akihiro Yamamura
CoRR, abs/0905.4325, 27 May 2009
Scientific journal, Quantum cryptographic technology (QCT) is expected to be a fundamental

technology for realizing long-term information security even against

as-yet-unknown future technologies. More advanced security could be achieved

using QCT together with contemporary cryptographic technologies. To develop and

spread the use of QCT, it is necessary to standardize devices, protocols, and

security requirements and thus enable interoperability in a multi-vendor,

multi-network, and multi-service environment. This report is a technical

summary of QCT and related topics from the viewpoints of 1) consensual

esta...

Photon-arrival detector with a controlled phase flip operation between a photon and a V-type atomic system
Kunihiro Kojima, Akihisa Tomita
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS, 26, 4, 836, 848, OPTICAL SOC AMER, Apr. 2009, [Peer-reviewed]
English, Scientific journal, We propose a photon-arrival detector (PAD), which detects the arrival of a signal photon and simultaneously projects the signal input state to a single-photon state, with an atom-cavity system. In this proposal, use of a V-type system as the intracavity atom is discussed for implementing the PAD since V-type systems have been widely studied in the field of solid state, enabling us to miniaturize and integrate that implementation. The performance of the proposed PAD is evaluated for a specific method of the detection process. The proposed PAD is capable of repeating the procedure for detecting the arrival of input photons and it has improved the detection probability so that it has a higher quantum efficiency than those of conventional photodetectors. (c) 2009 Optical Society of America

Proposal of an eavesdropping experiment for BB84 QKD protocol with 1 -> 3 phase-covariant quantum cloner
Yuta Okubo, Francesco Buscemi, Akihisa Tomita
QUANTUM COMMUNICATION, MEASUREMENT AND COMPUTING (QCMC), 1110, 355, +, AMER INST PHYSICS, 2009, [Peer-reviewed]
English, International conference proceedings, We propose an experimental implementation of a particular attack against the BB84 quantum key distribution protocol, utilizing a 1 -> 3 asymmetric phase-covariant quantum cloning. Our eavesdropping strategy is numerically optimized to extract the information about the input state. The proposed cloning circuit is easily implementable with linear optics and its performances turn out to be well close to the optimum attack.

Test and measurement on quantum key distribution systems
Akihisa Tomita
QUANTUM COMMUNICATIONS REALIZED II, 7236, SPIE-INT SOC OPTICAL ENGINEERING, 2009, [Peer-reviewed]
English, International conference proceedings, As in conventional communication systems, test and measurement play important roles in quantum key distribution (QKD) systems. Besides the observation that QKD protocols estimate the bound of information leakage from the measurement results on the transmission channel, test of quantum apparatus is necessary to ensure that the assumptions behind the security proof are satisfied in practice. Moreover, precise characterization of the device imperfection improves the final key rate, because one can specify the effect of the errors originated from the devices and sbtract it. However, careful consideration is required to guarantee that the test and measurement procedure will not open a loop-hole to the eavesdroppers.

Colourless Interferometric Technique for Large Capacity Quantum Key Distribution Systems by use of Wavelength Division Multiplexing
A. Tanaka, A. Tomita, A. Tajima
2009 35TH EUROPEAN CONFERENCE ON OPTICAL COMMUNICATION (ECOC), VDE VERLAG GMBH, 2009, [Peer-reviewed]
English, International conference proceedings, A novel interferometric technique is proposed for high-capacity quantum key distribution using wavelength division multiplexing. With additional phase modulation, we exhibited valid interference at any wavelengths while sharing interferometers over multi-channels, resulting in a low-cost configuration.

Ultra fast quantum key distribution over a 97 km installed telecom fiber with wavelength division multiplexing clock synchronization
Akihiro Tanaka, Mikio Fujiwara, Sae Woo Nam, Yoshihiro Nambu, Seigo Takahashi, Wakako Maeda, Ken-ichiro Yoshino, Shigehito Miki, Burm Baek, Zhen Wang, Akio Tajima, Masahide Sasaki, Akihisa Tomita
OPTICS EXPRESS, 16, 15, 11354, 11360, OPTICAL SOC AMER, Jul. 2008, [Peer-reviewed], [International Magazine]
English, Scientific journal, We demonstrated ultra fast BB84 quantum key distribution (QKD) transmission at 625 MHz clock rate through a 97 km field-installed fiber using practical clock synchronization based on wavelength-division multiplexing (WDM). We succeeded in over-one-hour stable key generation at a high sifted key rate of 2.4 kbps and a low quantum bit error rate (QBER) of 2.9%. The asymptotic secure key rate was estimated to be 0.78-0.82 kbps from the transmission data with the decoy method of average photon numbers 0, 0.15, and 0.4 photons/pulse. (C) 2008 Optical Society of America.

Statistical analysis of testing of an entangled state based on the Poisson distribution framework
Masahito Hayashi, Akihisa Tomita, Keiji Matsumoto
NEW JOURNAL OF PHYSICS, 10, IOP PUBLISHING LTD, Apr. 2008, [Peer-reviewed]
English, Scientific journal, A hypothesis testing scheme for entanglement has been formulated based on the Poisson distribution framework instead of the positive operator valued measure (POVM) framework. Three designs were proposed to test the entangled states in this framework. The designs were evaluated in terms of the asymptotic variance. It has been shown that the optimal time allocation between the coincidence and anti-coincidence measurement bases improves the conventional testing method. The test can be further improved by optimizing the time allocation between the anti-coincidence bases.

Experimental demonstration of quantum leader election in linear optics
Yuta Okubo, Xiang-Bin Wang, Yun-Kun Jiang, Seiichiro Tani, Akihisa Tomita
PHYSICAL REVIEW A, 77, 3, 032343, AMER PHYSICAL SOC, Mar. 2008, [Peer-reviewed]
English, Scientific journal, Linear optics is a promising candidate to enable the construction of quantum computers. A number of quantum protocols gates based on linear optics have been demonstrated. However, it is well known that these gates are nondeterministic and that higher order nonlinearity is necessary for deterministic operations. We found the quantum leader election protocol can be operated deterministically only with linear optics, and we have demonstrated the nearly deterministic operation which overcomes classical limit.

Quantum encoder and decoder for practical quantum key distribution using a planar lightwave circuit
Yoshihiro Nambu, Ken'ichiro Yoshino, Akihisa Tomita
JOURNAL OF MODERN OPTICS, 55, 12, 1953, 1970, TAYLOR & FRANCIS LTD, 2008, [Peer-reviewed]
English, Scientific journal, To launch quantum key distribution (QKD) into the commercial market, it is important to develop a system that is simpler and more reliable using current technology. This report proposes quantum encoders and decoders using a passive planar lightwave circuit (PLC) that is useful for implementing optical-fiber-based QKD systems. Our encoders and decoders are based on an asymmetric Mach-Zehnder interferometer and allow us to prepare and analyze various photonic time-bin qubits reliably. The system can be stable and polarization-insensitive merely by stabilizing and controlling the device temperature. Our PLC-based devices enables us to simplify the QKD system and increase its reliability.

Scalable QKD Network Using Simple Key-Management Technique with On-Demand Crypto-Key Supply
W. Maeda, A. Tanaka, S. Takahashi, A. Tajima, A. Tomita
2008 34TH EUROPEAN CONFERENCE ON OPTICAL COMMUNICATION (ECOC), IEEE, 2008, [Peer-reviewed]
English, International conference proceedings, We propose a simple key-management technique using novel crypto-key supply. We have also successfully demonstrated a five-node QKD network that uses dynamic quantum key generation and on-demand crypto-key supply.

Quantum key distribution systems and field trials
Akihiro Tanaka, Mikio Fujiwara, Sae Woo Nam, Yoshihiro Nambu, Seigo Takahashi, Wakako Maeda, Ken-ichiro Yoshino, Shigehito Miki, Burm Baek, Zhen Wang, Akio Tajima, Masahide Sasaki, Akihisa Tomita
2008 34TH EUROPEAN CONFERENCE ON OPTICAL COMMUNICATION (ECOC), IEEE, 2008, [Peer-reviewed]
English, International conference proceedings, Recent progress in quantum key distribution (QKD), regarding high-speed and long-distance experiments and field trials are briefly reviewed. We also introduce our research on the randomness of quantum key. We have confirmed a quantum key, as a crypto key, assures the required randomness with some dynamic compensation.

97-km QKD field trial using PLC-based one-way interferometers, SSPDs and WDM synchronization
Akihiro Tanaka, Mikio Fujiwara, Sae Woo Nam, Yoshihiro Nambu, Seigo Takahashi, Wakako Maeda, Ken'ichiro Yoshino, Shigehito Miki, Burm Back, Wang Zhen, Akio Tajima, Akihisa Tomita, Masahide Sasaki
2008 CONFERENCE ON OPTICAL FIBER COMMUNICATION/NATIONAL FIBER OPTIC ENGINEERS CONFERENCE, VOLS 1-8, 2759, +, OPTICAL SOC AMERICA, 2008, [Peer-reviewed]
English, International conference proceedings, The world's fastest BB84 QKD through a 97-km installed fiber was demonstrated using PLC-based interferometers, SSPDs and WDM synchronization. By suppressing nonlinear crosstalk induced by the multiplexed clock, QBER of 2.8% was obtained after 97-km transmission. (C) 2008 Optical Society of America.

Efficient generation of a photon pair in a bulk periodically poled potassium titanyl phosphate
Bao-Sen Shi, Chang Zhai, Guang-Can Guo, Yun-Kun Jiang, Akihisa Tomita
OPTICS COMMUNICATIONS, 278, 2, 363, 367, ELSEVIER SCIENCE BV, Oct. 2007, [Peer-reviewed]
English, Scientific journal, In this report, we demonstrate the efficient generation of collinearly propagating photon pairs in a bulk periodically poled potassium titanyl phosphate pumped by a cw laser. The detected coincidence counts are more than 7400/s with 3.58 mW pump power in a Hanbury-Brown-Twiss-type experiment. The estimated photon pair production rate is about 0.73 MHz/mW. This is very promising for some applications, such as quantum key distribution, proof of the Bell-inequality, preparation of single photon states in broadband wave packets, Franson-type interference and so on. (c) 2007 Elsevier B.V. All rights reserved.

Quantum information with Gaussian states
Xiang-Bin Wang, Tohya Hiroshima, Akihisa Tomita, Masahito Hayashi
PHYSICS REPORTS-REVIEW SECTION OF PHYSICS LETTERS, 448, 1-4, 1, 111, ELSEVIER SCIENCE BV, Aug. 2007, [Peer-reviewed], [Invited]
English, Scientific journal, Quantum optical Gaussian states are a type of important robust quantum states which are manipulatable by the existing technologies. So far, most of the important quantum information experiments are done with such states, including bright Gaussian light and weak Gaussian light. Extending the existing results of quantum information with discrete quantum states to the case of continuous variable quantum states is an interesting theoretical job. The quantum Gaussian states play a central role in such a case. We review the properties and applications of Gaussian states in quantum information with emphasis on the fundamental concepts, the calculation techniques and the effects of imperfections of the real-life experimental setups. Topics here include the elementary properties of Gaussian states and relevant quantum information device, entanglement-based quantum tasks such as quantum teleportation, quantum cryptography with weak and strong Gaussian states and the quantum channel capacity, mathematical theory of quantum entanglement and state estimation for Gaussian states. (C) 2007 Elsevier B.V. All rights reserved.

Practical quantum cryptosystem for metro area applications
Akio Tajima, Akihiro Tanaka, Wakako Maeda, Seigo Takahashi, Akihisa Tomita
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 13, 4, 1031, 1038, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 2007, [Peer-reviewed]
English, Scientific journal, A practical quantum cryptosystem prototype has been developed for metro area applications. It is of the size of a desktop and incorporates a temperature-independent optical interferometer, a highly reliable photon detector module, and random number generators. It supports various functions, including bit/frame synchronization, BB84 bit reconciliation, error correction and privacy amplification for quantum key generation, and encrypted communication. Two weeks of testing using a commercial fiber in the field demonstrated its ability to continuously generate final keys and to perform crypto communication using the keys.

Introduction to the issue on single photon counting: Detectors and applications
Mark A. Itzler, Sergio Cova, Michael Wahl, Akihisa Tomita
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 13, 4, 849, 851, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 2007, [Peer-reviewed]
English

Mode identification of high-quality-factor single-defect nanocavities in quantum dot-embedded photonic crystals
Masayuki Shirane, Shunsuke Kono, Jun Ushida, Shunsuke Ohkouchi, Naoki Ikeda, Yoshimasa Sugimoto, Akihisa Tomita
JOURNAL OF APPLIED PHYSICS, 101, 7, 073107, AMER INST PHYSICS, Apr. 2007, [Peer-reviewed]
English, Scientific journal, We investigate the quality (Q) factor and the mode dispersion of single-defect nanocavities based on a triangular-lattice GaAs photonic-crystal (PC) membrane, which contain InAs quantum dots (QDs) as a broadband emitter. To obtain a high Q factor for the dipole mode, we modulate the radii and positions of the air holes surrounding the nanocavity while keeping sixfold symmetry. A maximum Q of 17 000 is experimentally demonstrated with a mode volume of V=0.39(lambda/n)(3). We obtain a Q/V of 44 000(n/lambda)(3), one of the highest values ever reported with QD-embedded PC nanocavities. We also observe ten cavity modes within the first photonic band gap for the modulated structure. Their dispersion and polarization properties agree well with the numerical results. (c) 2007 American Institute of Physics.

Measurement of the off-diagonal geometric phase of a mixed-state photon via a Franson interferometer
Jian Li, Yun-Kun Jiang, Akihisa Tomita
PHYSICS LETTERS A, 362, 4, 269, 272, ELSEVIER SCIENCE BV, Mar. 2007, [Peer-reviewed]
English, Scientific journal, A geometric phase can be attached both diagonal and off-diagonal matrix elements of the density operator under parallel transport unitary operation. Here an experimental scheme is presented to measure the off-diagonal geometric phases for the mixed states of single photon using Franson-type interferometer. And two typical imperfections in real experimental condition are discussed. (c) 2006 Elsevier B.V. All rights reserved.

Quantum-nondemolition measurement of photon arrival using an atom-cavity system
Kunihiro Kojima, Akihisa Tomita
PHYSICAL REVIEW A, 75, 3, 032320, AMERICAN PHYSICAL SOC, Mar. 2007, [Peer-reviewed]
English, Scientific journal, A simple and efficient quantum-nondemolition measurement (QND) scheme is proposed in which the arrival of a signal photon is detected without affecting the qubit state. The proposed QND scheme functions even if the ancillary photon is replaced with weak light composed of vacuum and one-photon states. Although the detection scheme is designed for entanglement sharing applications, it is also suitable for general purification of a single-photon state.

Experimental demonstration of quantum leader election in linear optics　　　　　　　　　　　　　　　
Yuta Okubo, Xiang-Bin Wang, Akihisa Tomita
Optics InfoBase Conference Papers, Optical Society of America, 2007
English, International conference proceedings, We propose and demonstrate a new implementation of a quantum protocol which can operate deterministically only with linear optics. This protocol, which is called quantum leader election, exhibits the quantum advantages over classical protocol. © 2007 Optical Society of America.

Experimental demonstration of quantum leader election in linear optics　　　　　　　　　　　　　　　
Yuta Okubo, Xiang-Bin Wang, Akihisa Tomita
Optics InfoBase Conference Papers, Optical Society of America, 2007
English, International conference proceedings, We propose and demonstrate a new implementation of a quantum protocol which can operate deterministically only with linear optics. This protocol, which is called quantum leader election, exhibits the quantum advantages over classical protocol. © 2007 Optical Society of America.

Coherent control of exciton in a single InAs/GaAs quantum dot
Shunsuke Kono, Akihisa Tomita, Junichi Fujikata, Hideaki Saito, Kenichi Nishi
Optics InfoBase Conference Papers, 2007
International conference proceedings

Coherent control of exciton in a single InAs/GaAs quantum dot
Shunsuke Kono, Akihisa Tomita, Junichi Fujikata, Hideaki Saito, Kenichi Nishi
Conference on Lasers and Electro-Optics Europe - Technical Digest, 2007
International conference proceedings

The generation of polarization-entangled photon pairs using periodically poled lithium niobate waveguides in a fibre loop
Yun-Kun Jiang, Akihisa Tomita
JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 40, 2, 437, 443, IOP PUBLISHING LTD, Jan. 2007, [Peer-reviewed]
English, Scientific journal, We report the generation of polarization-entangled photon pairs at a telecom band using two cascade periodically poled lithium niobate waveguides in a fibre loop. The phase of an entangled photon state was stabilized automatically in our setup. The average visibility was about 93.5 +/- 2.6% for the entangled photon pairs after subtracting accidental coincidences in our measurement.

Hypothesis testing for an entangled state produced by spontaneous parametric down-conversion
Masahito Hayashi, Bao-Sen Shi, Akihisa Tomita, Keiji Matsumoto, Yoshiyuki Tsuda, Yun-Kun Jiang
PHYSICAL REVIEW A, 74, 6, AMERICAN PHYSICAL SOC, Dec. 2006, [Peer-reviewed]
English, Scientific journal, Generation and characterization of entanglement are crucial tasks in quantum information processing. A hypothesis testing scheme for entanglement has been formulated. Three designs were proposed to test the entangled photon states created by the spontaneous parametric down conversion. The time allocations between the measurement vectors were designed to consider the anisotropic deviation of the generated photon states from the maximally entangled states. The designs were evaluated in terms of the p value based on the observed data. It has been experimentally demonstrated that the optimal time allocation between the coincidence and anticoincidence measurement vectors improves the entanglement test. A further improvement is also experimentally demonstrated by optimizing the time allocation between the anticoincidence vectors. Analysis on the data obtained in the experiment verified the advantage of the entanglement test designed by the optimal time allocation.

Highly efficient polarization-entangled photon source using periodically poled lithium niobate waveguides
Yun-Kun Jiang, Akihisa Tomita
OPTICS COMMUNICATIONS, 267, 1, 278, 281, ELSEVIER SCIENCE BV, Nov. 2006, [Peer-reviewed]
English, Scientific journal, In our experiment, two channels of a 3 cm long periodically poled lithium niobate (PPLN) waveguide are used to generate the nondegenerate spontaneous parametric down conversion (NSPDC) photon pairs. We experimentally obtained the highly efficient polarization-entangled photon pairs by coherently combining two NSPDC processes, and keeping the phase stable over 30 min. We can also prepare two entangled photon pairs with different central wavelengths with the help of wavelength division multiplex. (c) 2006 Elsevier B.V. All rights reserved.

3 Quantum Info-Communications 3-2 Secure Communication with Quantum Cryptography
HASEGAWA TOSHIO, NISHIOKA TSUYOSHI, ISHIZUKA HIROKAZU, NAMBU YOSHIHIRO, TOMITA AKIHISA, TAJIMA AKIO
情報通信研究機構季報, 52, 3, 55-62, 96, 情報通信研究機構, 29 Sep. 2006
Japanese

Development of scanning near-field optical microscope working under cryogenic temperature and strong magnetic field
Akihiro Kirihara, Shunsuke Kono, Akihisa Tomita, Kazuo Nakamura
OPTICAL REVIEW, 13, 4, 279, 282, OPTICAL SOC JAPAN, Jul. 2006, [Peer-reviewed]
English, Scientific journal, We report on the development of a cantilever-based scanning near-field optical microscope (SNOM) working in an extreme environment, at cryogenic temperature around 10 K and under strong magnetic field up to 7 T. We designed a new optical system based on an infinite conjugate microscope, which extracts the near-field signal from a small aperture through a narrow chamber into free space as collimated light. Using this system, we successfully measured near-field and topographical images of a metal-hole sample simultaneously. Combining the local optical accessing technique with the external control of the electronic state, this SNOM system will be a powerful tool to study optical properties of semiconductor nanostructures. (c) 2006 The Optical Society of Japan.

One-way quantum key distribution system based on planar lightwave circuits
Yoshihiro Nambu, Ken'ichiro Yoshino, Akihisa Tomita
JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS, 45, 6A, 5344, 5348, JAPAN SOC APPLIED PHYSICS, Jun. 2006, [Peer-reviewed]
English, Scientific journal, We developed a one-way quantum key distribution (QKD) system based upon a planar lightwave circuit (PLC) interferometer. This interferometer is expected to be free from the backscattering inherent in commercially available two-way QKD systems and phase drift without active compensation. A key distribution experiment with spools of standard telecom fiber showed that the bit error rate was as low as 6% for a 100-km key distribution using an attenuated laser pulse with a mean photon number of 0.1 and was determined solely by the detector noise. This clearly demonstrates the advantages of our PLC-based one-way QKD system over two-way QKD systems for long distance key distribution.

Influence of pure dephasing by phonons on exciton-photon interfaces: Quantum microscopic theory
K Kojima, A Tomita
PHYSICAL REVIEW B, 73, 19, 195312, AMERICAN PHYSICAL SOC, May 2006, [Peer-reviewed]
English, Scientific journal, We have developed a full quantum microscopic theory to analyze the time evolution of transversal and longitudinal components of an exciton-single photon system coupled to bulk acoustic phonons. These components are subjected to two decay processes. One is radiative relaxation and the other is pure dephasing due to exciton-phonon interaction. The former results in a decay with an exponent linear to time, while the latter causes a faster initial decay than the radiative decay. We analyzed the dependence of the components on the duration of the input one-photon pulse, temperature, and radiative relaxation rates. Such a quantitative analysis is important for the developments of atom-photon interfaces which enable coherent transfer of quantum information between photons and atomic systems. We found that, for a GaAs spherical quantum dot in which the exciton interacts with bulk phonons, the maximal probability of the excited state can be increased up to 75%. This probability can be considered as the efficiency for quantum information transfer from photon to exciton.

Four photons interfering but showing the two-photon interference behaviour
BS Shi, A Tomita
JOURNAL OF MODERN OPTICS, 53, 7, 1003, 1009, TAYLOR & FRANCIS LTD, May 2006, [Peer-reviewed]
English, Scientific journal, It was shown by Riedmatten et al. [J. Mod. Opt. 51 1637 (2004)] that two different four-photon states could be generated according to the relation between the duration time of the pump laser T and the coherence time of the single photon from spontaneous parametric down-conversion (SPDC). In this paper, using four photons generated by the SPDC in a Periodically Poled Potassium Titanyl Phosphate (PPKTP), we experimentally observe the four-photon interference in a Michelson interferometer. All experimental results can be explained by the product of two independent two-photon interferences. Our experimental condition corresponds to the case of T >> t. This experiment clearly confirms the theory given by Riedmatten et al.

Time uncertainty of a photon pair creation in a bulk periodically poled potassium titanyl phosphate pumped by a femtosecond laser
BS Shi, A Tomita
NEW JOURNAL OF PHYSICS, 8, 38, IOP PUBLISHING LTD, Mar. 2006, [Peer-reviewed]
English, Scientific journal, Periodically poled crystals, such as periodically poled potassium titanyl phosphate (PPKTP) or lithium niobate, recently became more attractive because of their high efficiency for producing a photon pair in both cw and pulse cases. The time uncertainty of photon pair creation in a crystal is crucial for many important applications. In this paper, we experimentally demonstrate that the time uncertainty of the photon pair creation in a PPKTP crystal is much larger than the coherence time of a femtosecond pump laser by using phase-sensitive two-photon interference using a Michelson interferometer, which means the femtosecond laser cannot be considered as a clock that announces the creation of the photon pairs. The experimental results can be well explained by an effective Franson-type interferometer, if the PPKTP is regarded as a narrow band filter at the same time. Our experiment also shows a way by which the Franson-type two-photon interference phenomenon can be observed even if the coherence length of the pump laser is less than the path-length difference between the two interfering beams.

High quality factor photonic crystal micro-cavity design to utilize semiconductor nonlinearities
AV Gopal, A Tomita, S Lan, H Yamada, J Ushida
JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS, 45, 3A, 1612, 1616, JAPAN SOC APPLIED PHYSICS, Mar. 2006, [Peer-reviewed]
English, Scientific journal, Photonic crystal based microcavity structure with high quality factor (Q) and small mode volume is designed to make use of the semiconductor nonlinearities. Two and three-dimensional finite difference time domain calculations are performed, to study the dependence of Q on the defect size and shape and the position, shape and size of the nearest neighbor holes, to obtain the optimum structures. Structures are optimized to utilize semiconductor materials in the defect region to realize solid-state devices. The effect of nonlinearity in controlling the Q-factor is also reported.

Photonic realization of quantum information systems
Akihisa Tomita, Bao-Sen Shi
QUANTUM COMPUTATION AND INFORMATION: FROM THEORY TO EXPERIMENT, 102, 243, 275, SPRINGER-VERLAG BERLIN, 2006, [Peer-reviewed]
English, This Chapter to introduces research to implement quantum information systems with photonics. Photonics provides strong tools to realize systems working on a single qubit. Quantum key distribution systems using improved photon detectors have been developed for commercial products in two directions: longer distances and high speeds. A photonic circuit has demonstrated the quantum Fourier transform operation over 1024 qubit. Entangled photon genaration in spontaneous parametric down conversion has been also improved.

Experimental verification of fault tolerant quantum key distribution protocol
YK Jiang, XB Wang, BS Shi, A Tomita
OPTICS EXPRESS, 13, 23, 9415, 9421, OPTICAL SOC AMER, Nov. 2005, [Peer-reviewed]
English, Scientific journal, have investigated fault tolerant quantum key distribution experimentally, using four polarization encoding two-qubit states generated by spontaneous parametric down conversion. Collective noises on polarization and phase were simulated by passing the states through a half wave plate and a quarter wave pate. Error rate was calculated by measuring the two-qubit states in three basis. Our results show that the protocol is tolerant under collective random unitary noise. (c) 2005 Optical Society of America.

Excitonic molecule in a quantum dot: Photoluminescence lifetime of a single InAs/GaAs quantum dot
S Kono, A Kirihara, A Tomita, K Nakamura, J Fujikata, K Ohashi, H Saito, K Nishi
PHYSICAL REVIEW B, 72, 15, AMERICAN PHYSICAL SOC, Oct. 2005, [Peer-reviewed]
English, Scientific journal, Time dependence of the photoluminescence (PL) attributed to the exciton and biexciton in a single InAs/GaAs quantum dot is investigated at 4.3 K with microscopic spectroscopy. Dynamical behavior of the PL decay and the excitation intensity dependence of the exciton and biexciton PL are analyzed by rate equations assuming a cascading recombination from the biexciton to exciton state. The analysis shows that the biexciton lifetime is longer than the exciton lifetime. The estimated ratio of the biexciton lifetime to the exciton lifetime shows a molecular nature of the biexciton in the large quantum dots compared with the exciton Bohr radius.

Quantum information processing with fiber optics: Quantum Fourier transform of 1024 qubits
A Tomita
OPTICS AND SPECTROSCOPY, 99, 2, 204, 210, OPTICAL SOC AMER, Aug. 2005, [Peer-reviewed]
English, Scientific journal, Among a number of candidates, photons have advantages for implementing qubits: very weak coupling to the environment, the existing single photon measurement technique, and so on. Moreover, commercially available fiber-optic devices enable us to construct quantum circuits that consist of one-qubit operations (including classically controlled gates). Fiber optics resolves the mode matching problems in conventional optics and provides mechanically stable optical circuits. A quantum Fourier transform (QFF) followed by measurement was demonstrated with a simple circuit based on fiber optics. The circuit was shown to be robust against imperfections in the rotation gate. The error probability was estimated to be 0.01 per qubit, which corresponded to error-free operation for 100 qubits. The error probability can be further reduced to achieve successful QFT of 1024 qubits by taking the majority of the accumulated results. As is well known, QFT is a key function in quantum computations such as the final part of Shor's factorization algorithm. The present QFT circuit, in combination with controlled unitary gates, would make possible practical quantum computers. Possible schemes of realizing quantum computers in this line are explored. (c) 2005 Pleiades Publishing, Inc.

Quantum cryptography: A reliable security guard!: "Ultimate encryption" quantum mechanics guarantees security　　　　　　　　　　　　　　　
Akio Tajima, Akihisa Tomita
NEC Journal of Advanced Technology, 2, 3, 254, 259, Jun. 2005
English, Scientific journal, When a code based on the uncertainty principle of quantum mechanics is applied, an encrypted communication is realized that is absolutely impossible to be decrypted. This research topic, so called ultimate encryption, is currently more advanced than the quantum computer. NEC is currently at the forefront in this research area. Successes achieved to date are at the highest level in terms of length and speed of successive cryptographic key generation, under practical conditions.

Creation of a polarization W state using optical fibre multiports
BS Shi, A Tomita
JOURNAL OF MODERN OPTICS, 52, 5, 755, 761, TAYLOR & FRANCIS LTD, Mar. 2005, [Peer-reviewed]
English, Scientific journal, A scheme for experimental creation of a polarization W state is reported in this paper. This scheme is based on a common commercial multiport fibre coupler and single photon source and it is feasible using current technology. This scheme has a higher efficiency and can be easily extended to generate a multiphoton W state with a photon number larger than 4.

Time-resolved photoluminescence measurement of exciton and biexciton in an InAs/GaAs single quantum dot
Shunsuke Kono, Akihiro Kirihara, Akihisa Tomita, Kazuo Nakamura, Kenichi Nishi, Hideaki Saito, Junichi Fujikata, Keishi Ohashi
IQEC, International Quantum Electronics Conference Proceedings, 2005, 259, 260, 2005, [Peer-reviewed]
English, International conference proceedings, Dynamical behaviors of the exciton and biexciton luminescence at the wavelength of 1.18 μ from an InAs/GaAs single quantum dot are studied at 4.3 K. The rate equation analysis on the luminescence decays shows that the biexciton lifetime is longer than the exciton lifetime. The estimated lifetimes reflect molecular nature of the biexciton in the single quantum dot.

Temporal behaviour of field in high quality factor photonic crystal microcavity structure
AV Gopal, A Tomita, H Yamada, S Lan
OPTICS EXPRESS, 13, 2, 460, 467, OPTICAL SOC AMER, Jan. 2005, [Peer-reviewed]
English, Scientific journal, Temporal behaviour of incident pulse in high-quality (Q) factor photonic crystal microcavities are studied by two dimensional finite difference time domain calculations. For high-Q mode excitation, two periods of oscillation are observed in addition to the exponential decay corresponding to the cavity mode photon life time. Long and short period oscillations correspond to beats with low-Q mode (for short pulse widths) and to lattice periodicity for all pulse widths, respectively. For low-Q mode and off-resonant excitations, long period oscillations correspond to coupling to bandedge states. (C) 2005 Optical Society of America.

Highly efficient generation of pulsed photon pairs with bulk periodically poled potassium titanyl phosphate
BS Shi, A Tomita
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS, 21, 12, 2081, 2084, OPTICAL SOC AMER, Dec. 2004, [Peer-reviewed]
English, Scientific journal, We demonstrate efficient generation of collinearly propagating, degenerate pulsed photon pairs based on a bulk periodically poled potassium titanyl phosphate pumped by an ultrashort-pulse laser. Using a single-mode fiber as a spatial mode filter, we detect approximately 3200 coincidence counts per second per milliwatt pump power in a Hanbury-Brown-Twiss-type experiment. The estimated photon-pair production rate is approximately 1.05 MHz/mW pump power. This is very promising for the realization of sources for quantum communication and metrology. (C) 2004 Optical Society of America.

Highly efficient generation of pulsed photon pairs with bulk periodically poled potassium titanyl phosphate
BS Shi, A Tomita
JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS, 21, 12, 2081, 2084, OPTICAL SOC AMER, Dec. 2004, [Peer-reviewed]
English, Scientific journal, We demonstrate efficient generation of collinearly propagating, degenerate pulsed photon pairs based on a bulk periodically poled potassium titanyl phosphate pumped by an ultrashort-pulse laser. Using a single-mode fiber as a spatial mode filter, we detect approximately 3200 coincidence counts per second per milliwatt pump power in a Hanbury-Brown-Twiss-type experiment. The estimated photon-pair production rate is approximately 1.05 MHz/mW pump power. This is very promising for the realization of sources for quantum communication and metrology. (C) 2004 Optical Society of America.

Single-photon interference over 150 km transmission using silica-based integrated-optic interferometers for quantum cryptography
T Kimura, Y Nambu, T Hatanaka, A Tomita, H Kosaka, K Nakamura
JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS, 43, 9A-B, L1217, L1219, INST PURE APPLIED PHYSICS, Sep. 2004, [Peer-reviewed]
English, Scientific journal, We have demonstrated single-photon interference over 150 km using time-division interferometers for quantum cryptography, which were composed of two integrated-optic asymmetric Mach-Zehnder interferometers, and balanced gated-mode photon detectors. The observed fringe visibility was more than 80% after 150 km transmission.

Preparation of a pulsed polarization entangled photon pair via interference
BS Shi, A Tomita
OPTICS COMMUNICATIONS, 235, 4-6, 247, 252, ELSEVIER SCIENCE BV, May 2004, [Peer-reviewed]
English, Scientific journal, In this paper, we report on a scheme to generate a pulsed polarization entangled photon pair by coherently combining two type-I spontaneous parametric down-conversion (SPDC) processes via a Michelson interferometer. We use the interferometric technique to engineer the indistinguishability and the entanglement of a photon pair produced by an ultra-short pulse. Experimentally, the two photons created exhibit high-visibility quantum interference for both space-time and polarization variables. Such a source of highly entangled photon pair is useful in quantum information field. (C) 2004 Elsevier B.V. All rights reserved.

Antinormally ordered photodetection of continuous-mode field
K Usami, A Tomita, K Nakamura
INTERNATIONAL JOURNAL OF QUANTUM INFORMATION, 2, 1, 101, 117, WORLD SCIENTIFIC PUBL CO PTE LTD, Mar. 2004, [Peer-reviewed], [Invited]
English, Scientific journal, When the electromagnetic field is detected by stimulated emission, rather than by absorption, antinormally ordered photodetection can be realized. One of the distinct features of this photodetection scheme is its sensitivity to zero-point fluctuation due to the existence of the spontaneous emission. We have recently succeeded in experimentally demonstrating the antinormally ordered photodetection by exploiting nondegenerate stimulated parametric down-conversion process. To properly account for the experiment, the detection process needs to be treated with time-dependent and continuous-mode operators because of the broadband nature of the parametric down-conversion process and the wide spectrum of the pump that we used. Here, we theoretically analyze the antinormally ordered intensity correlation of the continuous-mode fields by pursuing the detection process in the Heisenberg picture. It is shown that the excess positive correlation due to zero-point fluctuation reduces because of the frequency-distinguishability of the two emitted photon pairs.

Measured quantum Fourier transform of 1024 qubits on fiber optics
A Tomita, K Nakamura
INTERNATIONAL JOURNAL OF QUANTUM INFORMATION, 2, 1, 119, 131, WORLD SCIENTIFIC PUBL CO PTE LTD, Mar. 2004, [Peer-reviewed]
English, Scientific journal, Quantum Fourier transform (QFT) is a key function to realize quantum computers. A QFT followed by measurement was demonstrated on a simple circuit based on fiber-optics. The QFT was shown to be robust against imperfections in the rotation gate. Error probability was estimated to be 0.01 per qubit, which corresponded to error-free operation on 100 qubits. The error probability can be further reduced by taking the majority of the accumulated results. The reduction of error probability resulted in a successful QFT demonstration on 1024 qubits.

Observation of antinormally ordered Hanbury Brown-Twiss correlations
K Usami, Y Nambu, BS Shi, A Tomita, K Nakamura
PHYSICAL REVIEW LETTERS, 92, 11, 113601, AMER PHYSICAL SOC, Mar. 2004, [Peer-reviewed]
English, Scientific journal, We have measured antinormally ordered Hanbury Brown-Twiss correlations for coherent states of the electromagnetic field by using a stimulated parametric down-conversion process. Photons were detected by stimulated emission, rather than by absorption, so that the detection responded not only to actual photons but also to zero-point fluctuations via spontaneous emission. The observed correlations were distinct from normally ordered ones as they showed excess positive correlations, i.e., photon bunching effects, which arose from the thermal nature of zero-point fluctuations.

Measured quantum Fourier transform of 1024 qubits on fiber optics
A Tomita, K Nakamura
INTERNATIONAL JOURNAL OF QUANTUM INFORMATION, 2, 1, 119, 131, WORLD SCIENTIFIC PUBL CO PTE LTD, Mar. 2004, [Peer-reviewed]
English, Scientific journal, Quantum Fourier transform (QFT) is a key function to realize quantum computers. A QFT followed by measurement was demonstrated on a simple circuit based on fiber-optics. The QFT was shown to be robust against imperfections in the rotation gate. Error probability was estimated to be 0.01 per qubit, which corresponded to error-free operation on 100 qubits. The error probability can be further reduced by taking the majority of the accumulated results. The reduction of error probability resulted in a successful QFT demonstration on 1024 qubits.

Observation of antinormally ordered Hanbury Brown-Twiss correlations
K Usami, Y Nambu, BS Shi, A Tomita, K Nakamura
PHYSICAL REVIEW LETTERS, 92, 11, AMER PHYSICAL SOC, Mar. 2004, [Peer-reviewed]
English, Scientific journal, We have measured antinormally ordered Hanbury Brown-Twiss correlations for coherent states of the electromagnetic field by using a stimulated parametric down-conversion process. Photons were detected by stimulated emission, rather than by absorption, so that the detection responded not only to actual photons but also to zero-point fluctuations via spontaneous emission. The observed correlations were distinct from normally ordered ones as they showed excess positive correlations, i.e., photon bunching effects, which arose from the thermal nature of zero-point fluctuations.

Quantum entanglement swapping with spontaneous parametric down-conversion
XB Wang, BS Shi, A Tomita, K Matsumoto
PHYSICAL REVIEW A, 69, 1, 014303, AMERICAN PHYSICAL SOC, Jan. 2004, [Peer-reviewed]
English, Scientific journal, Two remote. parties that have never interacted each other can be entangled through entanglement swapping operation done by a third party. Currently existing entanglement swapping experiments are done probabilistically by postselection, i.e., once a successful swapping is verified, the resultant entanglement is destructed. We propose a simple nonpostselection scheme to demonstrate the high-quality quantum entanglement swapping with the spontaneous parametric down-conversion process. Our scheme only requires the normal photon detectors which only distinguish the vacuum and nonvacuum Fock states.

Generation of a pulsed polarization entangled photon pair using a Sagnac interferometer
Bao-Sen Shi, Akihisa Tomita
Physical Review A - Atomic, Molecular, and Optical Physics, 69, 1, 4, 2004, [Peer-reviewed]
English, Scientific journal, In this paper, we report on a scheme to generate a pulsed polarization entangled photon pair through the use of a Sagnac interferometer. To demonstrate its workability, we experimentally obtained two-photon quantum interference for the polarization variable. The main advantage of this scheme is its exceptional stability, compared to other schemes based on the interferometric technique. It does not need any active or passive techniques to stabilize the interferometer, even the instrument is exposed to a relatively turbulent environment. © 2004 The American Physical Society.

1024-qubits-quantum Fourier transform on a fiber-optics circuit
A Tomita, K Nakamura
Quantum Informatics 2004, 5833, 239, 250, SPIE-INT SOC OPTICAL ENGINEERING, 2004, [Peer-reviewed]
English, International conference proceedings, Quantum Fourier transform (QFT) is an important subroutine in quantum computation. A QFT followed by measurement (MQFT), which appears in the final part of the phase estimation algorithm, was demonstrated on a simple circuit based on fiber-optics. The MQFT was shown to be robust against imperfections in the rotation gate. Error probability was estimated to be 0.01 per qubit, but could be further reduced by taking the majority of the accumulated results. The reduction of error probability resulted in a successful MQFT of 1024 qubits. The present results not only demonstrate the potential of fiber optics but provide an important clue for quantum computer design.

Generation of a pulsed polarization entangled photon pair using a Sagnac interferometer
BS Shi, A Tomita
PHYSICAL REVIEW A, 69, 1, AMER PHYSICAL SOC, Jan. 2004, [Peer-reviewed]
English, Scientific journal, In this paper, we report on a scheme to generate a pulsed polarization entangled photon pair through the use of a Sagnac interferometer. To demonstrate its workability, we experimentally obtained two-photon quantum interference for the polarization variable. The main advantage of this scheme is its exceptional stability, compared to other schemes based on the interferometric technique. It does not need any active or passive techniques to stabilize the interferometer, even the instrument is exposed to a relatively turbulent environment.

Single-photon interference experiment over 100km for quantum cryptography system using balanced gated-mode photon detector
H Kosaka, A Tomita, Y Nambu, T Kimura, K Nakamura
ELECTRONICS LETTERS, 39, 16, 1199, 1201, IEE-INST ELEC ENG, Aug. 2003, [Peer-reviewed]
English, Scientific journal, The authors demonstrate single-photon interference over 100 km using a balanced gated-mode photon detector and a plug and play system for quantum key distribution. The visibility with 0.1 photon/pulse was more than 80% after 100 km transmission. This corresponds to the fidelity of a quantum cryptography system of more than 90% and a QBER of less than 10%, satisfying the security criteria.

High performance photon detector for qubit discrimination in 1550nm
A Tomita
NEC RESEARCH & DEVELOPMENT, 44, 3, 290, 293, NEC CORPORATION, Jul. 2003, [Peer-reviewed]
English, Scientific journal, A photon detector combining the two avalanche photodiodes (APD) has been demonstrated for qubit discrimination in 1550nm. Spikes accompanied with the signals in gated-mode were canceled by balanced output from the two APDs. The spike cancellation made it possible to reduce the threshold in the discriminators, and thus the gate pulse voltage. The dark count probability and afterpulse probability were reduced to 7 x 10(-7) and 10(-4), respectively, without affecting the detection efficiency (11%) at 178K.

Quantum Fourier Transform with fiber optics
A Tomita
NEC RESEARCH & DEVELOPMENT, 44, 3, 282, 284, NEC CORPORATION, Jul. 2003, [Peer-reviewed]
English, Scientific journal, Quantum Fourier Transform (QFT) is a key ingredient of a number of quantum algorithms related to Shor's factoring algorithm. We demonstrate that a quantum Fourier transform followed by measurement in computational basis can be implemented with fiber-optic devices.

Generation of a pulsed polarization entangled-photon pair via a two-crystal geometry
BS Shi, A Tomita
PHYSICAL REVIEW A, 67, 4, AMER PHYSICAL SOC, Apr. 2003, [Peer-reviewed]
English, Scientific journal, In this paper, we report a scheme for the generation of a pulsed polarization entangled-photon pair in a two-crystal geometry. These two crystals are oriented with their optical axes aligned in a perpendicular plane, stacked in the vertical direction. The stack direction and the pass direction of pump laser are orthogonal. This scheme has some advantages compared to other schemes: no time compensation or narrow-band filter is needed, it is insensitive to the thickness of the nonlinear crystal. As a demonstration of its workability, we get more than 86% high visibility of two-photon quantum interference experimentally for the polarization variable, without any time compensation and narrow-band filter. This scheme may be a useful technique for the generation of a pulsed entangled-photon pair.

Generation of a pulsed polarization entangled-photon pair via a two-crystal geometry
Bao-Sen Shi, Akihisa Tomita
Physical Review A - Atomic, Molecular, and Optical Physics, 67, 4, 4, 2003, [Peer-reviewed]
English, Scientific journal, In this paper, we report a scheme for the generation of a pulsed polarization entangled-photon pair in a two-crystal geometry. These two crystals are oriented with their optical axes aligned in a perpendicular plane, stacked in the vertical direction. The stack direction and the pass direction of pump laser are orthogonal. This scheme has some advantages compared to other schemes: no time compensation or narrow-band filter is needed, it is insensitive to the thickness of the nonlinear crystal. As a demonstration of its workability, we get more than 86% high visibility of two-photon quantum interference experimentally for the polarization variable, without any time compensation and narrow-band filter. This scheme may be a useful technique for the generation of a pulsed entangled-photon pair. © 2003 The American Physical Society.

Quantum entanglement swapping with spontaneous parametric down conversion
W Xiang-Bin, BS Shi, A Tomita, K Matsumoto
QUANTUM INFORMATION AND COMPUTATION, 5105, 59, 67, SPIE-INT SOC OPTICAL ENGINEERING, 2003, [Peer-reviewed]
English, International conference proceedings, By quantum mechanics principle, two remote parties that have never interacted each other can be entangled through entanglement swapping operation done by a third party. Currently existing entanglement swapping experiments are done probabilistically by post-selection, i.e., once a successful swapping is verified, the resultant entanglement is destructed. We propose a simple non-post-selection scheme to probabilistically make the high quality quantum entanglement swapping with the spontaneous parametric down conversion (SPDC) process. After the swapping, two spatially separated parties who have never interacted each other are entangled through a single photon entangled state. Our scheme only requires the normal photon detectors which can distinguish the vacuum and non-vacuum Fock states.

Remote state preparation of an entangled state
BS Shi, A Tomita
JOURNAL OF OPTICS B-QUANTUM AND SEMICLASSICAL OPTICS, 4, 6, 380, 382, IOP PUBLISHING LTD, Dec. 2002, [Peer-reviewed]
English, Scientific journal, In this paper, we give a scheme for the remote state preparation of a chosen two-particle entangled state by a three-particle Greenberger-Horne-Zeilinger state, and propose a generalization to a chosen multiparticle entangled state. By this scheme, only one classical bit and one single-particle projective measurement are enough for the remote state preparation of some special chosen multiparticle entangled states.

Balanced, gated-mode photon detector for quantum-bit discrimination at 1550 nm
A Tomita, K Nakamura
OPTICS LETTERS, 27, 20, 1827, 1829, OPTICAL SOC AMER, Oct. 2002, [Peer-reviewed]
English, Scientific journal, A photon detector that combines two avalanche photodiodes (APDs) has been demonstrated for quantum-bit discrimination at 1550 nm. Spikes accompanied by signals in a gated mode were canceled by balanced output from the two APDs. The spike cancellation enabled one to reduce the threshold in the discriminators and thus the gate pulse voltage. The dark count probability and afterpulse probability were reduced to 7 X 10(-7) and 7 X 10(-4), respectively, without affecting the detection efficiency (11%) at 178 K. (C) 2002 Optical Society of America.

Teleportation of an unknown state by W state
BS Shi, A Tomita
PHYSICS LETTERS A, 296, 4-5, 161, 164, ELSEVIER SCIENCE BV, Apr. 2002, [Peer-reviewed]
English, Scientific journal, In this Letter, we show that W state can be used to realize the teleportation of an unknown state probabilistically. (C) 2002 Elsevier Science B.V. All rights reserved.

Detecting the inseparability and distillability of continuous variable states in Fock space
XB Wang, M Keiji, T Akihisa
PHYSICAL REVIEW LETTERS, 87, 13, 137903, AMER PHYSICAL SOC, Sep. 2001, [Peer-reviewed]
English, Scientific journal, The partial transposition (PT) operation is an efficient tool in detecting the inseparability of a mixed state. We give an explicit formula for the PT operation for the continuous variable states in Fock space. We then give the necessary and sufficient condition for the positivity of Gaussian operators. Based on this, a number of criteria on the inseparability and distillability for the multimode Gaussian states are naturally drawn. We finally give an explicit formula for the state in a subspace of a global Gaussian state. This formula, together with the known results for Gaussian states, gives the criteria for the inseparability and distillability in a subspace of the global Gaussian state.

Complete Bell state measurement with a solid state device
Akihisa Tomita
Physics Letters, Section A: General, Atomic and Solid State Physics, 282, 6, 331, 335, ELSEVIER SCIENCE BV, 30 Apr. 2001
English, Scientific journal, A solid state device, composed of two photon absorption crystals, rotators, and retarders, is proposed to discriminate all the four Bell states. Crystal symmetry and interference allow the absorption of a particular Bell state. The rotators and the retarders transform the other states to the state that can be detected. © 2001 Elsevier Science B.V.

Security of classical noise-based cryptograph
A Tomita, O Hirota
JOURNAL OF OPTICS B-QUANTUM AND SEMICLASSICAL OPTICS, 2, 6, 705, 710, IOP PUBLISHING LTD, Dec. 2000, [Peer-reviewed]
English, Scientific journal, We examine the security of a protocol on cryptographic key distribution proposed by Yuen and Kim (Yuen H P and Kim A M 1998 Phys. Lett. A 241 135). Theoretical and experimental analysis shows that, even if the eavesdropper can receive more photons than the legitimate receiver, secure key distribution is possible as long as the signal-to-noise ratio of the eavesdropper does not exceed eight times (9 dB) that of the receiver. Secure key distribution was demonstrated using conventional fibre optics. The secure key transmission rate in the experiment was estimated to be 2 Mb s(-1) at its maximum (0.04 bit per sender's bit). The present protocol has advantages over other quantum key distribution protocols in that it is more efficient and more easily implemented, but careful design and management are necessary to ensure the security of the cryptosystem.

Quantum key distribution using two coherent states of light and their superposition
Y Nambu, A Tomita, Y Chiba-Kohno, K Nakamura
PHYSICAL REVIEW A, 62, 1, 012312, AMER PHYSICAL SOC, Jul. 2000, [Peer-reviewed]
English, Scientific journal, Quantum-mechanical complementarity ensures the security of the key-distribution scheme reported by Brassard and Bennet in 1984 (BB84), but does not prohibit use of multi-photons as a signal carrier. We describe a novel BB84 scheme in which two nearly orthogonal coherent states carry the key, and the superposition of these states (cat states) protects the communication channel from eaves-dropping. Information leakage to eavesdroppers can be determined from the visibility of the interferential fringes in the distribution of the outcome when a certain quadrature component is measured through homodyne detection. The effect of channel loss and detector inefficiency is discussed.

Triple-exposure method for fabricating triangular-lattice photonic crystals
M Tokushima, H Kosaka, A Tomita, H Yamada
JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, 39, 7A, 4236, 4240, JAPAN J APPLIED PHYSICS, Jul. 2000, [Peer-reviewed]
English, Scientific journal, We have developed a triple-exposure method for defining a triangular-lattice pattern to form two-dimensional (2D) photonic-crystal (PC) waveguides operating at a 1.55-mu m wavelength. This method, in which three line-and-space (L/S) patterns are projected onto a wafer in three different directions, allows us to define a smaller-pitch triangular-lattice pattern compared to that definable by the conventional exposure method. When 0.61-mu m-pitch L/S masks were employed, an i-line stepper successfully resolved a 0.7-mu m-hole-pitch triangular-lattice pattern, which was at the resolution limit. A sharply bent line defect was introduced as a waveguide into the pattern by using US photomasks with the corresponding parts of the space areas removed. We applied this method to fabricate a triangular-lattice 2D PC with a hole pitch of 0.8 mu m and a minimum feature size of 0.05 mu m. The measured photonic bandgap (PBG) of this PC indicated that it operates at a near-1.55-mu m wavelength.

Splitting of triply degenerate refractive indices by photonic crystals
H Kosaka, A Tomita, T Kawashima, T Sato, S Kawakami
PHYSICAL REVIEW B, 62, 3, 1477, 1480, AMER PHYSICAL SOC, Jul. 2000, [Peer-reviewed]
English, Scientific journal, The refractive index is inherently triply degenerate in conventional crystals, but this degeneracy is lifted and the refractive index is split into a phase, a group, and a "fan" index by photonic crystals. These indices are explained as three different sections passing through the photonic band structure. We observed the fan index, which describes beam divergence and is defined by the local curvature of the equienergy section, range from near zero to almost infinity. This paper provides a unified description of photonic and conventional crystal optics, and may enable previously inconceivable optical designs.

Lightwave propagation through a 120 degrees sharply bent single-line-defect photonic crystal waveguide
M Tokushima, H Kosaka, A Tomita, H Yamada
APPLIED PHYSICS LETTERS, 76, 8, 952, 954, AMER INST PHYSICS, Feb. 2000, [Peer-reviewed]
English, Scientific journal, We have demonstrated 1.55 mu m wavelength lightwave propagation through a 120 degrees sharply bent waveguide formed in a triangular-lattice two-dimensional photonic crystal (2D PC). Such propagation has not previously been experimentally confirmed. The photonic crystal was fabricated in a silicon-on-insulator (SOI) wafer with the top silicon layer of the wafer used as a core layer. A 877-mu m-long single-line-defect waveguide was formed in the PC with a sharp 120 degrees bend near the middle of the waveguide. A tapered-hemispherical-end fiber was coupled to the input end of the waveguide for the light input, and the output from the other end of the waveguide was directly observed by scanning its near-field profile with another tapered-hemispherical-end fiber. (C) 2000 American Institute of Physics. [S0003-6951(00)01208-0].

Photonic-crystal spot-size converter
H Kosaka, T Kawashima, A Tomita, T Sato, S Kawakami
APPLIED PHYSICS LETTERS, 76, 3, 268, 270, AMER INST PHYSICS, Jan. 2000, [Peer-reviewed]
English, Scientific journal, We have demonstrated a spot-size converter (SSC) that is made of photonic crystals (PCs) and has a conversion ratio of 10:1 for a 1-mu m-wavelength light beam. Its real-spatial distribution was narrowed by intentionally broadening its wave vector distribution and increasing effective refractive index. The advantage of this PC-based SSC over conventional bulk-based SSC are compactness (monolithic integration), positional independence, and extremely deep depth of focus. This PC-SSC is a candidate for an interface between photonic-crystal waveguides and conventional optical waveguides. (C) 2000 American Institute of Physics. [S0003-6951(00)00903-7].

Splitting of triply degenerate refractive indices by photonic crystals
Hideo Kosaka, A. Tomita, T. Kawashima, T. Sato, S. Kawakami
Phys. Rev. B, 62, 3, 1477, 1480, 2000, [Peer-reviewed]
English, Scientific journal

Quantum key distribution using two coherent states of light and their superposition
Yoshihiro Nambu, Akihisa Tomita, Yoshie Chiba-Kohno, Kazuo Nakamura
Physical Review A - Atomic, Molecular, and Optical Physics, 62, 1, 11, 2000, [Peer-reviewed]
English, Scientific journal, Quantum-mechanical complementarity ensures the security of the key-distribution scheme reported by Brassard and Bennet in 1984 (BB84), but does not prohibit use of multi-photons as a signal carrier. We describe a novel BB84 scheme in which two nearly orthogonal coherent states carry the key, and the superposition of these states (cat states) protects the communication channel from eavesdropping. Information leakage to eavesdroppers can be determined from the visibility of the interferential fringes in the distribution of the outcome when a certain quadrature component is measured through homodyne detection. The effect of channel loss and detector inefficiency is discussed. © 2000 The American Physical Society.

Superprism phenomena in photonic crystals: Toward microscale lightwave circuits
H Kosaka, T Kawashima, A Tomita, M Notomi, T Tamamura, T Sato, S Kawakami
JOURNAL OF LIGHTWAVE TECHNOLOGY, 17, 11, 2032, 2038, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Nov. 1999, [Peer-reviewed]
English, Scientific journal, The superprism phenomenon, the dispersion of light 500 times stronger than the dispersion in conventional prisms, was demonstrated at optical wavelengths in photonic crystals (PC's) fabricated on Si. Drastic light-beam steering in the PC's was achieved by slightly changing the incident wavelength or angle, The scanning span reached 50 degrees with only a 1% shift of incident wavelength, and reached 140 degrees with only a 14 degrees shift of the incident angle at wavelengths around 1 mu m The propagation direction was quantitatively interpreted in terms of highly anisotropic dispersion surfaces derived by photonic-band calculation. The physics behind this demonstration will open a novel field called photonic crystalline optics. The application of these phenomena promises to enable the fabrication of integrated microscale lightwave circuits (mu LC's) on Si with large scale integrated (LSI)-compatible lithography techniques. Such mu LC's will allow more efficient use of wavelength resources when used in wavelength multiplexers/demultiplexers or dispersion compensators by enabling lower loss and broader bandwidth.

Effects of spectral broadening and cross relaxation on the gain saturation characteristics of quantum dot laser amplifiers
Y Nambu, A Tomita, H Saito, K Nishi
JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, 38, 9A, 5087, 5095, JAPAN J APPLIED PHYSICS, Sep. 1999, [Peer-reviewed]
English, Scientific journal, Spectral broadening and cross relaxation of an aggregate of highly carrier-injected quantum dots have been studied experimentally by evaluating the gain saturation and optical-wave mixing characteristics of traveling-wave laser amplifiers that include self-assembled 0.98-mu m InGaAs dots. Measured signal-gain spectra at room temperature showed that the gain decreases over the entire gain spectra when an intense and narrow-linewidth saturating signal is injected at a higher-order subband transition of the quantum dot. Differential-gain spectrum was calculated from the saturated and unsaturated signal gain spectra, and revealed asymmetric gain saturation with respect to the wavelength of the saturating signal. These results suggest that cross relaxation is so fast that the dot aggregate has a homogeneously broadened gain spectrum. Self-saturation and cross-saturation characteristics, and the detuning characteristics of highly nondegenerate four-wave mixing were also investigated. The latter revealed that the characteristic times of two cross-relaxation processes are about 1 ps and 100 fs.

Photonic crystals for micro lightwave circuits using wavelength-dependent angular beam steering
H Kosaka, T Kawashima, A Tomita, M Notomi, T Tamamura, T Sato, S Kawakami
APPLIED PHYSICS LETTERS, 74, 10, 1370, 1372, AMER INST PHYSICS, Mar. 1999, [Peer-reviewed]
English, Scientific journal, Light-beam steering that is extremely wavelength dependent has been demonstrated by using photonic crystals fabricated on Si. The scanning span reached 50 degrees with only a 1% shift of incident wavelength at around 1 mu m. The resulting angular dispersion is two orders of magnitude larger than that achieved with conventional prisms or gratings. The application of such superprism phenomena promises to enable the fabrication of integrated micro lightwave circuits that will allow more efficient use of wavelength resources when used in wavelength multiplexers/demultiplexers or dispersion compensators by enabling lower loss and broader bandwidth. (C) 1999 American Institute of Physics. [S0003-6951(99)00910-9].

Self-collimating phenomena in photonic crystals
H Kosaka, T Kawashima, A Tomita, M Notomi, T Tamamura, T Sato, S Kawakami
APPLIED PHYSICS LETTERS, 74, 9, 1212, 1214, AMER INST PHYSICS, Mar. 1999, [Peer-reviewed]
English, Scientific journal, We found that self-determining collimated light is generated in a photonic crystal fabricated on silicon. The divergence of the collimated beam is insensitive to that of the incident beam and much smaller than the divergence that would be generated in conventional Gaussian optics. The incident-angle dependence of the self-collimated light propagation including lens-like divergent propagation was interpreted in terms of the highly modulated dispersion surfaces with inflection points, where the curvature changes from downward to upward corresponding to respectively a concave/convex-lens case. This demonstration is an important step towards controlling beam profile in photonic crystal integrated light circuits and towards developing "photonic crystalline optics.'' (C) 1999 American Institute of Physics. [S0003-6951(99)02809-0].

Superprism phenomena in photonic crystals
H Kosaka, T Kawashima, A Tomita, M Notomi, T Tamamura, T Sato, S Kawakami
PHYSICAL REVIEW B, 58, 16, 10096, 10099, AMER PHYSICAL SOC, Oct. 1998, [Peer-reviewed]
English, Scientific journal, Extraordinary angle-sensitive light propagation, which we call a superprism phenomenon, was demonstrated at optical wavelength in photonic crystals with three-dimensional-periodic structure fabricated on Si substrate. The propagation beam was swung from - 90 degrees to + 90 degrees with a slight change in the incident angle within +/- 12 degrees. This effect together with wavelength sensitivity is at least two orders of magnitude stronger than that of the conventional prism. The incident-angle dependence including negative refraction and multiple beam branching was interpreted from highly anisotropic dispersion surfaces derived by photonic band calculation. These phe nomena will be available to fabricate microscale light circuits on Si with LSI-compatible lithography techniques. [S0163-1829(98)51840-1].

Conduction-band discontinuity of InAsP/InP heterojunction
T Anan, K Nishi, A Tomita, K Tokutome, S Sugou
JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, 37, 7, 3915, 3918, JAPAN J APPLIED PHYSICS, Jul. 1998, [Peer-reviewed]
English, Scientific journal, The band line-up of InAsP/lnP heterojunctions was investigated. The conduction-band discontinuity ratio Q(c) of strained InAsP alloys was determined by fitting the well thickness dependence of transition energies in InAsP quantum wells. This method does not require precise values of deformation potentials of InAsP, allowing mon reliable determination of the band offset. The obtained Q(c) value was 0.35, which is relatively small compared to previous reports. This value of the band offset is consistent with the predictions from the semi-empirical linear combination of atomic orbitals (LCAO) model.

Superprism phenomena in photonic crystals
KOSAKA H.
Phys. Rev., B, 58, 16, R10096, R10099, 1998

Evidence of nonuniform carrier distribution in multiple quantum well lasers
H Yamazaki, A Tomita, M Yamaguchi, Y Sasaki
APPLIED PHYSICS LETTERS, 71, 6, 767, 769, AMER INST PHYSICS, Aug. 1997, [Peer-reviewed]
English, Scientific journal, Carrier distribution in multiple quantum well (MQW) lasers is studied by measuring laser wavelength. The MQW is designed to contain wells with different thicknesses. The MQW laser wavelength agreed with the transition wavelength of the wells near the p-clad layer at room temperature. At low temperatures, however, the laser wavelength corresponded to the thicker wells. The results imply that hole localization takes place at room temperature but disappears at low temperatures. This shows the invalidity of the conventional capture/escape model. The importance of the mean free path of unbound carriers is pointed out. (C) 1997 American Institute of Physics.

Quantum-classical crossover in carrier transport
A Tomita
PHYSICS LETTERS A, 233, 1-2, 115, 120, ELSEVIER SCIENCE BV, Aug. 1997, [Peer-reviewed]
English, Scientific journal, Relaxation effects on the carrier transport have been studied using a density matrix approach. The analysis shows that the transport characteristics are determined by the ratio of the mean free path IF to the active region length L. The Landauer formula and the classical conductivity are obtained in the limits 2 pi l(F)/L much less than 1 and 2 pi l(F)/L much greater than 1. (C) 1997 Elsevier Science B.V.

Efficient exciton energy transfer between widely separated quantum wells at low temperatures
A Tomita, J Shah, RS Knox
PHYSICAL REVIEW B, 53, 16, 10793, 10803, AMER PHYSICAL SOC, Apr. 1996, [Peer-reviewed]
English, Scientific journal, Energy transfer between quantum wells is of fundamental interest and also contributes to the dynamical response of devices based on multiple quantum wells. We report the observation of efficient energy transfer at low temperatures between unequal GaAs quantum wells separated by a thick (10-30 nm) Al0.3Ga0.7As barrier. The transfer efficiency is about 30% for transfer from the narrow well to the wide well (Stokes transfer), about 10(-2)% for the anti-Stokes transfer, and nearly independent of temperature (2-80 K) and barrier thickness. Tunneling, thermal excitation, and impurity-related transitions cannot explain these observations. We present a calculation for transfer efficiency using Forster-type dipole-dipole interaction between excitons and between excitons and free carriers in quantum wells, and show that this dipole-dipole transfer process can reproduce the observed temperature dependence and the magnitudes of the transfer efficiency. This process has not been considered previously for energy transfer between quantum wells.

Many-body effects, space-charge potential, and valence-band mixing on the optical gain in quantum-well structures
A Tomita
PHYSICAL REVIEW B, 54, 8, 5609, 5619, AMERICAN PHYSICAL SOC, 1996
English, Scientific journal, The subband energy and gain in quantum wells are calculated with the local-density approximation, including valence-band mixing. The effects of the space-charge potential and the exchange-correlation potential are considered self-consistently. The space-charge potential is estimated to be as large as 45 meV for a 5-nm-thick Ga0.5In0.5P/(Al0.4Ga0.6)(0.5)In0.5P single quantum well with a carrier density of 4 x 10(12) cm(-2) at room temperature. It is shown that the exchange-correlation potential compensates the space-charge potential for holes to some extent in the present system. Comparison between the Hartree approximation (neglecting the exchange-correlation potential) and the full calculation shows the necessity of both the space-charge potential and the exchange-correlation potential to obtain accurate subband energies and optical gain. Some of the previous calculations on the many-body effects in quantum wells have not considered the space-charge potential. Thus, they are valid only for systems with weak space-charge potentials, for example, GaAs/AlxGa1-xAs quantum wells.

FEMTOSECOND HOLE RELAXATION IN N-TYPE MODULATION-DOPED QUANTUM-WELLS (VOL 48, PG 5708, 1993)
A TOMITA, J SHAH, JE CUNNINGHAM, SM GOODNICK, P LUGLI, SL CHUANG
PHYSICAL REVIEW B, 52, 7, 5445, 5445, AMER PHYSICAL SOC, Aug. 1995, [Peer-reviewed]
English

PROPOSAL OF STEP BARRIER STRUCTURES TO REDUCE HOLE LOCALIZATION IN MULTIPLE-QUANTUM-WELL STRUCTURES
A TOMITA
JOURNAL OF APPLIED PHYSICS, 77, 5, 2029, 2031, AMER INST PHYSICS, Mar. 1995, [Peer-reviewed]
English, Scientific journal

FREE-CARRIER EFFECT ON THE REFRACTIVE-INDEX CHANGE IN QUANTUM-WELL STRUCTURES
A TOMITA
IEEE JOURNAL OF QUANTUM ELECTRONICS, 30, 12, 2798, 2802, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Dec. 1994, [Peer-reviewed]
English, Scientific journal, Dielectric response of spatially inhomogeneous free carriers in quantum wells is studied by random phase approximation (RPA) for a high frequency (near infrared) light field, A general form for susceptibilities is derived for a guided optical mode, The analysis shows that a conventional treatment (Drude model) for the TE mode provides a good approximation, The susceptibility for the TM mode is almost the same as that for the TE mode, in spite of quantization of carrier motion, as long as the photon energy is much larger than the intersubband transition energy. The free carrier component of the refractive index change in quantum-well waveguides is thus isotropic near the band gap, Carrier confinement will not reduce the free carrier component in the linewidth enhancement factor around the lasing wavelength.

SPECTRAL HOLE-BURNING AND CARRIER-HEATING EFFECT ON THE TRANSIENT OPTICAL NONLINEARITY OF HIGHLY CARRIER-INJECTED SEMICONDUCTORS
Y NAMBU, A TOMITA
IEEE JOURNAL OF QUANTUM ELECTRONICS, 30, 9, 1981, 1994, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Sep. 1994, [Peer-reviewed]
English, Scientific journal, An improved density-matrix theory is developed that can treat both spectral hole burning and carrier heating self-consistently. Various intraband and interband relaxation terms characterized by different relaxation times and quasi-equilibrium distributions are introduced into the density-matrix equations within a relaxation-time approximation. Conservation of total number and energy densities of carrier systems in each band is considered to determine the quasi-equilibrium distributions. Formalism is applied to the calculation of the transient optical nonlinearity of highly carrier-injected semiconductors. Spectral hole burning and carrier-heating effects on the spectral and temporal characteristics are then clarified. In particular, the significant four-wave-mixing effect due to carrier heating is pointed out. An experiment that can be used to directly prove the existence of the carrier-heating effects on gain nonlinearity is also proposed.

FEMTOSECOND DYNAMICS OF NONTHERMAL HOLES IN N-MODULATION-DOPED QUANTUM-WELLS
A TOMITA, J SHAH, JE CUNNINGHAM, SM GOODNICK, P LUGLI, SL CHUANG
SEMICONDUCTOR SCIENCE AND TECHNOLOGY, 9, 5, 449, 452, IOP PUBLISHING LTD, May 1994, [Peer-reviewed]
English, Scientific journal, Nearly all femtosecond carrier relaxation studies are dominated by electron dynamics. We have investigated femtosecond hole dynamics by a judicious choice of experimental parameters: n-modulation-doped GaAs/AlGaAs quantum wells at low temperatures excited with small excess energy and low photoexcitation density. We show that holes are non-thermal for approximately the first 800 fs and determine the hole-electron energy loss rates by comparing experimental results with Monte Carlo simulations. These results represent the first definitive study of hole relaxation dynamics in a semiconductor.

FEMTOSECOND HOLE RELAXATION IN N-TYPE MODULATION-DOPED QUANTUM-WELLS
A TOMITA, J SHAH, JE CUNNINGHAM, SM GOODNICK, P LUGLI, SL CHUANG
PHYSICAL REVIEW B, 48, 8, 5708, 5711, AMERICAN PHYSICAL SOC, Aug. 1993
English, We present a study of femtosecond hole-relaxation dynamics in n-modulation-doped GaAs/AlxGa1-xAs quantum wells at low temperatures and low photoexcitation density. We conclude that holes are nonthermal for approximately the first 800 fs and determine the hole-electron energy loss rates by comparing experimental results with Monte Carlo simulations. These results represent a definitive study of hole scattering and relaxation processes in a semiconductor.

STRAIN EFFECT ON K-FACTOR, DIFFERENTIAL GAIN AND NONLINEAR GAIN COEFFICIENT FOR INGAAS/INGAASP STRAINED MULTIQUANTUM-WELL LASERS
JI SHIMIZU, A KIMURA, K NANIWAE, M NIDO, S MURATA, A TOMITA, A SUZUKI
ELECTRONICS LETTERS, 29, 7, 579, 581, IEE-INST ELEC ENG, Apr. 1993, [Peer-reviewed]
English, Scientific journal, The strain effect on the K factor, the differential gain and the nonlinear gain coefficient for InGaAs/InGaAsP tensile-strained and compressive-strained multiquantum well (MQW) lasers is experimentally investigated from the intrinsic modulation response. The differential gain increases with an increase in strain for both types of laser. The strain dependence of the nonlinear gain coefficient is not as high as that of the differential gain. Therefore, the K factor for strained MQW lasers mainly results from the differential gain.

INFLUENCE OF FREE CARRIER PLASMA EFFECT ON CARRIER-INDUCED REFRACTIVE-INDEX CHANGE FOR QUANTUM-WELL LASERS
S MURATA, A TOMITA, A SUZUKI
IEEE PHOTONICS TECHNOLOGY LETTERS, 5, 1, 16, 19, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jan. 1993, [Peer-reviewed]
English, Scientific journal, Influence of the free carrier component due to the plasma effect on carrier-induced refractive index change and its dependency on polarization for multiple-quantum-well (MQW) and bulk lasers are experimentally studied. The ratios of the component to the total index change, R(fc), are 0.6, 0.4, and 0.1 for 1.3-mum MQW, 1.3-mum bulk, and 0.8-mum MQW lasers, respectively. The TM/TE polarization ratios of the component, R(TM/TE), are 0.8 and 0.3 for 1.3-mum MQW and 0.8-mum MQW lasers. The relationship between the index change and the carrier overflow (to barrier and separate confinement heterostructure layers) for MQW lasers is also discussed. Large R(fc) and R(TM/TE) for the 1.3-mum MQW laser result from the carrier overflow.

HIGH-FREQUENCY MODULATION CHARACTERISTICS IN 1-BULLET-5-MU-M COMPRESSIVELY STRAINED MULTIQUANTUM WELL LASERS WITH LARGE NUMBER OF WELLS
S MURATA, K NANIWAE, J SHIMIZU, M NIDO, A TOMITA, A SUZUKI
ELECTRONICS LETTERS, 28, 15, 1456, 1457, IEE-INST ELEC ENG, Jul. 1992, [Peer-reviewed]
English, Scientific journal, The high-frequency modulation characteristics in 1.5-mu-m compressively strained multiquantum well (MQW) lasers with a large number of wells (N(w) = 20) are measured using an optical modulation technique. The damping K factor and differential gain are improved by increasing the number of wells. The nonlinear gain coefficient is nearly the same. For a 20 well MQW laser with 1.5% strain, the 3dB bandwidth is estimated to be 29 GHz at 15 mW output.

OPTICAL CONFINEMENT FACTOR DEPENDENCE OF K-FACTOR, DIFFERENTIAL GAIN, AND NONLINEAR GAIN IN 1.55-MU-M MQW AND STRAINED MQW LASERS
J SHIMIZU, H YAMADA, S MURATA, A TOMITA, M KITAMURA, A SUZUKI
IEEE TRANSACTIONS ON ELECTRON DEVICES, 38, 12, 2698, 2698, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Dec. 1991, [Peer-reviewed]
English

THZ OPTICAL-FREQUENCY CONVERSION OF 1GB/S-SIGNALS USING HIGHLY NONDEGENERATE 4-WAVE-MIXING IN AN INGAASP SEMICONDUCTOR-LASER
S MURATA, A TOMITA, J SHIMIZU, A SUZUKI
IEEE PHOTONICS TECHNOLOGY LETTERS, 3, 11, 1021, 1023, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Nov. 1991, [Peer-reviewed]
English, Scientific journal, THz-range optical-frequency conversion of 1 Gb/s-signals is demonstrated, for the first time, through the use of cavity-enhanced highly nondegenerate four-wave mixing (HNDFWM) in an InGaAsP semiconductor laser. This conversion is based on a subpicosecond ultrafast nonlinear gain process in the laser. The possibility of applying this phenomenon to an optical fiber dispersion compensator is also discussed.

OPTICAL-CONFINEMENT-FACTOR DEPENDENCIES OF THE K-FACTOR, DIFFERENTIAL GAIN, AND NONLINEAR GAIN COEFFICIENT FOR 1.55 MU-M INGAAS/INGAASP MQW AND STRAINED-MQW LASERS
J SHIMIZU, H YAMADA, S MURATA, A TOMITA, M KITAMURA, A SUZUKI
IEEE PHOTONICS TECHNOLOGY LETTERS, 3, 9, 773, 776, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Sep. 1991, [Peer-reviewed]
English, Scientific journal, Optical-confinement-factor GAMMA-dependencies of the K factor, differential gain dg / dN, and nonlinear gain coefficient epsilon, for 1.55-mu-m InGaAs/InGaAsP multiple-quantum-well (MQW) and compressively strained MQW lasers, were investigated experimentally. For both MQW and strained-MQW lasers, when GAMMA-is increased, the K factor is reduced, dg / dN is increased but epsilon-is almost constant. These results indicate that the GAMMA-dependence of the K factor mainly results from a change in dg / dN, and does not result from a change in epsilon. For the strained MQW lasers, the K factor, dg / dN and epsilon are, respectively, half as large, twice as large and the same as those for the MQW lasers, when both types of lasers have the same GAMMA-(= 0.05). This suggests that the strained MQW lasers with a large GAMMA-are expected to have a small K factor and thus be preferable in achieving large modulation bandwidths.

A NEW DENSITY-MATRIX THEORY FOR SEMICONDUCTOR-LASERS, INCLUDING NON-MARKOVIAN INTRABAND RELAXATION AND ITS APPLICATION TO NONLINEAR GAIN
A TOMITA, A SUZUKI
IEEE JOURNAL OF QUANTUM ELECTRONICS, 27, 6, 1630, 1641, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jun. 1991, [Peer-reviewed]
English, Scientific journal, A density matrix equation for semiconductor lasers has been derived from the microscopic equation of motion for electrons using a projection operator method. The effect of non-Markovian intraband relaxation has been found to be described by the autocorrelation functions of electron scattering terms in the microscopic interaction Hamiltonian. The obtained density matrix equation provides a systematic treatment for dynamical properties of semiconductor lasers, and the treatment can be performed by calculating the autocorrelation functions from available material parameters. A gain formula for arbitrary light output power has been derived from a single-mode steady-state nonperturbative solution. A simplified estimation employing a stochastic model has shown that non-Markovian intraband relaxation enhances both linear gain and nonlinear gain. The reduction of nonlinear gain effects is also discussed.

OBSERVATION OF HIGHLY NONDEGENERATE 4-WAVE-MIXING (GREATER-THAN 1 THZ) IN AN INGAASP MULTIPLE QUANTUM-WELL LASER
S MURATA, A TOMITA, J SHIMIZU, M KITAMURA, A SUZUKI
APPLIED PHYSICS LETTERS, 58, 14, 1458, 1460, AMER INST PHYSICS, Apr. 1991, [Peer-reviewed]
English, Scientific journal, Highly nondegenerate four-wave mixing (NDFWM) in the pump-probe detuning region of more than 1 THz has been observed in an InGaAsP multiple quantum well laser for the first time. Each of the probe and signal frequencies is closed to cavity resonance modes for the pump laser and those outputs are enhanced. The highly NDFWM process is based on a mechanism whose response time is less than 0.2 ps.

THZ OPTICAL FREQUENCY-CONVERSION OF 1GB/S SIGNALS USING HIGHLY NONDEGENERATE 4-WAVE-MIXING IN SEMICONDUCTOR-LASERS
S MURATA, A TOMITA, J SHIMIZU, A SUZUKI
IOOC-ECOC 91, PTS 1-4, 85, 88, SOC ELECTRICIENS ELECTRONICIENS RADIOELECTRICIENS, 1991, [Peer-reviewed]
English, International conference proceedings

5-1 ON-OFF CONTRAST INGAAS/INP MULTIPLE QUANTUM WELL FABRY-PEROT ETALON MODULATOR
A TOMITA, Y KOHGA, A SUZUKI, T TERAKADO, A AJISAWA
APPLIED PHYSICS LETTERS, 55, 18, 1817, 1819, AMER INST PHYSICS, Oct. 1989, [Peer-reviewed]
English, Scientific journal

DISPERSIVE-TYPE OPTICAL BISTABILITY IN A SELF-ELECTROOPTIC-EFFECT ETALON
A TOMITA, A SUZUKI, K KOMATSU
JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, 28, 8, 1523, 1524, JAPAN J APPLIED PHYSICS, Aug. 1989, [Peer-reviewed]
English

CARRIER-INDUCED LASING WAVELENGTH SHIFT FOR QUANTUM-WELL LASER-DIODES
A TOMITA, A SUZUKI
IEEE JOURNAL OF QUANTUM ELECTRONICS, 23, 7, 1155, 1159, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, Jul. 1987, [Peer-reviewed]
English, Scientific journal

OPTICAL FEEDBACK EFFECT ON BISTABLE LASER-DIODES
A TOMITA, A SUZUKI
OPTICAL AND QUANTUM ELECTRONICS, 19, S75, S82, CHAPMAN HALL LTD, Jul. 1987, [Peer-reviewed]
English, Scientific journal

TURN-OFF CHARACTERISTICS OF BISTABLE LASER DIODE
A TOMITA, T TERAKADO, A SUZUKI
JOURNAL OF APPLIED PHYSICS, 59, 6, 1839, 1842, AMER INST PHYSICS, Mar. 1986, [Peer-reviewed]
English, Scientific journal

Improvement of Diffraction Efficiency of Mode Demultiplexer Using Volume Hologram at 1550nm
柴 泰純, 岡本 淳, 林 穂高, 小川 和久, 富田 章久, 高畠 武敏, 後藤 優太, 品田 聡, 和田 尚也, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 119, 231, 105, 108, 17 Oct. 2019
電子情報通信学会, Japanese

Improvement of Diffraction Efficiency of Mode Demultiplexer Using Volume Hologram at 1550nm
柴 泰純, 岡本 淳, 林 穂高, 小川 和久, 富田 章久, 高畠 武敏, 後藤 優太, 品田 聡, 和田 尚也, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 119, 230, 105, 108, 17 Oct. 2019
電子情報通信学会, Japanese

量子暗号通信の最前線 (特別小特集 北で輝く光科学技術の研究)　　　　　　　　　　　　　　　
富田章久, 電子情報通信学会誌, 102, 10, 942, 946, Oct. 2019, [Peer-reviewed], [Invited], [Lead author]
Japanese, Introduction scientific journal

光による量子コンピュータの展望　　　　　　　　　　　　　　　
富田章久, 光アライアンス, 30, 7, 1, 5, Jul. 2019, [Invited], [Lead author]
Japanese, Introduction commerce magazine

Improvement of noise separation accuracy in image reconstruction in virtual phase conjugation based optical tomography
永吉 颯太, 岡本 淳, 後藤 優太, 小川 和久, 富田 章久, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 118, 449, 71, 74, 19 Feb. 2019
電子情報通信学会, Japanese

Improvement of noise separation accuracy in image reconstruction in virtual phase conjugation based optical tomography
永吉 颯太, 岡本 淳, 後藤 優太, 小川 和久, 富田 章久, 映像情報メディア学会技術報告 = ITE technical report, 43, 5, 71, 74, Feb. 2019
映像情報メディア学会, Japanese

High-Threshold Fault-Tolerant Quantum Computation with Analog Quantum Error Correction
Fukui Kosuke, Tomita Akihisa, Okamoto Atsushi, Fujii Keisuke, Butsuri, 74, 10, 720, 726, 2019, [Peer-reviewed], [Invited]

To implement large-scale quantum computation, the Gottesman-Kitaev-Preskill (GKP) qubit has been recognized as an important technological element. However, it is still challenging to experimentally generate the GKP qubit with the required squeezing level for large-scale quantum computation. To reduce this requirement, we develop a method to prevent the squeezing level from decreasing during the construction of the large-scale cluster. We show that our method with the analog quantum error correction can relax the required squeezing level within the reach of the current experimental technology.

, The Physical Society of Japan, Japanese, Introduction scientific journal

Experiment on digital image multiplexing/demultiplexing using virtual phase conjugation for high density holographic memory
後藤 優太, 岡本 淳, 小川 和久, 富田 章久, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 117, 432, 261, 265, 15 Feb. 2018
電子情報通信学会, Japanese

Experiment on digital image multiplexing/demultiplexing using virtual phase conjugation for high density holographic memory
後藤 優太, 岡本 淳, 小川 和久, 富田 章久, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 117, 431, 261, 265, 15 Feb. 2018
電子情報通信学会, Japanese

Experiment on digital image multiplexing/demultiplexing using virtual phase conjugation for high density holographic memory
後藤 優太, 岡本 淳, 小川 和久, 富田 章久, 映像情報メディア学会技術報告 = ITE technical report, 42, 4, 261, 265, Feb. 2018
映像情報メディア学会, Japanese

Experiment on selective mode conversion using dual-phase modulation
前田 智弘, 岡本 淳, 小川 和久, 富田 章久, 若山 雄太, 釣谷 剛宏, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 117, 196, 51, 55, 31 Aug. 2017
電子情報通信学会, Japanese

Separation Characteristics of Volume Holographic Spatial Mode Demultiplexer at 850 nm Wavelength
清水 新平, 岡本 淳, 水川 郁弥, 小川 和久, 富田 章久, 高畠 武敏, 品田 聡, 和田 尚也, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 117, 196, 45, 50, 31 Aug. 2017
電子情報通信学会, Japanese

Experiment on Spatial Mode Separation and Conversion Technique Using Volume Hologram
水川 郁弥, 岡本 淳, 後藤 優太, 清水 新平, 小川 和久, 富田 章久, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 116, 464, 157, 161, 20 Feb. 2017
電子情報通信学会, Japanese

Experiment on Virtual Shift Method for Alignment Error Reduction of Holographic Diversity Interferometry
山岸 信博, 岡本 淳, 野澤 仁, 後藤 優太, 小川 和久, 富田 章久, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 116, 464, 153, 156, 20 Feb. 2017
電子情報通信学会, Japanese

Characterization of multiplexing technique using virtual phase conjugation for high density holographic memory
後藤 優太, 岡本 淳, 小川 和久, 富田 章久, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 116, 463, 147, 152, 20 Feb. 2017
電子情報通信学会, Japanese

Experiment on Dual-Wavelength Method toward Spatial Mode De-Multiplexing with Volume Hologram in Optical Communication Wavelength Band
清水 新平, 岡本 淳, 水川 郁弥, 小川 和久, 富田 章久, 高畠 武敏, 品田 聡, 和田 尚也, 電子情報通信学会技術研究報告 = IEICE technical report : 信学技報, 116, 463, 163, 168, 20 Feb. 2017
電子情報通信学会, Japanese

Experiment on Dual-Wavelength Method toward Spatial Mode De-Multiplexing with Volume Hologram in Optical Communication Wavelength Band
清水 新平, 岡本 淳, 水川 郁弥, 小川 和久, 富田 章久, 高畠 武敏, 品田 聡, 和田 尚也, 映像情報メディア学会技術報告 = ITE technical report, 41, 5, 163, 168, Feb. 2017
映像情報メディア学会, Japanese

Experiment on Virtual Shift Method for Alignment Error Reduction of Holographic Diversity Interferometry
山岸 信博, 岡本 淳, 野澤 仁, 後藤 優太, 小川 和久, 富田 章久, 映像情報メディア学会技術報告 = ITE technical report, 41, 5, 153, 156, Feb. 2017
映像情報メディア学会, Japanese

Experiment on Spatial Mode Separation and Conversion Technique Using Volume Hologram
水川 郁弥, 岡本 淳, 後藤 優太, 清水 新平, 小川 和久, 富田 章久, 映像情報メディア学会技術報告 = ITE technical report, 41, 5, 157, 161, Feb. 2017
映像情報メディア学会, Japanese

Characterization of multiplexing technique using virtual phase conjugation for high density holographic memory
後藤 優太, 岡本 淳, 小川 和久, 富田 章久, 映像情報メディア学会技術報告 = ITE technical report, 41, 5, 147, 152, Feb. 2017
映像情報メディア学会, Japanese

Virtual interferogram-generation algorithm for high-accurate phase measurement using only two holograms
野澤 仁, 岡本 淳, 小川 和久, 富田 章久, 映像情報メディア学会技術報告 = ITE technical report, 40, 6, 303, 307, Feb. 2016
映像情報メディア学会, Japanese

C-3-34 Complex-amplitude Control Using Lens-less Type Spatial Cross Modulation Method
Kan Yuta, Okamoto Atsushi, Shibukawa Atsushi, Funakoshi Hisatoshi, Sakuma Hiroki, Tomita Akihisa, Proceedings of the Society Conference of IEICE, 2015, 1, 108, 108, 25 Aug. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-33 Reduction of Modal Crosstalk using Random Diffuser for Spatial Mode Conversion with Spatial Light Modulator
Ishii Koki, Okamoto Atsushi, Tsuritani Takehiro, Wakayama Yuta, Goto Yuta, Tomita Akihisa, Proceedings of the Society Conference of IEICE, 2015, 2, 197, 197, 25 Aug. 2015
The Institute of Electronics, Information and Communication Engineers, Japanese

EXPERIMENT ON THREE-DIMENSIONAL DISPLAY USING SPATIAL CROSS MODULATION METHOD WITH AN OPTICAL RANDOM DIFFUSER
H. Sakuma, A. Okamoto, A. Shibukawa, H. Funakoshi, Y. Goto, Y. Kan, A. Tomita, 2015 20TH MICROOPTICS CONFERENCE (MOC), 9386, 2015
We performed an experiment on a three-dimensional ( 3D) display using the spatial cross modulation (SCM) method with a single phase-only spatial light modulator and an optical random diffuser. The experimental result revealed the SCM has great potential to realize the 3D display with unprecedented high resolution and light utilization efficiency., IEEE, English

Dynamic wavefront compensation of signal beam in optical satellite communication system using phase conjugation technique
NISHIMAKI Kaori, OKAMOTO Atsushi, SHIBUKAWA Atsushi, TOMITA Akihisa, TAKAYAMA Yoshihisa, IEICE technical report. Circuits and systems, 114, 122, 87, 92, 09 Jul. 2014
We propose an optical satellite communication system which can dynamically compensate the signal beam wavefront using phase conjugation technique. The proposed system converts the signal beam wavefront into the phase conjugate wavefront using the double phase conjugate mirror formed by the photorefractive materials and communication beams. By using the time reversal wave and wavefront compensation effect produced by the phase conjugate wavefront, the automatic tracking of satellites and the background light reduction can be achieved at the same time. In addition, we consider the application of proposed method for the visible light communication on the ground., The Institute of Electronics, Information and Communication Engineers, English

B-10-58 Conversion and Extraction of Spatial Modes in a Multi-mode Fiber using Progressive Phase Conjugation Technique
Hirasaki Yuki, Okamoto Atsushi, Tomita Akihisa, Maeda Tomohiro, Proceedings of the IEICE General Conference, 2014, 2, 426, 426, 04 Mar. 2014
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-61 Reconfigurable Spatial Mode Conversion using a Spatial Light Modulator
Okamoto Atsushi, Hirasaki Yuki, Maeda Tomohiro, Zhao Yanfeng, Tomita Akihisa, Proceedings of the IEICE General Conference, 2014, 2, 429, 429, 04 Mar. 2014
The Institute of Electronics, Information and Communication Engineers, English

Recording and Reconstruction of 38-Level Spatial Quadrature-Amplitude Modulation Signal in Double-Referential Holographic Memory
ZUKERAN Keisuke, OKAMOTO Atsushi, TAKABAYASHI Masanori, SHIBUKAWA Atsushi, TOMITA Akihisa, ITE Technical Report, 37, 10, 1, 7, 15 Feb. 2013
We propose a double-referential holographic memory (DRHM) that can detect phase modulation signal by just entering reference beam into recording medium without additional system for generation and irradiation of the beam to interfere with signal beam. In this technique, by highly-accurate and simplified complex amplitude detection, high multi-level phase modulation signal and spatial quadrature-amplitude modulation (SQAM) signal become available, so it is expected that storage capacity and data transfer rate are greatly increased. In this paper, we describe the basic operation of the DRHM and evaluate multiple recording/reconstruction characteristics by numerical simulation. Besides, the method for furthermore improving reconstruction quality is newly proposed and its effectiveness is demonstrated., The Institute of Image Information and Television Engineers, Japanese

Improvement of Success Probability by Repeated Measurements in Entanglement Generation by Communication using Squeezed Light
松岡 史晃, 富田 章久, 岡本 淳, 量子情報技術研究会資料, 27, 73, 78, 27 Nov. 2012
量子情報技術時限研究専門委員会, Japanese

A Protocol of the Quantum Relay Using Quantum Group Secret Sharing and Distributed Computing
鈴木 貴士, 富田 章久, 岡本 淳, 量子情報技術研究会資料, 27, 16, 18, 27 Nov. 2012
量子情報技術時限研究専門委員会, English

2重位相共役鏡を用いた光衛星間通信システムにおける2値位相板による背景光低減手法
西牧可織, 岡本淳, 富田章久, 若山雄太, 高山佳久, 電気・情報関係学会北海道支部連合大会講演論文集(CD-ROM), 2012, ROMBUNNO.24, 20 Oct. 2012
Japanese

B-10-88 Stabilization for Long-term Operation of High-speed Wavelength-division Multiplexing Quantum Key Distribution System
Yoshino Ken-ichiro, Fujiwara Mikio, Tomita Akihisa, Sasaki Masahide, Tajima Akio, Proceedings of the Society Conference of IEICE, 2012, 2, 270, 270, 28 Aug. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

A-7-12 Quantum relay method using group secret sharing with quantum states
Suzuki Takashi, Tomita Akihisa, Okamoto Atushi, Proceedings of the IEICE General Conference, 2012, 180, 180, 06 Mar. 2012
The Institute of Electronics, Information and Communication Engineers, Japanese

A field test of high-speed wavelength-division multiplexing quantum key distribution system
YOSHINO Ken-ichiro, FUJIWARA Mikio, TANAKA Akihiro, TAKAHASHI Seigo, NAMBU Yoshihiro, TOMITA Akihisa, MIKI Shigehito, YAMASHITA Taro, WANG Zhen, SASAKI Masahide, TAJIMA Akio, Technical report of IEICE. OCS, 111, 411, 35, 38, 26 Jan. 2012
A high-speed quantum key distribution system was developed with the wavelength-division multiplexing (WDM) technique and dedicated key distillation hardware engines. Two interferometers for encoding and decoding are shared over eight wavelengths to reduce the system's size, cost, and control complexity. The key distillation engines can process a huge amount of data from the WDM channels by using a 1-Mbit block in real time. We demonstrated a 3-channel WDM system that simultaneously uses avalanche photodiodes and superconducting single photon detectors. We achieved 12-hour continuous key generation with a secure key rate of 208 kbps through a 45-km field fiber with 14.5-dB loss., The Institute of Electronics, Information and Communication Engineers, Japanese

Acquisition and tracking in inter-satellite communication with dynamically reconfigurable optical device
NISHIMAKI Kaori, OKAMOTO Atsushi, WAKAYAMA Yuta, TOMITA Akihisa, TAKAYAMA Yoshihisa, 電子情報通信学会技術研究報告. SAT, 衛星通信, 111, 336, 137, 140, 05 Dec. 2011
We propose an innovative acquisition and tracking system using a dynamically reconfigurable optical device for optical inter-satellite communication without growing in size and complications due to directivity control of laser beams. The proposed system provides relaxation of acquisition accuracy and suppression of beam divergence by utilization of phase conjugation to form an inter-satellite communication link. This paper reports on the temporal and spatial characteristics of our system for various incident beam angles to consider relaxation of acquisition accuracy and tracking speed., The Institute of Electronics, Information and Communication Engineers, English

B-10-34 Holographic wavelength filter with ultra-broad tuning range for reconfigurable optical add-drop multiplexer
Wakayama Yuta, Okamoto Atsushi, Tomita Akihisa, Proceedings of the Society Conference of IEICE, 2011, 2, 247, 247, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-35 Recording and Reconstruction of LP Modes in Volume Holographic Mode Multiplexer
Aoki Kohei, Okamoto Atsushi, Tomita Akihisa, Honma Satoshi, Proceedings of the Society Conference of IEICE, 2011, 2, 248, 248, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-36 Experiment on LP Modes Excitation with Multi-mode Fiber for Mode Division Multiplex Communication
Soma Daiki, Okamoto Atsushi, Oda Takuya, Tomita Akihisa, Sato Kunihiro, Proceedings of the Society Conference of IEICE, 2011, 2, 249, 249, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

BCI-1-7 Spatial mode excitation and separation for mode division multiplexing technology
Okamoto Atsushi, Soma Daiki, Oda Takuya, Tomita Akihisa, Wakayama Yuta, Proceedings of the Society Conference of IEICE, 2011, 1, "SS, 39"-"SS-40", 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-29 A High Speed Quantum Key Distribution System based on Colorless Interferometric Technique and Hardware Key Distillation
Tanaka Akihiro, Fujiwara Mikio, Yoshino Kenichiro, Takahasi Seigo, Nambu Yoshihiro, Tomita Akihisa, Miki Shigehito, Yamashita Taro, Wang Zhen, Sasaki Masahide, Tajima Akio, Proceedings of the Society Conference of IEICE, 2011, 2, 242, 242, 30 Aug. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

Analysis of LP Mode Separation on All-optical Demultiplexer with Dynamic Multiple Holograms for Mode Division Multiplexing Communication
ODA Takuya, OKAMOTO Atsushi, SOMA Daiki, TOMITA Akihisa, IEICE technical report, 111, 183, 75, 80, 18 Aug. 2011
This study aims at the realization of mode division multiplexing (MDM) communication using multi-mode fiber by all-optical mode demultiplexer consisted of the dynamic multiple holograms. In this paper, we consider the separation of three multiplexed MDM signals, and investigate a mode separation characteristic for six combinations composed of three LP modes. In addition, to improve the separation performance of the all-optical mode demultiplexer, we clarify the dependence of the mode separation on the intensity ratio by varying of the two light intensities used in the hologram recording. Moreover, we show that the signal-to-noise ratio is greatly improved by choosing the combination of LP modes and intensity ratio adequately., The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-58 Experiment on Separating of LP Modes with Dynamic Multiplex Holographic Element for Mode Division Multiplex Communication
Soma Daiki, Okamoto Atsushi, Oda Takuya, Tomita Akihisa, Proceedings of the IEICE General Conference, 2011, 2, 367, 367, 28 Feb. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-69 Enhancement of quantum-key distribution speed using a key distillation engine
Tanaka Akihiro, Fujiwara Mikio, Tomita Akihisa, Tajima Akio, Proceedings of the IEICE General Conference, 2011, 2, 378, 378, 28 Feb. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-98 Ultra-broad tuning range of reconfigurable optical add-drop multiplexer using dynamic holographic diffraction grating
Wakayama Yuta, Okamoto Atsushi, Tomita Akihisa, Proceedings of the IEICE General Conference, 2011, 2, 407, 407, 28 Feb. 2011
The Institute of Electronics, Information and Communication Engineers, Japanese

Secure Photonic Networks : Potential Attacks and Security Enhancement
KITAYAMA Ken-ichi, SASAKI Masahide, ARAKI Soichiro, TSUBOKAWA Makoto, TOMITA Akihisa, INOUE Kyo, HARASAWA Katsuyoshi, NAGASAKO Yuki, TAKADA Atsushi, 電子情報通信学会技術研究報告. PN, フォトニックネットワーク, 110, 431, 65, 68, 21 Feb. 2011
We address emerging threats to the security of photonic networks as these networks become heterogeneous being opened to the upper layers, other operators, and end users. We review the potential threats, mainly loss of the confidentiality of user data transmitted through optical fibers and disturbances of network control, both of which could seriously damage the entire network. We then propose a novel conceptual model of a secure photonic network by introducing a quantum key distribution (QKD) network to its legacy_structure. Secure keys generated by the QKD network are managed by key management agents (KMAs) and used to encrypt not only user data but also control signals. The KMAs cooperate with the Generalized Multi-Protocol Label Switching (GMPLS) controller for secure path provisioning and drive photonic and modern crypto engines in appropriate combinations. Finally, we present a roadmap of a deployment scenario, starting from niche applications such as mission critical and business applications and the next. Digital cinema distribution through a photonic network is presented as an example of a niche application., The Institute of Electronics, Information and Communication Engineers, Japanese

Development of a key distillation hardware engine for high speed quantum-key distribution
TANAKA Akihiro, FUJIWARA Mikio, TOMITA Akihisa, TAJIMA Akio, IEICE technical report, 110, 392, 25, 30, 20 Jan. 2011
Not only the improvement of single photon transmission performance, but also the improvement of the subsequent signal processing comes to be a key challenge in implementation of quantum key distribution system. In particular, speeding up the signal processing and enhancement of the code length are indispensable. We have developed dedicated hardware engine, key distillation engine, to overcome the challenge. In this paper, we report performance of the key distillation board., The Institute of Electronics, Information and Communication Engineers, Japanese

BI-1-11 Hybrid entanglement as a link between free-space and fiber communication
Yoshino Ken-ichiro, Fujiwara Mikio, Toyoshima Morio, Sasaki Masahide, Nambu Yoshihiro, Tomita Akihisa, Proceedings of the IEICE General Conference, 2010, 1, "SS, 93"-"SS-94", 02 Mar. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

CI-1-6 Performance of hybrid entanglement photon pair source for quantum key distribution
藤原 幹生, 吉野 健一郎, 豊嶋 守生, 南部 芳弘, 佐々木 雅英, 富田 章久, Proceedings of the IEICE General Conference, 2010, 1, "SS, 10"-"SS-11", 02 Mar. 2010
The Institute of Electronics, Information and Communication Engineers, Japanese

23pTB-2 POVM for practical single-photon avalanche photodiode
Tsujino Kenji, Tomita Akihisa, Meeting abstracts of the Physical Society of Japan, 65, 1, 215, 215, 01 Mar. 2010
The Physical Society of Japan (JPS), Japanese

Large Capacity Quantum Key Distribution Systems by use of Wavelength Division Multiplexing with a low-cost and easily-controlled configuration : Colourless Interferometric Technique for sharing interferometers over multi-channels
TANAKA Akihiro, TOMITA Akihisa, TAJIMA Akio, IEICE technical report, 109, 400, 61, 66, 21 Jan. 2010
1 Mbps final key generation rate after 50 km transmission requires to multiplex more than 8 quantum channels because of operating frequency limit on photon detectors. A novel interferometric technique is proposed to achieve the multiplexing using wavelength division multiplexing. With additional phase modulation, we exhibited valid interference at any wavelengths while sharing interferometers over multi-channels, resulting in a low-cost and easily-controllable configuration., The Institute of Electronics, Information and Communication Engineers, Japanese

Single Photon Quantum Key Distribution System
TOMITA Akihisa, Japanese journal of optics, 39, 1, 10, 16, 10 Jan. 2010
応用物理学会分科会日本光学会, Japanese

Improvement of Diffraction Efficiency for All-Optical Tunable Wavelength Filter by Apodized Photorefractive Grating
Y. Wakayama, A. Okamoto, A. Tomita, H. Nihei, S. Honma, K. Suzuki, 2010 15TH OPTOELECTRONICS AND COMMUNICATIONS CONFERENCE (OECC), 692, +, 2010
A great improvement of diffraction efficiency in all-optical tunable holographic wavelength filters is shown by activating four corners of the material with photorefractive apodization method by optimizing the intensity ratio of the writing beams., IEICE-INST ELECTRONICS INFORMATION & COMMUNICATION ENGINEERS, English

Experiment on Dynamic Reconfigurable Diffraction Element for Mode Division Multiplex Communication
A. Okamoto, D. Soma, Y. Wakayama, A. Tomita, K. Sato, A. A. Grabar, 2010 15TH OPTOELECTRONICS AND COMMUNICATIONS CONFERENCE (OECC), 302, +, 2010
Light speed mode de-multiplexing is demonstrated by 3-dimentional phase matching for multiple spatial modes through holographic multiplexing technique. Separating of around 60-70% was achieved in experiments of separating multiplexed three modes by dynamic reconfigurable diffraction., IEICE-INST ELECTRONICS INFORMATION & COMMUNICATION ENGINEERS, English

量子光学がおもしろい 量子もつれ光子対の発生=パラメトリックダウンコンバージョン光子対発生の原理と実際=
富田章久, 光アライアンス, 20, 10, 5-9, 29, 01 Oct. 2009
日本工業出版, Japanese, Introduction commerce magazine

Issues for the implementation of quantum key distribution : Required performance for the signal processing
TANAKA Akihiro, TAJIMA Akio, TOMITA Akihisa, IEICE technical report, 109, 212, 53, 60, 22 Sep. 2009
Not only the improvement of single photon transmission performance, but also the improvement of the subsequent signal processing comes to be a key problem in implementation of quantum key distribution system. In particular, speeding up the signal processing and enhancement of the code length are indispensable. In this paper, we review the required performance on the signal processing., The Institute of Electronics, Information and Communication Engineers, Japanese

C-4-12 Modeling afterpulse in gated mode avalanche photodiodes
Tomita Akihisa, Nakata Takeshi, Mizuki Emiko, Takahashi Seigo, Makita Kikuo, Proceedings of the Society Conference of IEICE, 2008, 1, 218, 218, 02 Sep. 2008
The Institute of Electronics, Information and Communication Engineers, Japanese

Fast Quantum Cryptography System Using Single Photon Communication
TOMITA AKIHISA, TAJIMA AKIO, レーザー研究, 36, 8, 487-492, 492, 15 Aug. 2008
レ-ザ-学会, Japanese

Fast quantum cryptography system using single photon communication
TAJIMA Akio, TANAKA Akihiro, YOSHINO Ken-ichiro, MAEDA Wakako, NAMBU Yoshihiro, TAKAHASHI Seigo, TOMITA Akihisa, IEICE technical report, 108, 83, 51, 56, 12 Jun. 2008
Quantum key distribution (QKD) systems based on single photon transmission can generate unconditionally secure common key between remote users. Improvement of QKD performance, particularly on key generation rate, has been required to meet current network traffic. High key generation rate is realized by increasing photon detection rate and by reducing error rate. Therefore, a high-speed QKD system should be equipped with low loss receivers with high visibility, highly efficient photon detectors with small dark count probability, and a stable clock synchronization system with low stray light to the quantum signals. A solution for these issues are given by employing planar lightwave circuit interferometers, single photon detection circuits and modules, and clock synchronization based on wavelength division multiplexing (WDM) technique., The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-38 High-efficiency single photon detector using a self-training discriminator for high-speed quantum cryptosystems
Takahashi Seigo, Tajima Akio, Tomita Akihisa, Proceedings of the IEICE General Conference, 2008, 2, 321, 321, 05 Mar. 2008
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-40 Experimental quantification of the Faked State Attack against a quantum cryptosystem
Eguchi Akihiro, Tanaka Akihiro, Takahashi Seigo, Tomita Akihisa, Maeda Wakako, Hamamoto Kiichi, Tajima Akio, Proceedings of the IEICE General Conference, 2008, 2, 323, 323, 05 Mar. 2008
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-39 97-km Quantum Key Distribution Field Trial
Maeda Wakako, Fujiwara Mikio, Sae Woo Nam, Nambu Yoshihiro, Takahashi Seigo, Tanaka Akihiro, Yoshino Ken-ichiro, Miki Shigehito, Baek Burm, Wang Zhen, Tajima Akio, Tomita Akihisa, Sasaki Masahide, Proceedings of the IEICE General Conference, 2008, 2, 322, 322, 05 Mar. 2008
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-27 BB84 Quantum Cryptography encoder with the flexibility to realize all XYZ basis using 2×2 interferometer and dual-drive intensity modulator
Tanaka Akihiro, Yoshino Ken'ichiro, Nambu Yoshihiro, Tajima Akio, Tomita Akihisa, Proceedings of the Society Conference of IEICE, 2007, 2, 221, 221, 29 Aug. 2007
The Institute of Electronics, Information and Communication Engineers, Japanese

24aRG-13 Quantum nondemolition measurement of photon-arrival for Bell state measurement
Kojima Kunihiro, Tomita Akihisa, Meeting abstracts of the Physical Society of Japan, 62, 2, 197, 197, 21 Aug. 2007
The Physical Society of Japan (JPS), Japanese

基礎からの量子光学 第20回 量子暗号鍵配布の安全性理論
富田章久, Optronics, 26, 308, 147-155, 155, 10 Aug. 2007
オプトロニクス社, Japanese

量子暗号鍵配布システムの実際
富田章久, 富田章久, 電子情報通信学会論文誌 A, J90-A, 5, 358-366, 366, 01 May 2007
量子暗号鍵配布(QKD)は無条件安全な暗号鍵を共有する技術である.QKDの実用化に向けて実環境での安定動作と安全性の保証が課題となる.BB84プロトコルを用いたQKD装置について,鍵生成の原理・実装法を解説した後,気温の変動や機械的振動を伴う環境において安定な動作を行うための干渉計の温度無依存化,量子光信号の同期化技術,デコイ法による,半導体レーザ光源を用いた現実的な設定における安全性保証付きの鍵生成について最近の成果を報告する., The Institute of Electronics, Information and Communication Engineers, Japanese

Quantum key distribution with guaranteed security by weak coherent light
HAYASHI Masahito, TOMITA Akihisa, HASEGAWA Jun, HIROSHIMA Tohya, IEICE technical report, 106, 595, 143, 148, 09 Mar. 2007
Security formulas of quantum key distribution (QKD) with imperfect resources are obtained for finite-length code when the decoy method is applied. This analysis is useful for guaranteeing the security for the implemented QKD system. Our formulas take account into the effect of the vacuum state and dark counts in the detector., The Institute of Electronics, Information and Communication Engineers, English

CS-7-2 Single Photon Detector for Quantum Cryptosystem
Tajima Akio, Takahashi Seigo, Tomita Akihisa, Proceedings of the IEICE General Conference, 2007, 2, "S, 17"-"S-18", 07 Mar. 2007
The Institute of Electronics, Information and Communication Engineers, Japanese

CS-7-4 A Single Photon Detection Circuit for a High-speed Quantum Cryptosystem
Takahashi Seigo, Tajima Akio, Tomita Akihisa, Proceedings of the IEICE General Conference, 2007, 2, "S, 21"-"S-22", 07 Mar. 2007
The Institute of Electronics, Information and Communication Engineers, Japanese

19pXG-2 Monogamy Inequality for Distributed Gaussian Entanglement
Hayashi M., Tomita A., Hiroshima T., Hasegawa J., Meeting abstracts of the Physical Society of Japan, 62, 1, 144, 144, 28 Feb. 2007
The Physical Society of Japan (JPS), Japanese

21pXK-9 Experimental realization of quantum leader election on linear optics
Okubo Y., Wang X.B., Tomita A., Meeting abstracts of the Physical Society of Japan, 62, 1, 177, 177, 28 Feb. 2007
The Physical Society of Japan (JPS), Japanese

Entanglement photon, generation and application to the optics. The generation of the entanglement photon by the parametric down conversion.
TOMITA AKIHISA, TOMITA AKIHISA, 光技術コンタクト, 44, 11, 621-629, 654, 20 Nov. 2006
日本オプトメカトロニクス協会, Japanese

B-10-18 Realizing Decoy State on a High-Speed Quantum Cryptosystem
Tanaka Akihiro, Tomita Akihisa, Maeda Wakako, Takahashi Seigo, Tajima Akio, Proceedings of the Society Conference of IEICE, 2006, 2, 212, 212, 07 Sep. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-44 An Estimation Method of Noise Carrier Incidence Rates of an APD for a Quantum Cryptosystem
Takahashi Seigo, Nakata Takeshi, Tajima Akio, Tomita Akihisa, Proceedings of the IEICE General Conference, 2006, 2, 364, 364, 08 Mar. 2006
The Institute of Electronics, Information and Communication Engineers, Japanese

Quantum information device and system technology in NEC : Innovations of information technology with using quantum mechanics
YOROZU Shinichi, NAKAMURA Yasunobu, TOMITA Akihisa, IEICE technical report, 105, 575, 31, 35, 20 Jan. 2006
Quantum information technology is attracting great expectations and attentions for its innovative operating principles. In this paper, we report on a recent progress of quantum computing technology and quantum cryptography technology in NEC Corporation., The Institute of Electronics, Information and Communication Engineers, Japanese

A-6-13 Security of modified BB84 quantum cryptography without basis sift
Tomita Akihisa, Proceedings of the Society Conference of IEICE, 2005, 165, 165, 07 Sep. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

C-4-27 Compact, Low-power-consumption Dual-APD Photon Receiver Modules for Quantum Cryptography
Takeuchi Takeshi, Takahashi Seigo, Tomita Akihisa, Tajima Akio, Proceedings of the Society Conference of IEICE, 2005, 1, 308, 308, 07 Sep. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

Hypothesis testing for an entangled state produced by spontaneous parametric down conversion (Statistical Inference of Records and Related Statistics)
Tsuda Yoshiyuki, Shi Bao-Sen, Tomita Akihisa, Hayashi Masahito, Matsumoto Keiji, Jiang Yun-Kun, RIMS Kokyuroku, 1439, 221, 224, Jul. 2005
Kyoto University, Japanese

A minimized Photon Receiver for a high-speed quantum cryptosystem
TAKAHASHI Seigo, TANAKA Akihiro, MAEDA Wakako, TAJIMA Akio, TAKEUCHI Takeshi, TOMITA Akihisa, Technical report of IEICE. LQE, 105, 143, 61, 66, 17 Jun. 2005
A practical quantum cryptosystem which security is guaranteed by quantum physics is strongly required. A high-speed and small size single photon detector for a quantum key distribution (QKD), which is one of the kernel technologies of quantum cryptosystem, is developed. As the quantum cryptosystem is a part of backbone communication equipment, the reliability and packaging into a small form factor cabinet is indispensable. The long time operation stability and enough low QBER of the developed single photon detector is confirmed through evaluations., The Institute of Electronics, Information and Communication Engineers, Japanese

A High-Speed Quantum Cryptosystem
Tajima Akio, Tanaka Akihiro, Maeda Wakako, Takahashi Seigo, Takeuchi Takeshi, Tomita Akihisa, Nambu Yoshihiro, Technical report of IEICE. OCS, 105, 76, 7, 11, 16 May 2005
Quantum cryptosystems, which security is guaranteed by quantum physics, are promising candidate for next generation impregnable optical networks. However, there are problems to realize practical quantum cryptosystems. Temperature independence, high-speed operation, and long-time stable operation should be established for practical use. We developed key technologies to solve these problems, and have realized a high-speed quantum cryptosystem in a compact PCI base chassis., The Institute of Electronics, Information and Communication Engineers, Japanese

Photodetector Sensitive to the Vacuum Fluctuations-Quantum Counter-
USAMI KOJI, NAMBU YOSHIHIRO, TOMITA AKIHISA, NAKAMURA KAZUO, 日本物理学会誌, 60, 5, 363, 367, 05 May 2005
When the electromagnetic field is detected by stimulated emission, rather than by absorption, antinormally ordered photodetection can be realized. One of the distinct features of this photodetection scheme, i.e., a quantum counter, is its sensitivity to vacuum fluctuations. We have recently succeeded in experimentally demonstrating the quantum counter by exploiting stimulated parametric down-conversion. Here, we present some interesting physics concerning vacuum fluctuations and quantum measurement processes through our experiment on the quantum counter., The Physical Society of Japan (JPS), Japanese

B-10-62 A High-speed Quantum Cryptosystem (4) : Miniaturized Photon Receiver
Takahashi Seigo, Takeuchi Takeshi, Tanaka Akihiro, Maeda Wakako, Tajima Akio, Tomita Akihisa, Proceedings of the IEICE General Conference, 2005, 2, 352, 352, 07 Mar. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

B-10-59 A High-Speed Quantum Cryptosystem (1) : Architecture
Tajima Akio, Tanaka Akihiro, Maeda Wakako, Takahashi Seigo, Takeuchi Takeshi, Tomita Akihisa, Nambu Yoshihiro, Proceedings of the IEICE General Conference, 2005, 2, 349, 349, 07 Mar. 2005
The Institute of Electronics, Information and Communication Engineers, Japanese

量子アルゴリズムの新地平 半古典的量子フーリエ変換 ショアのアルゴリズムの後半部を実現
富田章久, 数理科学, 42, 6, 20-24, 24, 01 Jun. 2004
サイエンス社, Japanese

Generatioin of Entangled Photon Pairs
TOMITA AKIHISA, 光学, 33, 5, 264-270, 270, 10 May 2004
応用物理学会分科会日本光学会, Japanese

Optical Fiber Transmission over 100km in Quantum Cryptography System with High Sensitivity Photon Detectors
TOMITA AKIHISA, NAMBU YOSHIHIRO, NAKAMURA KAZUO, NEC技報, 56, 12, 54-57, 57, 25 Dec. 2003
日本電気, Japanese

Quantum Information Processing and New-Type Moore's Law. Speedup by Parallelism via Quantum Bits.
IMAI HIROSHI, MATSUMOTO KEIJI, TOMITA AKIHISA, 情報処理, 43, 11, 1204-1209, 1209, 15 Nov. 2002
量子コンピュータは，暗号セキュリティの諸問題をきわめて効率的に解けることが理論的に示されたことによって，次世代計算モデルとして期待されている．これらは，量子力学原理を情報処理に活用するものであるが，量子力学というところがとっつきにくかったりする．本稿では，そのようなある種の壁を乗り越えて，量子コンピュータ研究が今のコンピュータ科学の延長線上にもあることを簡明に解説することを試みる．, Information Processing Society of Japan (IPSJ), Japanese

計算機科学の新展開 超並列・量子情報・分子計算・計算科学 大規模量子計算の可能性
松本啓史, 富田章久, 今井浩, Comput Today, 19, 5, 11-16, 10, 01 Sep. 2002
サイエンス社, Japanese

Practical Quantum Cryptography
NAMBU Yoshihiro, TOMITA Akihisa, The Journal of the Institute of Electronics,Information and Communication Engineers, 85, 8, 606, 612, 01 Aug. 2002
量子暗号技術は,従来通信システムにおいて根本的な解決が難しいセキュリティ問題を解決する可能性を秘め,近年多くの量子科学及び情報科学研究者の関心を引いている.中でも光を用いた量子暗号システムは,早期の実用化が期待されている.本稿では現状光通信技術の応用により実現可能な,実用的光量子暗号システムの研究動向を,その実装技術的側面に重きを置いて紹介する., The Institute of Electronics, Information and Communication Engineers, Japanese

Reply to "Comment on: Teleportation of an unknown state by W state" - [Phys. Lett. A 300 (2002) 324]
BS Shi, A Tomita, PHYSICS LETTERS A, 300, 4-5, 538, 539, Aug. 2002
ELSEVIER SCIENCE BV, English, Others

Quantum Information Sciences: New Paradigm for Information Processing. Practical Quantum Cryptography.
NAMBU YOSHIHIRO, TOMITA AKIHISA, 電子情報通信学会誌, 85, 8, 606-612, 01 Aug. 2002
Japanese

量子暗号
富田 章久, オペレーションズ・リサーチ : 経営の科学 = [O]perations research as a management science [r]esearch, 47, 5, 322, 327, 01 May 2002
公益社団法人日本オペレーションズ・リサーチ学会, Japanese

量子情報処理パラダイム 2 量子暗号
富田章久, オペレーションズ・リサーチ, 47, 5, 322-327, 528, 01 May 2002
Japanese

Experimental investigation of pulsed entangled photons and photonic quantum channels
Y Nambu, K Usami, A Tomita, S Ishizaka, T Hiroshima, Y Tsuda, K Matsumoto, K Nakamura, QUANTUM OPTICS IN COMPUTING AND COMMUNICATIONS, 4917, 13, 24, 2002
The development of key devices and systems in quantum information technology, such as entangled,particle sources, quantum gates and quantum cryptographic systems, requires a reliable and well-established method for characterizing how well the devices or systems work. We report our recent work on experimental characterization of pulsed entangled photonic states and photonic quantum channels, using the methods of state and process tomography. By using state tomography, we could reliably evaluate the states generated from a two-photon source under development and develop a highly entangled pulsed photon source. We are also devoted to characterization of single-qubit and two-qubit photonic quantum channels. Characterization of typical single-qubit decoherence channels has been demonstrated using process tomography. Characterization of two-qubit channels, such as classically correlated channels and quantum mechanically correlated channels is under investigation. These characterization techniques for quantum states and quantum processes will be useful for developing photonic quantum devices and for improving their performances., SPIE-INT SOC OPTICAL ENGINEERING, English, Summary international conference

量子コンピュータと量子情報技術の未来 研究の最前線から 光による量子ゲートの実現について
富田章久, Comput Today, 18, 5, 17-23, 41, 01 Sep. 2001
サイエンス社, Japanese

Experimental evaluation of entanglement in polarization-entangled photon pair using quantum tomography
Usami K., Nambu Y., Ishizaka S., Hiroshima T., Tomita A., Nakamura K., Meeting abstracts of the Physical Society of Japan, 56, 1, 110, 110, 09 Mar. 2001
The Physical Society of Japan (JPS), Japanese

光による量子ゲートの実現について
富田章久, Computer Today, 105, 17, 23, 2001

Complete Bell State measurement with controlled absorption
Tomita Akihisa, Meeting abstracts of the Physical Society of Japan, 55, 2, 99, 99, 10 Sep. 2000
The Physical Society of Japan (JPS), Japanese

Principles and Practical methods on quantum cryptography
Tomita Akihisa, Proceedings of the Society Conference of IEICE, 2000, 374, 375, 07 Sep. 2000
The Institute of Electronics, Information and Communication Engineers, Japanese

Security analysis and experiment on the noise based cryptography
Tomita Akihisa, Hirota Osamu, Proceedings of the Society Conference of IEICE, 2000, 169, 169, 07 Sep. 2000
The Institute of Electronics, Information and Communication Engineers, Japanese

The present status and problems on quantum cryptography
Tomita Akihisa, Nambu Yoshihiro, Nakamura Kazuo, Proceedings of the Society Conference of IEICE, 2000, 353, 354, 07 Sep. 2000
The Institute of Electronics, Information and Communication Engineers, Japanese

25pA-10 Quantum cryptographic apparatus with self-checking device inside
Mayers Dominic, Kohno Yoshie, Nambu Yoshihiro, Tomita Akihisa, Meeting abstracts of the Physical Society of Japan, 54, 2, 112, 112, 03 Sep. 1999
The Physical Society of Japan (JPS), Japanese

28a-ZE-6 Self-collimating phenomena in photonic crystals
Kosaka H., Tomita A., Kawashima T., Sato T., Kawakami S., Notomi M., Tamamura T., Meeting abstracts of the Physical Society of Japan, 54, 1, 177, 177, 15 Mar. 1999
The Physical Society of Japan (JPS), Japanese

Photonic crystals toward PLC applications-μLC
Kosaka H., Kawashima T., Tomita A., Notomi M., Tamamura T., Sato T., Kawakami S., Proceedings of the IEICE General Conference, 1999, 1, 267, 267, 08 Mar. 1999
The Institute of Electronics, Information and Communication Engineers, Japanese

Super-prism phenomena in photonic crystals
Kosaka H., Tomita A., Kawashima T., Sato T., Kawakami S., Notomi M., Tamamura T., Proceedings of the Society Conference of IEICE, 1998, 1, 420, 421, 07 Sep. 1998
The Institute of Electronics, Information and Communication Engineers, Japanese

Anomalous bending phenomena in 3D photonic crystals
KOSAKA H., TOMITA A., KAWASHIMA T., SATO T., KAWAKAMI S., NOTOMI M., TAMAMURA T., Meeting abstracts of the Physical Society of Japan, 53, 2, 263, 263, 05 Sep. 1998
The Physical Society of Japan (JPS), Japanese

Optical frequency difference measurement by fourth order interference
Tomita Akihisa, Proceedings of the IEICE General Conference, 1998, 1, 419, 419, 06 Mar. 1998
The Institute of Electronics, Information and Communication Engineers, Japanese

27p-ZL-9 Intraband relaxation of real photo-excited carriers and its influence on the nonlinear optical spectra
NAMBU Y, TOMITA A, 秋の分科会講演予稿集, 1992, 2, 422, 422, 14 Sep. 1992
The Physical Society of Japan (JPS), Japanese

MQW etalon spatial light modulators.
TOMITA AKIHISA, KOGA YUJI, SUZUKI AKIRA, 光学, 18, 12, 689-690, Dec. 1989
Japanese

量子コンピュータ/イジング型コンピュータ研究開発最前線　　　　　　　　　　　　　　　
富田章久, 福井浩介, 第2章第6節
情報機構, 25 Feb. 2019, [Contributor]

量子情報工学　　　　　　　　　　　　　　　
富田 章久
森北出版, Feb. 2017, [Single work]

基礎からの量子光学　　　　　　　　　　　　　　　
富田 章久, 量子暗号鍵配布の安全性理論
オプトロニクス社, 2009, [Joint work]

量子情報通信 ―基礎から最前線まで―　　　　　　　　　　　　　　　
田島 章雄, 南部 芳弘, 富田 章久, 量子暗号 メトロネットワーク展開へ向けたシステム安定化・小型化技術
オプトロニクス社, 2006, [Joint work]

Quantum Computation and Information - From Theory to Experiment,; Topics in Applied Physics, Vol. 102　　　　　　　　　　　　　　　
TOMITA Akihisa, Realization of Quantum Information System
Springer Verlag, 2006, [Joint work]

情報セキュリティハンドブック　　　　　　　　　　　　　　　
今井浩, 富田章久, 小林弘志, 第2編第8章 量子暗号
オーム社, 2004, [Joint work]

量子情報理論の基本を学ぶ　　　　　　　　　　　　　　　
富田章久
電子情報通信学会 光通信システム研究会 OCSサマースクール, 22 Jul. 2022, Japanese, Public discourse
[Invited]

Practical Methods for Security Certification of Quantum Key Distribution　　　　　　　　　　　　　　　
Akihisa Tomita
Quantum 2.0 Conference and Exhibition, 14 Jun. 2022, English, Invited oral presentation
[Invited]

量子情報技術の拓く未来へ～量子技術で社会，ビジネスはこう変わる～　　　　　　　　　　　　　　　
富田章久, 村井信哉, 山田秀昭
量子コンピューティングEXPO2022春, 13 May 2022, Japanese, Public discourse
[Invited]

量子の未来をとことん語り合う～日本は量子でGAFAを超える～　　　　　　　　　　　　　　　
伊藤公平, 富田章久, 島田太郎
TOSHIBA OPEN SESSIONS Session 2, 04 Mar. 2022, Japanese, Nominated symposium
[Invited]

量子暗号通信の最前線　　　　　　　　　　　　　　　
富田章久
第12回 光・電波フォーラム, 16 Nov. 2021, Japanese, Public discourse
[Invited]

Improvement of mode compensation accuracy using a random diffuser in progressive phase conjugation,　　　　　　　　　　　　　　　
Zeyu Shen, Shuanglu Zhang, Atsushi Okamoto, Akihisa Tomita
International Symposium on Imaging, Sensing, and Optical Memory, Oct. 2021, English, Oral presentation

Improvement in Diffraction Efficiency of Volume Holographic Mode De-Multiplexer with Dual Wavelength method by Using Thick Medium　　　　　　　　　　　　　　　
Yuya Kuroda, Atsushi Okamoto, Akihisa Tomita, Taketoshi Takahata, Satoshi Shinada, Yuta Goto, Naoya Wada
International Symposium on Imaging, Sensing, and Optical Memory, Oct. 2021, English, Oral presentation

3D super-resolution projection using single-lens spatial cross-modulation method　　　　　　　　　　　　　　　
Xinruinan Zhang, Atsushi Okamoto, Hisatoshi Funakoshi, Akihisa Tomita
International Symposium on Imaging, Sensing, and Optical Memory, Oct. 2021

光量子情報技術の最新動向
富田章久
電子情報通信学会ソサエティ大会, 17 Sep. 2021, Japanese, Nominated symposium
12434369, [Invited]

量子鍵配送の高速化に向けた光検出器の研究開発
富田章久
第82回応用物理学会秋季学術講演会, 10 Sep. 2021, Japanese, Nominated symposium
34192453, [Invited]

量子暗号の基礎と最近の動向
富田章久
第63回 ISSスクエア水平ワークショップ, 21 May 2021, Japanese, Nominated symposium
12434369, [Invited]

仮想位相共役による超解像複素振幅計測における検出器精度依存性評価　　　　　　　　　　　　　　　
今井凌也, 岡本淳, 川島聡, 小川和久, 富田章久
第56 回応用物理学会北海道支部 第17 回日本光学会北海道支部 合同学術講演会, Jan. 2021, Japanese, Oral presentation

モード拡散多重通信における2 光波間の最小識別距離の評価　　　　　　　　　　　　　　　
張帥, 岡本淳, 前田智弘, 沈澤宇, 小川和久, 富田章久
第56 回応用物理学会北海道支部 第17 回日本光学会北海道支部 合同学術講演会, Jan. 2021, Japanese, Oral presentation

空間光変調器を用いた数モードマルチコアファイバのジョイントスイッチの実験　　　　　　　　　　　　　　　
阿部祐太, 岡本 淳, 小川和久, 富田章久, 相馬大樹, 吉兼 昇, 釣谷剛宏
光通信システム研究会 (OCS), Jan. 2021, Japanese, Oral presentation

体積ホログラム分波器における補償波面の解像度に対する特性　　　　　　　　　　　　　　　
阿部祐太, 岡本 淳, 小川和久, 富田章久, 相馬大樹, 吉兼 昇, 釣谷剛宏
光通信システム研究会 (OCS), Jan. 2021, Japanese, Oral presentation

事前・事後選択系における物理量の複素揺らぎの定式化と実験的検証　　　　　　　　　　　　　　　
阿部夏来, 小川和久, 小林弘和, 富田章久
第43回量子情報技術研究会, Dec. 2020, Japanese, Oral presentation

時間領域における光子の超短波動関数の直接測定
小川和久, 岡崎巧実, 小林弘和, 中西俊博, 富田章久
第43回量子情報技術研究会, Dec. 2020, Japanese, Oral presentation
34194389

Quantum random number generator integrated into a transmitter of BB84 QKD systems
W. Zhang, A. Tomita, K. Ogawa, A. Okamoto
20th Asian Quantum Information Science Conference, Dec. 2020, English, Oral presentation
34191611

体積ホログラムにおける媒質の厚さと回折効率との関係性　　　　　　　　　　　　　　　
黒田 裕也, 岡本 淳, 林 穂高, 高畠 武敏, 品田 聡, 後藤 優太, 和田 尚也, 小川 和久, 富田 章久
電気・情報関係学会北海道支部連合大会, 07 Nov. 2020, Japanese, Oral presentation

光量子情報技術－数学を具現化する技術と実装を救う数学－
富田章久
数学と科学、工学の協働に関する連続セミナー, 04 Nov. 2020, Japanese, Public discourse
34191611, [Invited]

体積ホログラム分波器の出力信号の波面補償方法　　　　　　　　　　　　　　　
林 穂高, 岡本 淳, 高畠武敏, 品田 聡, 後藤優太, 和田尚也, 小川和久, 富田章久
光通信システム研究会 (OCS), Nov. 2020, Japanese, Oral presentation

Gaussian distribution in optical tomographic imaging using virtual phase conjugation　　　　　　　　　　　　　　　
Jiang Mengying, Atsushi Okamoto, Kazuhisa Ogawa, Akihisa Tomita
International Symposium on Imaging, Sensing, and Optical Memory, Nov. 2020, English

Numerical simulation of switching of signal path of FM-MCFs by SLM　　　　　　　　　　　　　　　
Yuta Abe, Atsushi Okamoto, Kazuhisa Ogawa, Akihisa Tomita, Daiki Soma, Yuta Wakayama, Takehiro Tsuritani
International Symposium on Imaging, Sensing, and Optical Memory, Nov. 2020, English, Oral presentation

uper-resolution Measurement By Virtual Phase Conjugation Using A Small Number Of Pixels For Optical Detection　　　　　　　　　　　　　　　
Satoshi Kawashima, Atsushi Okamoto, Kazuhisa Ogawa, Akihisa Tomita
International Symposium on Imaging, Sensing, and Optical Memory, Nov. 2020, English, Oral presentation

Mode Compensation using Progressive Phase Conjugation Technique　　　　　　　　　　　　　　　
Zeyu Shen, Atsushi Okamoto, Shuanglu Zhang, Kazuhisa Ogawa, Akihisa Tomita
International Symposium on Imaging, Sensing, and Optical Memory, Nov. 2020, English, Oral presentation

Improving the reconstruction quality of spatial mode exchange technique by adjusting the diffusion angle of diffuser　　　　　　　　　　　　　　　
Shuanglu Zhang, Atsushi Okamoto, Kazuhisa Ogawa, Akihisa Tomita, Taketoshi Takahata, Satoshi Shinada, Yuta Goto, Naoya Wada
International Symposium on Imaging, Sensing, and Optical Memory, Nov. 2020, English, Oral presentation

Activities on Quantum Information Technology in Japan
Akihisa Tomita
Quantum 2020, 20 Oct. 2020, English, Nominated symposium
34191611, [Invited]

Implementation Security Certification of Quantum Key Distribution Devices
Akihisa Tomita
Topical Conference on Quantum Communication and Security, 18 Dec. 2019, English, Invited oral presentation
34191611, [Invited]

Quantum Key Distribution in the Real World　　　　　　　　　　　　　　　
Akihisa Tomita
JSAP-OSA Joint Symposia 2019, 21 Sep. 2019, English, Invited oral presentation
[Invited]

光による量子コンピュータの実現に向けて　　　　　　　　　　　　　　　
富田章久, 福井浩介
電子情報通信学会ソサイエティ大会, 11 Sep. 2019, Japanese, Nominated symposium
[Invited]

A Long-term Secure Data Transmission and Storage Network Based on Quantum Key Distribution
Akihisa Tomita
Advanced Photonics: Congress Signal Processing in Photonic Communications, 31 Jul. 2019, English, Invited oral presentation
11498398, [Invited]

Implementation Security Certification of a Quantum Key Distribution System through Device Characterization　　　　　　　　　　　　　　　
TOMITA Akihisa
Optical Fiber Communications Conference and Exhibition, 06 Mar. 2019, English, Invited oral presentation
[Invited], [International presentation]

Quantum Key Distribution - How do you know it's secure　　　　　　　　　　　　　　　
TOMITA Akihisa
Okinawa School in Physics 2018: Coherent Quantum Dynamics, 01 Oct. 2018, English, Public discourse
[Invited], [International presentation]

High-Threshold Fault-Tolerant Quantum Computation with Analog Quantum Error Correction　　　　　　　　　　　　　　　
Kosuke Fukui, Akihisa Tomita, Atshushi Okamoto, Keisuke Fujii
18th Asian Quantum Information Science Conference, 08 Sep. 2018, English, Oral presentation
[International presentation]

State tomographic characterization of BB84 states generated by a Dual-Parallel Modulator　　　　　　　　　　　　　　　
Weiyang Zhang, Akihisa Tomita, Yu Kadosawa, Kuzihisa Ogawa, Atsushi Okamoto
8th International Conference on Quantum Cryptography, 30 Aug. 2018, English, Poster presentation
[International presentation]

量子暗号鍵配送ー最近の研究開発動向　　　　　　　　　　　　　　　
富田 章久
物性研短期研究会 量子情報・物性の新潮流, 03 Aug. 2018, Japanese, Public discourse
[Invited], [Domestic Conference]

Characterization and Security Certification of a Practical Quantum Key Distribution System　　　　　　　　　　　　　　　
TOMITA Akihisa
Laser Physics Workshop 2018, 17 Jul. 2018, English, Invited oral presentation
[Invited], [International presentation]

State tomographic characterization of BB84 states generated by a Dual-Parallel Modulator　　　　　　　　　　　　　　　
Weiyang Zhang・Akihisa Tomita, Yu Kadosawa, Kazuhisa Ogawa
The 38th meeting of quantum information technology, 05 Jun. 2018, English, Oral presentation
[Domestic Conference]

システムの時間発展によって作られる複素数値のqubitプローブを用いた取得法　　　　　　　　　　　　　　　
小川和久, 安彦 修, 小林弘和, 中西俊博, 岡崎巧実, 富田章久
第38回量子情報技術研究会, 05 Jun. 2018, Japanese, Oral presentation
[Domestic Conference]

BB84 Quantum Key Distribution system with a Dual-Parallel Modulator　　　　　　　　　　　　　　　
張 維楊, 廉澤 悠, 富田 章久, 小川 和久, 岡本 淳
第37回量子情報技術研究会 (QIT37), Nov. 2017, English, Poster presentation
[Domestic Conference]

アナログ情報を活用した耐故障性連続量量子計算　　　　　　　　　　　　　　　
福井 浩介, 富田 章久, 岡本 淳, 藤井 啓祐
第37回量子情報技術研究会 (QIT37), Nov. 2017, Japanese, Oral presentation
[Domestic Conference]

An efficient countermeasure against correlated intensity fluctuations in optical pulses on high-speed decoy BB84 QKD systems　　　　　　　　　　　　　　　
Akihisa Tomita, Ken-Ichiro Yoshino, Mikio Fujiwara, Tatsuya Sumiya, Toshihiko Sasaki, Kensuke Nakata, Akio Tajima, Masato Koashi, Masahiro Takeoka, Masahide Sasaki
7th International Conference on Quantum Cryptography （Qcrypt2017), 21 Sep. 2017, English, Oral presentation
[International presentation]

Analog quantum error correction with encoding a qubit into an oscillator　　　　　　　　　　　　　　　
FUKUI Kosuke, TOMITA Akihisa, OKAMOTO Atsushi
17th Asian Quantum Information Science Conference, 06 Sep. 2017, English, Oral presentation
[International presentation]

Decoy State BBM92 Quantum Key Distribution Protocol with Multi-photon-event Rejectio　　　　　　　　　　　　　　　
MIN Liang, TOMITA Akihisa, OKAMOTO Atsushi
17th Asian Quantum Information Science Conference, 05 Sep. 2017, English, Poster presentation
[International presentation]

Classical observation of a focused beam spot beyond the diffraction limit by using the time-reversal symmetry of quantum mechanics　　　　　　　　　　　　　　　
OGAWA Kazuhisa, KOBAYASHI Hirokazu, TOMITA Akihisa
第36回量子情報技術研究会, 30 May 2017, Japanese, Oral presentation
[Domestic Conference]

Method of reducing the propagation of errors in a controlled-phase gate with the GKP qubit　　　　　　　　　　　　　　　
FUKUI Kosuke, TOMITA Akihisa, OKAMOTO Atsushi
QIT, 29 May 2017, Japanese, Poster presentation
[Domestic Conference]

Topological One-Way Quantum Computation with the GKP Code States using Highly-Reliable Post-Selected Measurement　　　　　　　　　　　　　　　
Kosuke Fukui, Akihisa Tomita, Atsushi Okamoto
The 20th Conference on Quantum Information Processing, 16 Jan. 2017, English, Poster presentation
[International presentation]

連続量状態を用いたTwo-qubit bit flip codeの提案　　　　　　　　　　　　　　　
福井 浩介, 富田 章久, 岡本 淳
第35回量子情報技術研究会, 24 Nov. 2016, Japanese, Poster presentation
[Domestic Conference]

コヒーレント状態の重ね合わせを利用した微小な位相シフト検出　　　　　　　　　　　　　　　
松岡 史晃, 富田 章久, 鹿野 豊
第35回量子情報技術研究会, 24 Nov. 2016, Japanese, Poster presentation
[Domestic Conference]

Practical quantum key distribution　　　　　　　　　　　　　　　
TOMITA Akihisa
4th ETSI/IQC Workshop on Quantum-Safe Cryptography, 20 Sep. 2016, English, Invited oral presentation
[Invited], [International presentation]

Security of the Bennett 1992 Quantum Key Distribution Protocol Estimating Eavesdropper’s Information Without the Bit Error Rate　　　　　　　　　　　　　　　
Toshiyuki Nakamura, Kensuke Nakata, Akihisa Tomita, Kazuhisa Ogawa, Atsushi Okamoto
6th International Conference on Quantum Cryptography, 12 Sep. 2016, English, Poster presentation
[International presentation]

Robust Quantum Key Distribution Systems Using a Dual-Parallel Modulator　　　　　　　　　　　　　　　
Yu Kadosawa, Kensuke Nakata, Akihisa Tomita, Kazuhisa Ogawa, Atsushi Okamoto
6th International Conference on Quantum Cryptography, 12 Sep. 2016, English, Poster presentation
[International presentation]

Quantum Key Distribution Network for Multiple Applications　　　　　　　　　　　　　　　
Akio Tajima, Takashi Kondoh, Takao Ochi, Mikio Fujiwara, Ken-Ichiro Yoshino, Hiromi Iizuka, Toshio Sakamoto, Akihisa Tomita, Ei Shimamura, Shione Asami, Masahide Sasaki
The 16th Asian Quantum Information Science Conference (AQIS2016), 28 Aug. 2016, English, Oral presentation
[Domestic Conference]

An approximated single photon state generation from coherent states entangled with qubits by measuring qubits　　　　　　　　　　　　　　　
Fumiaki Matsuoka, Akihisa Tomita
The 16th Asian Quantum Information Science Conference (AQIS2016), 28 Aug. 2016, English, Poster presentation
[International presentation]

Intensity fluctuation suppression in a decoy-state quantum key distribution transmitter　　　　　　　　　　　　　　　
Kensuke Nakata, Akihisa Tomita, Yu Kadosawa, Kazuhisa Ogawa, Atushi Okamoto
The 16th Asian Quantum Information Science Conference (AQIS2016), 28 Aug. 2016, English, Poster presentation
[International presentation]

Fault-tolerant quantum computation using maximum-likelihood decoder with the GKP code states　　　　　　　　　　　　　　　
Kosuke Fukui, Akihisa Tomita, Atsushi Okamoto
The 16th Asian Quantum Information Science Conference, 28 Aug. 2016, English, Poster presentation
[International presentation]

Noise-tolerant post-selected measurement using noise margin with GKP code state　　　　　　　　　　　　　　　
Kosuke Fukui, Akihisa Tomita, Atsushi Okamoto
The International Conference on Quantum Communication, Measurement and Computing (QCMC), 04 Jul. 2016, English, Poster presentation
[International presentation]

連続量状態を用いた長距離全光量子通信プロトコルの提案　　　　　　　　　　　　　　　
福井 浩介, 富田 章久, 岡本 淳
第34回量子情報技術研究会, 30 May 2016, Japanese, Poster presentation
[Domestic Conference]

連続変数と離散変数のハイブリッド系における離散変数の測定による連続変数量子状態制御　　　　　　　　　　　　　　　
松岡 史晃, 富田 章久
第34回量子情報技術研究会, 30 May 2016, Japanese, Oral presentation
[Domestic Conference]

Security Certification for QKD Implementation　　　　　　　　　　　　　　　
TOMITA Akihisa
UK-Japan Quantum Technology Workshop, 27 Mar. 2016, English, Invited oral presentation
[Invited], [International presentation]

Bennett 1992の盗聴量解析　　　　　　　　　　　　　　　
中村 敏幸, 中田 賢祐, 富田 章久, 小川 和久, 岡本 淳
第63回応用物理学会春季学術講演会, 19 Mar. 2016, Japanese, Oral presentation
[Domestic Conference]

量子暗号鍵配付システム の現状と将来の応用　　　　　　　　　　　　　　　
富田 章久
nano tech 2016 ロボティック・シンポジウム, 27 Jan. 2016, Japanese, Public discourse
[Invited], [Domestic Conference]

Entanglement Generation by Communication using Phase-Squeezed Light with Photon Loss　　　　　　　　　　　　　　　
Fumiaki Matsuoka, Akihisa Tomita
The 19th Conference on Quantum Information Processing, 10 Jan. 2016, English, Poster presentation
[International presentation]

Proposal for quantum error correction with continuous variables　　　　　　　　　　　　　　　
Kosuke Fukui, Akihisa Tomita, Atsushi Okamoto
The 19th Conference on Quantum Information Processing (QIP), 10 Jan. 2016, English, Poster presentation
[International presentation]

Nested Modulatorを用いたロバストなデコイ状態の生成　　　　　　　　　　　　　　　
中田 賢祐, 富田 章久, 岡本 淳
第33回量子情報技術研究会, 24 Nov. 2015, Japanese, Oral presentation
[Domestic Conference]

位相スクイーズド光を用いた通信による原子間もつれ形成と光子損失　　　　　　　　　　　　　　　
松岡 史晃, 富田 章久
第33回量子情報技術研究会, 24 Nov. 2015, Japanese, Oral presentation
[Domestic Conference]

Robust Quantum State Generation with a Dual-Parallel Modulator and Its Application for Quantum Key Distribution Systems　　　　　　　　　　　　　　　
Yu Kadosawa, Kensuke Nakata, Akihisa Tomita, Atsushi Okamoto
5th International Conference on Quantum Cryptography, 30 Sep. 2015, English, Poster presentation
[International presentation]

Toward a security certificated communication systems　　　　　　　　　　　　　　　
TOMITA Akihisa
5th International Conference on Quantum Cryptography (Qcrypt2015), 30 Sep. 2015, English, Invited oral presentation
[Invited], [International presentation]

Cluster-based architecture for fault-tolerant quantum computing with continous-variable cluster states　　　　　　　　　　　　　　　
FUKUI Kousuke, TOMITA Akihisa, OKAMOTO Atsushi
The 15th Asian Quantum Information Science Conference (AQIS2015), 25 Aug. 2015, English, Poster presentation
[International presentation]

Generation of phase-squeezed state using conditional displacement operation　　　　　　　　　　　　　　　
MATSUOKA Fumiaki, TOMITA Akihisa
The 15th Asian Quantum Information Science Coference (AQIS2015), 25 Aug. 2015, English, Poster presentation
[International presentation]

Security certification of practical quantum key distribution systems　　　　　　　　　　　　　　　
TOMITA Akihisa
The 15 th Asian Quantum Information Science Conference (AQIS2015), 25 Aug. 2015, English, Invited oral presentation
[Invited], [International presentation]

デュアルパラレル変調器によるロバストな量子状態の生成と量子鍵配送装置への応用　　　　　　　　　　　　　　　
廉澤 悠, 中田 賢祐, 富田 章久, 岡本 淳
第32回量子情報技術研究会, 25 May 2015, Japanese, Poster presentation
[Domestic Conference]

条件付き変位操作と事後選択による位相スクイーズド状態の生成　　　　　　　　　　　　　　　
松岡 史晃, 富田 章久
第32回量子情報技術研究会, 25 May 2015, Japanese, Poster presentation
[Domestic Conference]

Intensity fluctuation of a gain-switched semiconductor laser for a quantum key distribution system　　　　　　　　　　　　　　　
NAKATA Kensuke, TOMITA Akihisa, OKAMOTO Atsushi
第32回量子情技術研究会, 25 May 2015, Japanese, Oral presentation
[Domestic Conference]

Practical quantum key distribution -security certification and performance improvement　　　　　　　　　　　　　　　
TOMITA Akihisa
UK-Japan Quantum Technology Workshop, 23 Mar. 2015, English, Invited oral presentation
[International presentation]

Generation and evaluation of the physical random number using a gain-switched semiconductor laser　　　　　　　　　　　　　　　
T. Seki, A. Tomita, A. Okamoto
The International Conference on Quantum Communication, Measurement and Computing (QCMC2012), 02 Nov. 2014, English, Poster presentation
[International presentation]

Improvement of Success Probability by Squeezed Light in Weak Value Amplifi cation for Single-Photon-Level Nonlinearity　　　　　　　　　　　　　　　
F. Matsuoka, A. Tomita, A. Okamoto
The International Conference on Quantum Communication, Measurement and Computing (QCMC2012), 02 Nov. 2014, English, Poster presentation
[International presentation]

Toward implementation of nonlinear dynamics on a quantum system　　　　　　　　　　　　　　　
TOMITA Akihisa
Mathematics and Physics of Interacting Quantum Systems (MPIQS), 23 Oct. 2014, English, Invited oral presentation
[Invited], [International presentation]

Quantization of Classical Nonlinear Dynamics with Feedback Control　　　　　　　　　　　　　　　
Kiyotaka Nomizu, A. Tomita, A. Okamoto
The 14th Asian Quantum Information Science Conference (AQIS2014), 20 Aug. 2014, English, Poster presentation
[International presentation]

Entanglement swapping with bound entanglement　　　　　　　　　　　　　　　
Kenta Sugiyama, A. Tomita, A. Okamoto
The 14th Asian Quantum Information Science Conference (AQIS2014), 20 Aug. 2014, English, Poster presentation
[International presentation]

High-speed bridge circuit for InGaAs avalanche photodiode single-photon detector　　　　　　　　　　　　　　　
Hirofumi Hashimoto, Akihisa Tomita, Atsushi Okamoto
Photonic West, Advances in Photonics of Quantum Computing, Memory, and Communication VII, 01 Feb. 2014, English, Oral presentation
[International presentation]

Evaluation of the phase correlation between the optical pulses for transmission in quantum key distribution　　　　　　　　　　　　　　　
Toshiya Kobayashi, Akihisa Tomita, Atsushi Okamoto
Photonic West, Advances in Photonics of Quantum Computing, Memory, and Communication VII, 01 Feb. 2014, English, Oral presentation
[International presentation]

GHz駆動が可能なバランス型単一光子検出器の設計とその評価　　　　　　　　　　　　　　　
橋本裕史, 富田章久, 岡本 淳
第29回量子情報技術研究会, 18 Nov. 2013, Japanese, Poster presentation
[Domestic Conference]

単一光子非線形性に対する弱値増幅の厳密な解析　　　　　　　　　　　　　　　
松岡史晃, 富田章久, 岡本 淳
第29回量子情報技術研究会, 18 Nov. 2013, Japanese, Oral presentation
[Domestic Conference]

10GHzでの量子鍵配送における送信光パルス間の位相相関の評価　　　　　　　　　　　　　　　
小林俊哉, 富田章久, 岡本 淳
第29回量子情報技術研究会, 13 Nov. 2013, Japanese, Poster presentation
[Domestic Conference]

Characterization of d-dim frequency entanled photon pairs　　　　　　　　　　　　　　　
HASHIMOTO Hirofumi, TOMITA Akihisa, OKAMOTO Atsushi
平成25年度電気・情報関係学会北海道支部連合大会, 19 Oct. 2013, Japanese, Oral presentation
[Domestic Conference]

A practical unconditionally secure communication system based on quantum key distribution　　　　　　　　　　　　　　　
TOMITA Akihisa
The 2nd International Joint Workshop on Intelligent Convergence Technology, 23 Jul. 2013, English, Invited oral presentation
[Invited], [International presentation]

A proposal of an entangler for a photon qubit to a superconductor qubit with a semiconductor quantum dot　　　　　　　　　　　　　　　
NAKATA Knsuke, TOMITA Akihisa, OKAMOTO Atsush
Laser Physics Workshop 2013, 17 Jul. 2013, English, Oral presentation
[Invited], [International presentation]

Long-term Field Demonstration of WDM Quantum Key Distribution System with Stabilization Control　　　　　　　　　　　　　　　
Yoshino Kennichiro, Ochi T, Fujiwara Mikio, Tomita Akihisa, Sasaki Masahide, Tajima Akio
The 10th Conference on Lasers and Electro-Optics Pacific Rim(CLEO-PR 2013), 03 Jul. 2013, English, Oral presentation
[International presentation]

Improvement of Success Probability by Squeezed Light in Weak Value Amplifi cation for Single-Photon-Level Nonlinearity　　　　　　　　　　　　　　　
MATSUOKA Fumiaki, TOMITA Akihisa, OKAMOTO Atsushi
The 10th Conference on Lasers and Electro-Optics Pacific Rim(CLEO-PR 2013), 03 Jul. 2013, English, Poster presentation
[International presentation]

Improvement of sucess probability by squeezed light in weak value amplification for single-photon-level nonlinearity　　　　　　　　　　　　　　　
Matsuoka Fumiaki, TOMITA Akihisa, Atsushi Okamoto
The 28th Quantum Information Technology, 27 May 2013, Japanese, Oral presentation
[Domestic Conference]

Recording and Reconstruction of 38-Level Spatial Quadrature-Amplitude Modulation Signal in Double-Referential Holographic Memory
ZUKERAN KEISUKE, OKAMOTO ATSUSHI, TAKABAYASHI MASANORI, SHIBUKAWA ATSUSHI, TOMITA AKIHISA
映像情報メディア学会技術報告, 15 Feb. 2013, Japanese

Entanglement generation between a superconducting qubit and a photonic qubit　　　　　　　　　　　　　　　
NAKATA Kensuke, TOMITA Akihisa, OKAMOTO Atsushi
The 3rd The International Conference on Quantum Information and Technology (ICQIT2013), 18 Jan. 2013, English, Oral presentation
[International presentation]

Quantum and Classical Technologies for a secure photonic network　　　　　　　　　　　　　　　
TOMITA Akihisa
The 3rd The International Conference on Quantum Information and Technology (ICQIT2013), 16 Jan. 2013, English, Invited oral presentation
[Invited], [International presentation]

異波長記録再生法を用いた体積ホログラムによる空間モード分離実験
川端謙斗, 岡本淳, 本間聡, 富田章久, 若山雄太
応用物理学会北海道支部・日本光学会北海道地区合同学術講演会講演予稿集, 11 Jan. 2013, Japanese

2重参照型ホログラフィックメモリにおける39値空間直交振幅変調信号の記録再生
瑞慶覧圭佑, 岡本淳, 渋川敦史, 富田章久
応用物理学会北海道支部・日本光学会北海道地区合同学術講演会講演予稿集, 11 Jan. 2013, Japanese

ホログラフィックダイバーシティ干渉計を用いた高精度な位相計測に向けた複素振幅評価アルゴリズム
若山雄太, 岡本淳, 野澤仁, 富田章久
応用物理学会北海道支部・日本光学会北海道地区合同学術講演会講演予稿集, 11 Jan. 2013, Japanese

デジタル空間スペクトル拡散技術によるホログラフィックメモリにおける記憶容量の飛躍的向上
渋川敦史, 岡本淳, 若山雄太, 富田章久
応用物理学会北海道支部・日本光学会北海道地区合同学術講演会講演予稿集, 11 Jan. 2013, Japanese

スクイーズド光を用いた原子間もつれ形成時の3量子状態識別
松岡史晃, 富田章久, 岡本淳
応用物理学会北海道支部・日本光学会北海道地区合同学術講演会講演予稿集, 11 Jan. 2013, Japanese

Development of a Quantum Key Distribution System and Network　　　　　　　　　　　　　　　
TOMITA Akihisa
Asia-Pacific Conference & Workshop on Quantum Information Science 2012, 03 Dec. 2012, English, Invited oral presentation
[Invited], [International presentation]

A Quantum Relay using Quantum Scret Sharing　　　　　　　　　　　　　　　
SUZUKI Takashi, TOMITA Akihisa
第27回量子情報技術研究会, Nov. 2012, English, Oral presentation

Evaluation of the phase correlation between the optical pulses for transmission in quantum key distribution　　　　　　　　　　　　　　　
小林 俊哉, 富田 章久, 岡本 淳
量子情報技術研究会, Nov. 2012, Japanese, Oral presentation

Improvement of Success Probability by Repeated Measurements in Entanglement Generation by Communication using Squeezed Light　　　　　　　　　　　　　　　
松岡 史晃, 富田 章久, 岡本 淳
量子情報技術研究会, Nov. 2012, Japanese, Oral presentation

Entanglement between superconducting qubits and photon qubits　　　　　　　　　　　　　　　
中田賢佑, 富田章久, 岡本 淳
量子情報技術研究会, Nov. 2012, Japanese, Oral presentation

2重参照型ホログラフィにおける分散型位相シフト法によるシンボル誤り率の改善
瑞慶覽圭佑, 岡本淳, 富田章久, 渋川敦史, 文仙正俊
電気・情報関係学会北海道支部連合大会講演論文集(CD−ROM), 20 Oct. 2012, Japanese

異波長記録再生法を用いた体積ホログラムによるLPモードの分離
川端謙斗, 岡本淳, 本間聡, 富田章久, 若山雄太
電気・情報関係学会北海道支部連合大会講演論文集(CD−ROM), 20 Oct. 2012, Japanese

体積ホログラムを用いたモード合波器による空間モード多重励振
青木洸平, 岡本淳, 富田章久, 若山雄太
電気・情報関係学会北海道支部連合大会講演論文集(CD−ROM), 20 Oct. 2012, Japanese

2重位相共役鏡を用いた光衛星間通信システムにおける2値位相板による背景光低減手法
西牧可織, 岡本淳, 富田章久, 若山雄太, 高山佳久
電気・情報関係学会北海道支部連合大会講演論文集(CD−ROM), 20 Oct. 2012, Japanese

A Protocol of the Quantum Relay using Quantum Group Secret Sharing　　　　　　　　　　　　　　　
Takashi Suzuki, Akihisa Tomita, Atsushi Okamoto
The 2nd Annual Coference on Quantum Cryptography, 10 Sep. 2012, English, Poster presentation
[International presentation]

波長多重による高速量子鍵配送システムの長時間連続運転に向けた安定化制御
吉野健一郎, 藤原幹生, 富田章久, 佐々木雅英, 田島章雄
電子情報通信学会大会講演論文集, 28 Aug. 2012, Japanese

Entanglement Generation by Communication Using Squeezed State　　　　　　　　　　　　　　　
Fumiaki Matsuoka, Akihisa Tomita, Atsushi Okamoto
12th Asian Quantum Information Science Conference, 23 Aug. 2012, English, Oral presentation
[International presentation]

WDM quantum key distribution system using dual-mode single photon detectors　　　　　　　　　　　　　　　
K. Yoshino, M. Fujiwara, A. Tanaka, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, A. Tajima
11th Intl. Conference on Quantum Communication, Measurement and Computing, 30 Jul. 2012, English, Poster presentation
[International presentation]

Field Demonstration of High-speed Wavelengthdivision Multiplexing Quantum Key Distribution System and its Stabilized Operation　　　　　　　　　　　　　　　
Ken-Ichiro Yoshino, Mikio Fujiwara, Akihiro Tanaka, Seigo Takahashi, Yoshihiro Nambu, Akihisa Tomita, Shigehito Miki, Taro Yamashita, Zhen Wang, Masahide Sasaki, Akio Tajima
The 17th OptoElectronics and Communications Conference, 02 Jul. 2012, English, Oral presentation
[International presentation]

深宇宙通信システムへの応用を目指したサブ・ガイガーモードInGaAs APD単一光子検出器の開発
宮本義人, 辻野賢治, 片岡淳, 富田章久
電子情報通信学会大会講演論文集, 06 Mar. 2012, Japanese

量子状態を用いたグループ秘密分散を利用した量子リレー方式
鈴木貴士, 富田章久, 岡本淳
電子情報通信学会大会講演論文集, 06 Mar. 2012, Japanese

Sub‐Geiger mode APD光子検出器における光子計数特性
辻野賢治, 富田章久
日本物理学会講演概要集, 05 Mar. 2012, Japanese

モード分割多重通信用の空間モード励振・分離技術
岡本淳, 若山雄太, 青木洸平, 本間聡, 富田章久, 小田拓哉, 相馬大樹
応用物理学関係連合講演会講演予稿集(CD−ROM), 29 Feb. 2012, Japanese

A field test of high-speed wavelength-division multiplexing quantum key distribution system
YOSHINO KEN'ICHIRO, FUJIWARA MIKIO, TANAKA AKIHIRO, TAKAHASHI SEIGO, NAMBU YOSHIHIRO, TOMITA AKIHISA, MIKI SHIGEHITO, YAMASHITA TARO, WANG ZHEN, SASAKI MASAHIDE, TAJIMA AKIO
電子情報通信学会技術研究報告, 19 Jan. 2012, Japanese

ホログラフィックダイバーシティ干渉法による複素振幅計測のための2チャンネルアルゴリズムの提案
野澤仁, 岡本淳, 若山雄太, 富田章久, 高林正典
Optics & Photonics Japan講演予稿集(CD−ROM), 2012, Japanese

量子鍵配送技術の現状と空間伝送への応用
吉野健一郎, 南部芳弘, 田島章雄, 坂本俊夫, 越智貴夫, 名川飛鳥, 浅見潮音, 富田章久, 藤原幹生, 豊嶋守生, 佐々木雅英
宇宙科学技術連合講演会講演集(CD−ROM), 2012, Japanese

大幅な記録密度向上を目指した多層化コリニアホログラフィックメモリに関する研究
渋川敦史, 岡本淳, 富田章久, 高林正典
電気・情報関係学会北海道支部連合大会講演論文集(CD−ROM), 22 Oct. 2011, Japanese

ホログラフィック・ダイバーシティ干渉法による空間直交変調信号の検出
岡本淳, 九里佳祐, 富田章久, 高林正典
電気・情報関係学会北海道支部連合大会講演論文集(CD−ROM), 22 Oct. 2011, Japanese

自己参照型ホログラフィックメモリのための位相コード多重法
高林正典, 岡本淳, 富田章久
電気・情報関係学会北海道支部連合大会講演論文集(CD−ROM), 22 Oct. 2011, Japanese

再構成可能アドドロップ多重化装置のための超広波長可変域を有するホログラフィック波長フィルタ
若山雄太, 岡本淳, 富田章久
電子情報通信学会大会講演論文集, 30 Aug. 2011, Japanese

モード分割多重通信に向けた空間モードの励振・分離技術
岡本淳, 相馬大樹, 小田拓哉, 富田章久, 若山雄太
電子情報通信学会大会講演論文集, 30 Aug. 2011, Japanese

波長無依存干渉計と高速鍵抽出基盤を用いた高速量子暗号システム
田中聡寛, 藤原幹生, 吉野健一郎, 高橋成五, 南部芳弘, 富田章久, 三木茂人, 山下太郎, WANG Zhen, 佐々木雅英, 田島章雄
電子情報通信学会大会講演論文集, 30 Aug. 2011, Japanese

モード分割多重通信に向けたマルチモードファイバを用いたLPモード励振実験
相馬大樹, 岡本淳, 小田拓哉, 富田章久, 佐藤邦宏
電子情報通信学会大会講演論文集, 30 Aug. 2011, Japanese

体積ホログラムを用いたモードマルチプレクサにおけるLPモードの記録と再生
青木洸平, 岡本淳, 富田章久, 本間聡
電子情報通信学会大会講演論文集, 30 Aug. 2011, Japanese

Analysis of LP Mode Separation on All-optical Demultiplexer with Dynamic Multiple Holograms for Mode Division Multiplexing Communication
ODA TAKUYA, OKAMOTO ATSUSHI, SOUMA DAIKI, TOMITA AKIHISA
電子情報通信学会技術研究報告, 18 Aug. 2011, Japanese

サブ・ガイガーモード動作InGaAs APD単一光子検出器の高速化
宮本義人, 辻野賢治, 片岡淳, 富田章久
応用物理学会学術講演会講演予稿集(CD−ROM), 16 Aug. 2011, Japanese

東京QKDネットワークにおける超伝導ナノワイヤ単一光子検出システムの応用
WANG Z, 藤原幹生, 三木茂人, 山下太郎, 寺井弘高, 田中聡寛, 吉野健一郎, 高橋成五, 南部芳弘, 田島章雄, 富田章久, 本庄利守, 武居弘樹, 玉木潔, 都倉康弘, 佐々木雅英
応用物理学会学術講演会講演予稿集(CD−ROM), 16 Aug. 2011, Japanese

量子暗号
富田章久
応用物理学関係連合講演会講演予稿集(CD−ROM), 09 Mar. 2011, Japanese

量子鍵配送ネットワークにおける鍵管理システム
田中聡寛, 百目木智康, 藤原幹生, 富田章久, 佐々木雅英, 田島章雄
応用物理学関係連合講演会講演予稿集(CD−ROM), 09 Mar. 2011, Japanese

空間‐ファイバ統合型量子ネットワーク
藤原幹生, 吉野健一郎, 豊嶋守生, 南部芳弘, 佐々木雅英, 富田章久
応用物理学関係連合講演会講演予稿集(CD−ROM), 09 Mar. 2011, Japanese

動的ホログラフィック回折格子を用いた再構成可能アドドロップ多重化装置における超広波長可変幅
若山雄太, 岡本淳, 富田章久
電子情報通信学会大会講演論文集, 28 Feb. 2011, Japanese

モード分割多重通信に向けた動的多重ホログラフィック素子によるLPモードの分離実験
相馬大樹, 岡本淳, 小田拓哉, 富田章久
電子情報通信学会大会講演論文集, 28 Feb. 2011, Japanese

鍵抽出高速化エンジンによる量子鍵配送の高速化
田中聡寛, 藤原幹生, 富田章久, 田島章雄
電子情報通信学会大会講演論文集, 28 Feb. 2011, Japanese

セキュアフォトニックネットワーク—その脅威と対策—
北山研一, 佐々木雅英, 荒木壮一郎, 坪川真, 富田章久, 井上恭, 原澤克嘉, 長迫勇樹, 高田篤
電子情報通信学会技術研究報告, 21 Feb. 2011, Japanese

Development of a key distillation hardware engine for high speed quantum-key distribution
TANAKA AKIHIRO, FUJIWARA MIKIO, TOMITA AKIHISA, TAJIMA AKIO
電子情報通信学会技術研究報告, 20 Jan. 2011, Japanese

ホログラフィックダイバーシティ干渉法を用いた位相分布計測
九里佳祐, 岡本淳, 高林正典, 富田章久
Optics & Photonics Japan講演予稿集(CD−ROM), 2011, Japanese

超広域フレキシブルWDMへ向けたダイナミックROADMのためのホログラフィック波長フィルタ
若山雄太, 岡本淳, 富田章久
光通信システムシンポジウム講演予稿集, 2011, Japanese

モード分割多重通信に向けたマルチモードファイバの空間モード励振
相馬大樹, 岡本淳, 小田拓哉, 富田章久, 佐藤邦宏
光通信システムシンポジウム講演予稿集, 2011, Japanese

超高密度記録に向けた多層化コリニアホログラフィにおけるページ配置の最適設計
渋川敦史, 岡本淳, 富田章久, 高林正典, 佐藤邦宏
Optics & Photonics Japan講演予稿集, 08 Nov. 2010, Japanese

多値位相信号と強度変調符号によるハイブリッド変調型ホログラフィックメモリに関するエラー低減特性
高林正典, 岡本淳, 富田章久
Optics & Photonics Japan講演予稿集, 08 Nov. 2010, Japanese

サブ・ガイガーモード動作Si APDを用いた高効率・光子検出器の検討
辻野賢治, 富田章久, 富田章久
応用物理学会学術講演会講演予稿集(CD−ROM), 30 Aug. 2010, Japanese

量子ドット単一光子源を用いた1.55μm帯量子鍵配付実験(II)
南部芳弘, 竹本一矢, 宮澤俊之, 和久井健太郎, 廣瀬真一, 臼杵達哉, 高津求, 横山直樹, 吉野健一郎, 富田章久, 萬伸一, 佐久間芳樹, 荒川泰彦, 荒川泰彦
応用物理学会学術講演会講演予稿集(CD−ROM), 30 Aug. 2010, Japanese

低暗電流InGaAs APDを用いたサブ・ガイガーモード光子検出器の検証
宮本義人, 辻野賢治, 片岡淳, 富田章久, 富田章久
応用物理学会学術講演会講演予稿集(CD−ROM), 30 Aug. 2010, Japanese

量子ドット単一光子源を用いた1.55μm帯量子鍵配付実験(I)
竹本一矢, 南部芳弘, 宮澤俊之, 和久井健太郎, 廣瀬真一, 臼杵達哉, 高津求, 横山直樹, 吉野健一郎, 富田章久, 萬伸一, 佐久間芳樹, 荒川泰彦, 荒川泰彦
応用物理学会学術講演会講演予稿集(CD−ROM), 30 Aug. 2010, Japanese

Sub‐Geiger mode InGaAs APDを用いた光子検出器の動作検証
辻野賢治, 宮本義人, 片岡淳, 富田章久
応用物理学関係連合講演会講演予稿集(CD−ROM), 03 Mar. 2010, Japanese

空間伝送とファイバ伝送をつなぐハイブリッドエンタングルメント
吉野健一郎, 藤原幹生, 豊嶋守生, 佐々木雅英, 南部芳弘, 富田章久
電子情報通信学会大会講演論文集, 02 Mar. 2010, Japanese

空間‐ファイバ統合型ハイブリッド量子もつれ光源
藤原幹生, 吉野健一郎, 豊嶋守生, 南部芳弘, 佐々木雅英, 富田章久
電子情報通信学会大会講演論文集, 02 Mar. 2010, Japanese

アバランシェフォトダイオード光子検出器におけるPOVMの再検証
辻野賢治, 富田章久
日本物理学会講演概要集, 01 Mar. 2010, Japanese

Large Capacity Quantum Key Distribution Systems by use of Wavelength Division Multiplexing with a low-cost and easily-controlled configuration-Colourless Interferometric Technique for sharing interferometers over multi-channels-
TANAKA AKIHIRO, TOMITA AKIHISA, TAJIMA AKIO
電子情報通信学会技術研究報告, 21 Jan. 2010, Japanese

Single Photon Quantum Key Distribution System
TOMITA AKIHISA, TOMITA AKIHISA
光学, 10 Jan. 2010, Japanese

Issues for the implementation of quantum key distribution-Required performance for the signal processing-
TANAKA AKIHIRO, TAJIMA AKIO, TOMITA AKIHISA, TOMITA AKIHISA
電子情報通信学会技術研究報告, 22 Sep. 2009, Japanese

結晶性改良を施したメサ型InAlAs‐APDの光子検出特性
中田武志, 水木恵美子, 高橋成五, 牧田紀久夫, 富田章久, 富田章久
応用物理学会学術講演会講演予稿集, 08 Sep. 2009, Japanese

WDMによるQKD大容量化の為の干渉計波長無依存化技術
田中聡寛, 富田章久, 田島章雄
電子情報通信学会大会講演論文集, 01 Sep. 2009, Japanese

非対称型マッハ・ツェンダー干渉計による1.5μm帯単一光子干渉
竹本一矢, 和久井健太郎, 廣瀬真一, 高津求, 横山直樹, 南部芳弘, 富田章久, 萬伸一, 佐久間芳樹, 宮澤俊之, 臼杵達哉, 荒川泰彦, 荒川泰彦, 荒川泰彦
応用物理学関係連合講演会講演予稿集, 30 Mar. 2009, Japanese

単一光子検出用メサ型InAlAs‐APDのゲートモード動作とアフターパルス測定
中田武志, 水木恵美子, 高橋成五, 牧田紀久夫, 富田章久, 富田章久
応用物理学会学術講演会講演予稿集, 02 Sep. 2008, Japanese

ゲートモードアバランシェフォトダイオード光子検出器のアフターパルスモデル
富田章久, 富田章久, 中田武志, 水木恵美子, 高橋成五, 牧田紀久夫
電子情報通信学会大会講演論文集, 02 Sep. 2008, Japanese

オンデマンド鍵供給に基づく鍵管理方式を用いたQKDネットワーク
前田和佳子, 田中聡寛, 高橋成五, 田島章雄, 富田章久, 富田章久
電子情報通信学会大会講演論文集, 02 Sep. 2008, Japanese

Fast Quantum Cryptography System using Single Photon Communication
TAJIMA AKIO, TANAKA AKIHIRO, YOSHINO KEN'ICHIRO, MAEDA WAKAKO, NAMBU YOSHIHIRO, TAKAHASHI SEIGO, TOMITA AKIHISA
電子情報通信学会技術研究報告, 05 Jun. 2008, Japanese

セルフトレーニング型単一光子検出器の最適動作点の決定手法
高橋成五, 江口明大, 浜本貴一, 田島章雄, 富田章久, 富田章久
応用物理学関係連合講演会講演予稿集, 27 Mar. 2008, Japanese

97km量子鍵配送フィールドトライアル
前田和佳子, 藤原幹生, NAM Sae Woo, 南部芳弘, 高橋成五, 田中聡寛, 吉野健一郎, 三木茂人, BAEK Burm, WANG Zhen, 田島章雄, 富田章久, 富田章久, 佐々木雅英
電子情報通信学会大会講演論文集, 05 Mar. 2008, Japanese

高速量子暗号用高効率光子検出器
高橋成五, 田島章雄, 富田章久, 富田章久
電子情報通信学会大会講演論文集, 05 Mar. 2008, Japanese

量子暗号装置に対するFaked State攻撃定量化実験
江口明大, 田中聡寛, 高橋成五, 富田章久, 富田章久, 前田和佳子, 浜本貴一, 田島章雄
電子情報通信学会大会講演論文集, 05 Mar. 2008, Japanese

Sapce quantum cryptographic communication mission
TOMITA AKIHISA, NAMBU YOSHIHIRO, YOSHINO KEN'ICHIRO, TOYOSHIMA MORIO, TAKAYAMA YOSHIHISA, KUNIMORI YUKIO, FUJIWARA MIKIO, SASAKI MASAHIDE
宇宙科学技術連合講演会講演集(CD−ROM), 2008, Japanese

High-efficiency single photon detector combined with an ultra-small APD module and a self-training discriminator for high-speed quantum cryptosystems
Seigo Takahashi, Akio Tajima, Akihisa Tomita
Tech. Digest of 13th Microoptics Conference (MOC07) Post-deadline papers, PD-1, p.p. 2-3 (2007), 27 Dec. 2007, Oral presentation
A single-photon avalanche detector (SPAD) for high-speed quantum-key
generation has successfully been developed. It has the highest photon detection
repetition frequency and the lowest dark count rate in the world, as a
board-mountable sub-system. The SPAD consists of an ultra-small dual-avalanche
photodiode (APD) module and a novel discriminator. The APD module design is
consistent with cooling capability and high-frequency characteristics. The new
module has a 3 GHz bandwidth enabling 1 GHz gate-pulse repetition. The
bandwidth is extended 15-fold relative to the most wideband peltier cool...

Experimental demonstration of quantum leader election in linear optics
Yuta Okubo, Xiang-Bin Wang, Yun-Kun Jiang, Seiichiro Tani, Akihisa Tomita
27 Sep. 2007, Oral presentation
Linear optics is a promising candidate to enable the construction of quantum
computers. A number of quantum protocols gates based on linear optics have been
demonstrated. However, it is well-known that these gates are non-deterministic
and that higher order nonlinearity is necessary for deterministic operations.
We found the quantum leader election protocol (QLE) can be operated
deterministically only with linear optics, and we have demonstrated the nearly
deterministic operation which overcomes classical limit.

光子検出用二段メサ型InAlAsアバランシェ・フォトダイオード
中田武志, 水木恵美子, 牧田紀久夫, 富田章久, 富田章久
応用物理学会学術講演会講演予稿集, 04 Sep. 2007, Japanese

InAs/GaAs単一量子ドットにおけるラビ振動
河野俊介, 河野俊介, 富田章久, 富田章久, 藤方潤一, 斎藤英彰, 西研一
応用物理学会学術講演会講演予稿集, 04 Sep. 2007, Japanese

電荷制御型InAs量子ドットの磁気分光測定
桐原明宏, 河野俊介, 藤方潤一, 大河内俊介, 大河内俊介, 富田章久, 富田章久, 萬伸一
応用物理学会学術講演会講演予稿集, 04 Sep. 2007, Japanese

2×2干渉計と2電極MZ変調器を用いたXYZ全基底に対応可能なBB84量子暗号送信器
田中聡寛, 吉野健一郎, 南部芳弘, 田島章雄, 富田章久
電子情報通信学会大会講演論文集, 29 Aug. 2007, Japanese

原子—共振器系を用いた量子非破壊2光子ベル状態測定
小島邦裕, 小島邦裕, 富田章久, 富田章久
日本物理学会講演概要集, 21 Aug. 2007, Japanese

単一量子ドットと単一欠陥フォトニック結晶ナノ共振器の結合
白根昌之, 河野俊介, 池田直樹, 池田直樹, 大河内俊介, 杉本喜正, 杉本喜正, 富田章久, 富田章久
応用物理学関係連合講演会講演予稿集, 27 Mar. 2007, Japanese

H1型フォトニック結晶共振器におけるパーセル効果
河野俊介, 白根昌之, 池田直樹, 杉本喜正, 大河内俊介, 大河内俊介, 富田章久, 富田章久
応用物理学関係連合講演会講演予稿集, 27 Mar. 2007, Japanese

電荷制御可能なInAs量子ドットデバイスによる高次荷電励起子発光の観測
桐原明宏, 藤方潤一, 大河内俊介, 大河内俊介, 河野俊介, 富田章久, 富田章久, 萬伸一
応用物理学関係連合講演会講演予稿集, 27 Mar. 2007, Japanese

三元APDのアフターパルス評価:光検出信号とダーク検出信号後のアフターパルス出現確率の比較
中田武志, 水木恵美子, 牧田紀久夫, 高橋成五, 富田章久, 富田章久
応用物理学関係連合講演会講演予稿集, 27 Mar. 2007, Japanese

InAs/GaAs単一量子ドットのコヒーレント制御
河野俊介, 富田章久, 富田章久, 藤方潤一, 斎藤英彰, 西研一
応用物理学関係連合講演会講演予稿集, 27 Mar. 2007, Japanese

量子暗号通信における光子検出器
田島章雄, 高橋成五, 富田章久, 富田章久, 富田章久
電子情報通信学会大会講演論文集, 07 Mar. 2007, Japanese

量子暗号の高速化に向けた単一光子検出回路の提案
高橋成五, 田島章雄, 富田章久, 富田章久
電子情報通信学会大会講演論文集, 07 Mar. 2007, Japanese

高速量子暗号システムでのDecoy法設計
田中聡寛, 富田章久, 富田章久, 前田和佳子, 高橋成五, 田島章雄
電子情報通信学会大会講演論文集, 07 Sep. 2006, Japanese

H1型フォトニック結晶共振器のモード同定
白根昌之, 牛田淳, 河野俊介, 池田直樹, 杉本喜正, 大河内俊介, 富田章久
応用物理学会学術講演会講演予稿集, 29 Aug. 2006, Japanese

高Q値H1型フォトニック結晶共振器
白根昌之, 河野俊介, 池田直樹, 杉本喜正, 大河内俊介, 富田章久
応用物理学会学術講演会講演予稿集, 29 Aug. 2006, Japanese

量子暗号用APDのノイズキャリア発生率評価
高橋成五, 中田武志, 田島章雄, 富田章久, 富田章久
電子情報通信学会大会講演論文集, 08 Mar. 2006, Japanese

Quantum information device and system technology in NEC-Innovations of information technology with using quantum mechanics-
YOROZU SHIN'ICHI, NAKAMURA YASUNOBU, NAKAMURA YASUNOBU, TOMITA AKIHISA, TOMITA AKIHISA
電子情報通信学会技術研究報告, 20 Jan. 2006, Japanese

基底照合を行わないBB84型量子暗号の安全性
富田章久, 富田章久
電子情報通信学会大会講演論文集, 07 Sep. 2005, Japanese

量子暗号通信用小型低消費電力Dual‐APD単一光子受信モジュール
竹内剛, 高橋成五, 富田章久, 富田章久, 田島章雄
電子情報通信学会大会講演論文集, 07 Sep. 2005, Japanese

A minimized Photon Receiver for a high-speed quantum cryptosystem
TAKAHASHI SEIGO, TANAKA AKIHIRO, MAEDA WAKAKO, TAJIMA AKIO, TAKEUCHI TAKESHI, TOMITA AKIHISA, TOMITA AKIHISA
電子情報通信学会技術研究報告, 17 Jun. 2005, Japanese

A High-Speed Quantum Cryptosystem
TAJIMA AKIO, TANAKA AKIHIRO, MAEDA WAKAKO, TAKAHASHI SEIGO, TAKEUCHI TAKESHI, TOMITA AKIHISA, TOMITA AKIHISA, NAMBU YOSHIHIRO
電子情報通信学会技術研究報告, 16 May 2005, Japanese

InAs/GaAs単一量子ドットの蛍光寿命測定
河野俊介, 桐原明宏, 富田章久, 中村和夫, 西研一, 斉藤英彰, 藤方潤一, 大橋啓之
応用物理学関係連合講演会講演予稿集, 29 Mar. 2005, Japanese

単一方向型量子暗号通信システムの開発
南部芳弘, 富田章久, 田島章雄, 中村和夫
応用物理学関係連合講演会講演予稿集, 29 Mar. 2005, Japanese

高速量子暗号通信システムの開発(4)〜小型光子受信器の開発〜
高橋成五, 竹内剛, 田中聡寛, 前田和佳子, 田島章雄, 富田章久
電子情報通信学会大会講演論文集, 07 Mar. 2005, Japanese

高速量子暗号通信システムの開発(1)—アーキテクチャ—
田島章雄, 田中聡寛, 前田和佳子, 高橋成五, 竹内剛, 富田章久, 南部芳弘
電子情報通信学会大会講演論文集, 07 Mar. 2005, Japanese

量子暗号鍵配布システムの実用技術の開発
田中総寛, 前田和佳子, 田島章雄, 富田章久, 竹内剛, 高橋成五, 南部芳弘
電子情報通信学会大会講演論文集, 08 Sep. 2004, Japanese

全ファイバー系による4光波混合による強度相関ビーム対の発生
数井賢治, 富田章久
応用物理学関係連合講演会講演予稿集, 28 Mar. 2004, Japanese

InAs/GaAs量子ドットの単一ドット分光
河野俊介, 中島とも子, 富田章久, 中村和夫, 石田真彦, 藤方潤一, 横田均, 大橋啓之, 西研一
応用物理学会学術講演会講演予稿集, 30 Aug. 2003, Japanese

偏光干渉による紫外領域サブピコ秒光パルスのパルス幅測定
数井賢治, 南部芳弘, 富田章久
応用物理学関係連合講演会講演予稿集, 27 Mar. 2002, Japanese

雑音を用いた暗号を実現法の安全性解析と実証
富田章久, 広田修
電子情報通信学会大会講演論文集, 07 Sep. 2000, Japanese

量子暗号の原理と実現法
富田章久
電子情報通信学会大会講演論文集, 07 Sep. 2000, Japanese

量子暗号の現状と課題
富田章久, 南部芳弘, 中村和夫
電子情報通信学会大会講演論文集, 07 Sep. 2000, Japanese

コヒーレント2状態と猫状態を用いたBB84暗号プロトコル
南部芳弘, 富田章久, 河野芳江, 中村和夫
量子情報技術研究会資料, 18 Nov. 1999, Japanese

信頼性チェック可能な量子暗号装置
河野芳江, 南部芳弘, 富田章久, MAYERS D
日本物理学会講演概要集, 03 Sep. 1999, Japanese

フォトニック結晶の異方性分散応用 レンズ・コリメータ現象
小坂英男, 川嶋貴之, 富田章久, 納富雅也, 玉村敏昭, 佐藤尚, 川上彰二郎
応用物理学関係連合講演会講演予稿集, 28 Mar. 1999, Japanese

フォトニック結晶中の異常光伝播 セルフコリメーション
小坂英男, 富田章久, 川嶋貴之, 佐藤尚, 川上彰二郎, 納富雅也, 玉村敏昭
日本物理学会講演概要集, 15 Mar. 1999, Japanese

フォトニック結晶のPLC応用へ向けて μLC
小坂英男, 川嶋貴之, 富田章久, 納富雅也, 玉村敏昭, 佐藤尚, 川上彰二郎
電子情報通信学会大会講演論文集, 08 Mar. 1999, Japanese

3次元フォトニック結晶における異常屈折現象
小坂英男, 富田章久, 川嶋貴之, 佐藤尚, 川上彰二郎, 納富雅也, 玉村敏昭
日本物理学会講演概要集, 05 Sep. 1998, Japanese

Super-prism phenomena in photonic crystals.
KOSAKA HIDEO, TOMITA AKIHISA, KAWASHIMA TAKAYUKI, SATO TAKASHI, KAWAKAMI SHOJIRO, NOTOMI MASAYA, TAMAMURA TOSHIAKI
電子情報通信学会大会講演論文集, Sep. 1998, Japanese

Four Wave Mixing Characteristics in a Carrier Injected Quantum-dot System.
NAMBU YOSHIHIRO, TOMITA AKIHISA, SAITO HIDEAKI, NISHI KEN'ICHI
応用物理学会学術講演会講演予稿集, Sep. 1998, Japanese

Gain Saturation Characteristics in a Carrier Injected Quantum. Dot System.
NAMBU YOSHIHIRO, TOMITA AKIHISA, SAITO HIDEAKI, NISHI KEN'ICHI
応用物理学会学術講演会講演予稿集, Sep. 1998, Japanese

Anisotropic Dispersion in Photonic Crystals. Super-Prism Phenomena.
KOSAKA HIDEO, TOMITA AKIHISA, KAWASHIMA TAKAYUKI, SATO TAKASHI, KAWAKAMI SHOJIRO, NOTOMI MASAYA, TAMAMURA TOSHIAKI
応用物理学会学術講演会講演予稿集, Sep. 1998, Japanese

Optical frequency difference measurement by fourth order interference.
富田章久
電子情報通信学会大会講演論文集, Mar. 1998, Japanese

Efficient Four-wave Mixing using a Quantum-dot Laser Amplifier.
NANBU YOSHIHIRO, TOMITA AKIHISA
応用物理学関係連合講演会講演予稿集, Mar. 1998, Japanese

Analysis of dominant mechanisms determining characteristic temperature for semiconductor laser.
NANBU YOSHIHIRO, TOMITA AKIHISA
応用物理学会学術講演会講演予稿集, Oct. 1997, Japanese

Controlling hole distribution in InAsP/InGaAlAs quantum wells by InP spacer.
TOMITA AKIHISA
応用物理学会学術講演会講演予稿集, Oct. 1997, Japanese

Temperature Characteristics of long-wavelength strained QW LDs in the low temperature region.
NANBU YOSHIHIRO, ANAN TAKAYOSHI, TOKUTOME KEIICHI, TOMITA AKIHISA
応用物理学関係連合講演会講演予稿集, Mar. 1997, Japanese

Temperature dependence of the gain in strained InAsP/InGaAsP quantum wells.
TOMITA AKIHISA
応用物理学関係連合講演会講演予稿集, Mar. 1997, Japanese

Carrier Transport between quantum wells under dissipation.
TOMITA AKIHISA
応用物理学関係連合講演会講演予稿集, Mar. 1996, Japanese

Temperature-charactaristics of AlGaInP self-pulsation-LD.
SAWANO HIROYUKI, UENO YOSHIYASU, TOMITA AKIHISA, HOTTA HITOSHI, MIYASAKA FUMIHITO, KOBAYASHI KEN'ICHI
応用物理学関係連合講演会講演予稿集, Mar. 1995, Japanese

A proposal of step barrier structures to improve the hole transport in multiple quantum well lasers.
TOMITA AKIHISA
応用物理学関係連合講演会講演予稿集, Mar. 1994, Japanese

1.5.MU.m-band High-speed and Low-.ALPHA.-parameter Semiconductor Lasers with Strained Quantum Wells.
仁道正明, 木村明隆, 清水淳一, 村田茂, 難波江宏一, 富田章久, 鈴木明
電子情報通信学会大会講演論文集, Sep. 1993, Japanese

Free Carrier Contribution on Dielectric Properties in Quantum Well Structures.
富田章久
応用物理学会学術講演会講演予稿集, Sep. 1993, Japanese

Strain Effect on the Differential Gain and the Nonlinear Gain Coefficient for InGaAs/InGaAsP Strained Multiple Quantum Well Lasers.
清水淳一, 木村明隆, 難波江宏一, 仁道正明, 村田茂, 富田章久, 鈴木明
応用物理学関係連合講演会講演予稿集, Mar. 1993, Japanese

Performance enhancement of 1.5 .MU.m band semiconductor lasers using deformed diffraction grating.Speed enhancement.
NIDO MASAAKI, MURATA SHIGERU, NAHAE KOICHI, SHIMIZU JUN'ICHI, TOMITA AKIHISA, SUZUKI AKIRA
電気学会光・量子デバイス研究会資料, 08 Dec. 1992, Japanese

Intraband relaxation of real photo-excited carriers and its influence on the nonlinear optical spectra.
南部芳弘, 富田章久
日本物理学会講演概要集(分科会), Sep. 1992, Japanese

Analysis of transient nonlinear gain spectra of highly excited semiconductors.
南部芳弘, 富田章久
応用物理学会学術講演会講演予稿集, Sep. 1992, Japanese

Analysis of carrier relaxation dynamics considering carrier number/energy conservation in semiconductor.
南部芳弘, 富田章久
応用物理学関係連合講演会講演予稿集, Mar. 1992, Japanese

THz Optical Frequency Conversion of Gb/s Signals Using Highly Nondegenerate Four-Wave Mixing in an InGaAsP laser.
村田茂, 富田章久, 清水淳一, 鈴木明
応用物理学会学術講演会講演予稿集, Oct. 1991, Japanese

High-frequency modulation characteristics of 1.55um band strained-multi-quantum-well lasers with a large number of wells.
難波江宏一, 村田茂, 仁道正明, 清水淳一, 富田章久, 鈴木明
応用物理学会学術講演会講演予稿集, Oct. 1991, Japanese

Highly nondegenerate (>1THz) four-wave mixing in a Fabry-Perot semiconductor laser.
村田茂, 富田章久, 清水淳一, 鈴木明
応用物理学関係連合講演会講演予稿集, Mar. 1991, Japanese

A Density matrix Theory for Semiconductor Lasers Considering Non-Markoffian Intraband Relaxation. II.
富田章久, 鈴木明
応用物理学関係連合講演会講演予稿集, Mar. 1991, Japanese

A density matrix theory on semiconductor lasers considering non-markoffian intraband relaxation.
富田章久, 鈴木明
応用物理学会学術講演会講演予稿集, Sep. 1990, Japanese

Non-linear gain in semicondutor laser rate equations.
富田章久, 鈴木明
応用物理学会学術講演会講演予稿集, Sep. 1990, Japanese

量子力学　　　　　　　　　　　　　　　
北海道大学

ナノ材料テクノロジー　　　　　　　　　　　　　　　
東京工業大学

光デバイス学特論　　　　　　　　　　　　　　　
北海道大学

応用数学II　　　　　　　　　　　　　　　
北海道大学

Research and Development for Construction of a Global Quantum Cryptography Network
ICT重点技術の研究開発プロジェクト
Sep. 2020 - Mar. 2025
Akihisa Tomita
Ministry of Internal Affairs and Communications, Hokkaido University, Principal investigator

Direct measurement of time-frequency quantum channels of photons for the construction of the quantum internet infrastructure
Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C)
01 Apr. 2021 - 31 Mar. 2024
小川 和久, 富田 章久
本研究では, 個々の量子系を量子通信によりコヒーレントに接続し, 将来的に大規模な量子情報処理を目指す「量子インターネット」構想を念頭に置き, 各量子系を光子の時間-周波数自由度に量子情報をエンコードして伝送する際の, 送りたい光子の時間-周波数状態の準備, および量子チャネル通過に伴う未知の変換 (プロセス) , 光子を検出して状態を評価する際の測定を評価する手段として, 「直接測定」という独自のアプローチを提案し, 実証することを目指している.
まず最初の段階の2つの課題として, 「（扱いやすい別の物理自由度での）直接測定という独自アプローチの検証」と「光子の時間-周波数状態の実験的操作」を設定した. 前者に関しては, 直接測定の理論は物理系の種類にはよらず一般的に成立する理論であるため, 多少扱いが難しい光子の時間-周波数状態を用いた実験を最初に行うのではなく, それに先立って, 扱いやすい光の偏光状態を用いて, 状態・プロセス・測定の直接測定が実現することを実験的に検証し, 量子トモグラフィと比較して遜色ない精度で実行できることを示した. 後者に関しては, 実際に100フェムト秒以下の超短パルス光子の時間-周波数自由度を操作して状態の生成およびその状態の直接測定を実験的に実現した.
これらの実験結果については, 前者は現在論文執筆中であり, 後者についてはOptical Express Vol. 29, Issue 13, pp. 19403-19416 (2021)において発表された.
Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (C), Osaka University, 21K04915

Photonics and Quantum Technology for Society 5.0
Cross-ministerial Strategic Innovation Promotion Program
Sep. 2018 - Mar. 2023
Akihisa Tomita
Cabinet office of Japan, Hokkaido University, Coinvestigator

百年以上の超長期秘匿性を保証する情報通信ネットワーク基盤技術
科学研究費助成事業（基盤研究（S))
Jul. 2018 - Mar. 2023
富田 章久
日本学術振興会, Principal investigator, Competitive research funding

量子鍵配送デバイス評価技術
ImPACT
Apr. 2015 - Mar. 2019
TOMITA Akihisa
Cabinet Office, Government of Japan, Principal investigator, Competitive research funding

セキュアフォトニックネットワーク技術に関する研究開発 課題イ　　　　　　　　　　　　　　　
contract research project
2010 - 2015
Tamaki Kiyoshi
NICT, Competitive research funding

量子情報・光子状態の生成・制御・検出技術　　　　　　　　　　　　　　　
ＪＳＴ戦略的創造研究推進制度（総括実施型） （創造科学技術推進事業：ＥＲＡＴＯ）
2005 - 2010
光を用いた量子情報処理手法・デバイス・システムの研究開発。特に量子暗号鍵配布、光量子ゲートなど
Competitive research funding

Single Photon detection, Generation, Measurement, Control of quantum states, Optical quantum gates　　　　　　　　　　　　　　　
JST Basic Research Programs (Exploratory Research for Advanced Technology :ERATO)
Competitive research funding

量子鍵配送システム用の送信装置，受信装置，量子鍵配送方法，および量子鍵配送プログラム　　　　　　　　　　　　　　　
Patent right, 吉野 健一郎, 藤原 幹生, 佐々木 雅英, 富田 章久
特願2016-176364, 09 Sep. 2016

量子暗号鍵出力装置，量子暗号鍵通信システム及び量子暗号鍵出力方法　　　　　　　　　　　　　　　
Patent right, 富田 章久, 中田 賢祐
特願2016-173063, 05 Sep. 2016

乱数列発生装置，量子暗号送信機及び量子暗号通信システム　　　　　　　　　　　　　　　
Patent right, 富田 章久, 中田 賢祐
特願2016-019552, 04 Feb. 2016

Time-bin polarization format exchange technique for entangled optical source　　　　　　　　　　　　　　　
Patent right, Mikio Fujiwara, Morio Toyoshima, Masahide Sasaki, Akihisa Tomita, Yoshihiro Nambu, Ken-ichiro Yoshino
US 12/979,707, 28 Oct. 2012
US8509446 B2
13 Aug. 2013

光ファイバ通信方法
Patent right, 富田 章久, 日本電気株式会社
特願平2-094514, 10 Apr. 1990
特開平3-292018, 24 Dec. 1991
特許第3006023号
26 Nov. 1999
201103060240174690

半導体レーザ装置
Patent right, 富田 章久, 日本電気株式会社
特願平3-043107, 08 Mar. 1991
特開平4-280490, 06 Oct. 1992
特許第2943359号
25 Jun. 1999
201103007133642631

光ファイバ分散補償方法
Patent right, 富田 章久, 日本電気株式会社
特願平2-100601, 17 Apr. 1990
特開平3-296728, 27 Dec. 1991
特許第2910144号
09 Apr. 1999
201103008312629253

光ファイバ分散補償方法
Patent right, 富田 章久, 日本電気株式会社
特願平2-100602, 17 Apr. 1990
特開平3-296729, 27 Dec. 1991
特許第2910145号
09 Apr. 1999
201103039566009770

光半導体材料
Patent right, 富田 章久, 日本電気株式会社
特願平1-187733, 19 Jul. 1989
特開平3-051813, 06 Mar. 1991
特許第2540949号
25 Jul. 1996
201103005292204391

光変調器
Patent right, 富田 章久, 日本電気株式会社
特願昭63-027461, 10 Feb. 1988
特開平1-204018, 16 Aug. 1989
特許第2513265号
30 Apr. 1996
201103014597701840

半導体レーザ装置
Patent right, 富田 章久, 日本電気株式会社
特願平3-339665, 28 Nov. 1991
特開平5-152677, 18 Jun. 1993
200903006245770115

アバランシェフォトダイオード
Patent right, 富田 章久, 日本電気株式会社
特願昭63-291583, 18 Nov. 1988
特開平2-137376, 25 May 1990
201103069385240558

光双安定半導体装置
Patent right, 富田 章久, 光技術研究開発株式会社
特願昭62-229717, 16 Sep. 1987
特開平1-073786, 20 Mar. 1989
201103089570220882

光子検出器とそれを用いた量子通信装置及び量子計算機　　　　　　　　　　　　　　　
Patent right
特許4132860号

量子回路および量子計算機　　　　　　　　　　　　　　　
Patent right
特許4030838号

量子回路　　　　　　　　　　　　　　　
Patent right
特許第3981969号

単一光子発生装置　　　　　　　　　　　　　　　
Patent right
特許第3422482号

光変調器　　　　　　　　　　　　　　　
Patent right
特許第2513265号

アバランシエフオトダイオート　　　　　　　　　　　　　　　
Patent right
特許第2100015号

光ファイバ分散補償方法　　　　　　　　　　　　　　　
Patent right
特許第2910144号

波長計測装置及び核波長計測装置を備えた波長制御装置　　　　　　　　　　　　　　　
Patent right
特許第2976919号