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Akihisa Tomita
Faculty of Information Science and Technology　Electronics for Informatics　Advanced Electronics
Professor
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Last Updated :2024/05/09

Researcher Profile and Settings

Affiliation

  • Faculty of Information Science and Technology　Electronics for Informatics　Advanced Electronics

Job Title

  • Professor

Degree

  • Ph.D（The University of Tokyo）

URL

J-Global ID

Research Interests

  • quantum cryptography   光通信   光物性   量子情報   量子光学   Photonics   Optical Properties of semiconductors   Quantum Information   Quantum Optics   

Research Areas

  • Manufacturing technology (mechanical, electrical/electronic, chemical engineering) / Communication and network engineering
  • Nanotechnology/Materials / Optical engineering and photonics
  • Natural sciences / Semiconductors, optical and atomic physics

Educational Organization

Academic & Professional Experience

  • 2010/04 - Today Hokkaido University, Graduate School of Information Science and Technology Professor
  • 2009/04 - 2010/03 University of Tsukuba Professor
  • 2005/11 - 2010/03 NEC Senior Principal Researcher
  • 2005/10 - 2010/03 ERATO-SORST, JST Group Leader, Quantum Computing and Information Project
  • 2003/10 - 2008/03 Tokyo Institute of Technology Associate Professor
  • 1998/07 - 2005/10 NEC Fundamental Research Laboratories Principal Researcher
  • 2000/10 - 2005/09 ERATO, JST Group Leader, Quantum Computing and Information Project
  • 1984/04 - 1998/06 NEC, Optoelectronics Research Laboratories
  • 1991/09 - 1992/08 AT&T Bell Research Laboratories visiting researcher

Education

  •        - 1984  The University of Tokyo
  •        - 1982  The University of Tokyo  Faculty of Science  Department of Physics

Association Memberships

  • 日本物理学会   電子情報通信学会   応用物理学会   アメリカ光学会   The Physical Society of Japan   The Japanese Society of Applied Physics   Information and Communication Engineers   The Institute of Electronics   Optical Society of America   

Research Activities

Published Papers

  • Haobo Ge, Akihisa Tomita, Atsushi Okamoto, Kazuhisa Ogawa
    Optics Letters 49 (4) 822 - 822 0146-9592 2024/02/05 
    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.
  • Nobuhiro Yamagishi, Atsushi Okamoto, Akihisa Tomita
    Optical Review 30 (2) 217 - 231 1340-6000 2023/03/28
  • Haobo Ge, Akihisa Tomita, Atsushi Okamoto, Kazuhisa Ogawa
    IEEE Transactions on Quantum Engineering 4 1 - 8 2023
  • Zeyu Shen, Atsushi Okamoto, Shuanglu Zhang, Akihisa Tomita
    Optical Review 1340-6000 2022/09/12 [Refereed]
  • Shuanglu Zhang, Atsushi Okamoto, Yuta Abe, Ryo Watanabe, Akihisa Tomita, Daiki Soma, Yuta Wakayama, Takehiro Tsuritani
    Optics Express 29 (24) 39096 - 39106 2021/11/22 [Refereed][Not invited]
     
    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.
  • Kazuhisa Ogawa, Natsuki Abe, Hirokazu Kobayashi, Akihisa Tomita
    Physical Review Research 3 (3) 2021/07/21 [Refereed][Not invited]
  • Kazuhisa Ogawa, Takumi Okazaki, Hirokazu Kobayashi, Toshihiro Nakanishi, Akihisa Tomita
    Optics Express 29 (13) 19403 - 19403 2021/06/21 [Refereed][Not invited]
     
    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.
  • 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 1340-6000 2021/04 [Refereed][Not invited]
     
    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.
  • Kazuhisa Ogawa, Hirokazu Kobayashi, Akihisa Tomita
    Physical Review A 101 (4) 2469-9926 2020/04/17 [Refereed]
  • Weiyang Zhang, Yu Kadosawa, Akihisa Tomita, Kazuhisa Ogawa, Atsushi Okamoto
    OPTICS EXPRESS 28 (9) 13965 - 13977 1094-4087 2020/04 [Refereed][Not invited]
     
    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
  • 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 2020/01/31
  • 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 0277-786X 2020 
    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 2019/08 [Refereed][Not invited]
  • 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 2019/07 [Refereed][Not invited]
  • Akihisa Tomita
    ADVANCED QUANTUM TECHNOLOGIES 2 (5-6) 2019/06 [Refereed]
     
    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.
  • K. Ogawa, O Yasuhiko, H. Kobayashi, T. Nakanishi, A. Tomita
    New Journal of Physics 21 (4) 043013 - 043013 2019/04 [Refereed][Not invited]
  • 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 2018/10 [Refereed][Not invited]
  • Goto Yuta, Okamoto Atsushi, Takabayashi Masanori, Ogawa Kazuhisa, Tomita Akihisa
    OPTICAL REVIEW 25 (5) 549 - 554 1340-6000 2018/10 [Refereed][Not invited]
  • Fukui Kosuke, Tomita Akihisa, Okamoto Atsushi
    PHYSICAL REVIEW A 98 (2) 2469-9926 2018/08/22 [Refereed][Not invited]
  • Kosuke Fukui, Akihisa Tomita, Atsushi Okamoto, Keisuke Fujii
    Physical Review X 8 (2) 2160-3308 2018/05/25 [Refereed][Not invited]
     
    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.
  • Yuta Goto, Atsushi Okamoto, Atsushi Shibukawa, Kazuhisa Ogawa, Akihisa Tomita
    Optics Express 26 (4) 3779 - 3790 1094-4087 2018/02/19 [Refereed][Not invited]
     
    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.
  • Kazuhisa Ogawa, Hirokazu Kobayashi, Akihisa Tomita
    Physical Review A 97 (2) 2469-9934 2018/02/13 [Refereed][Not invited]
     
    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.
  • Shimpei Shimizu, Atsushi Okamoto, Fumiya Mizukawa, Kazuhisa Ogawa, Akihisa Tomita, Taketoshi Takahata, Satoshi Shinada, Naoya Wada
    Applied Optics 57 (2) 146 - 153 2155-3165 2018/01/10 [Refereed][Not invited]
     
    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.
  • Shimpei Shimizu, Atsushi Okamoto, Fumiya Mizukawa, Kazuhisa Ogawa, Akihisa Tomita, Taketoshi Takahata, Satoshi Shinada, Naoya Wada
    22nd Microoptics Conference, MOC 2017 2017- 260 - 261 2018/01/02 [Refereed][Not invited]
     
    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.
  • Yuta Goto, Atsushi Okamoto, Kazuhisa Ogawa, Akihisa Tomita
    22nd Microoptics Conference, MOC 2017 2017- 358 - 359 2018/01/02 [Refereed][Not invited]
     
    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.
  • Tomohiro Maeda, Atsushi Okamoto, Kazuhisa Ogawa, Akihisa Tomita, Yuta Wakayama, Takehiro Tsuritani
    22nd Microoptics Conference, MOC 2017 2017- 68 - 69 2018/01/02 [Refereed][Not invited]
     
    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.
  • Shogo Hoshino, Atsushi Okamoto, Yuta Goto, Kazuhisa Ogawa, Akihisa Tomita, Yuta Wakayama, Takehiro Tsuritani
    Proc. SPIE 10561 105610D  2018/01 [Refereed][Not invited]
  • Zihan Zhou, Atsushi Okamoto, Shimpei Shimizu, Akihisa Tomita, Kazuhisa Ogawa, Yuta Wakayama, Takehiro Tsuritani
    Proc. SPIE 10561 105610H  0277-786X 2018/01 [Refereed][Not invited]
     
    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.
  • Ken-ichiro Yoshino, Mikio Fujiwara, Kensuke Nakata, Tatsuya Sumiya, Toshihiko Sasaki, Masahiro Takeoka, Masahide Sasaki, Akio Tajima, Masato Koashi, Akihisa Tomita
    npj Quantum Information Springer Nature 4 (1) 2018/01 [Refereed][Not invited]
     
    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.
  • Kosuke Fukui, Akihisa Tomita, Atsushi Okamoto
    PHYSICAL REVIEW LETTERS 119 (18) 0031-9007 2017/11 [Refereed][Not invited]
     
    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.
  • Yuta Goto, Atsushi Okamoto, Masanori Takabayashi, Kazuhisa Ogawa, Akihisa Tomita
    JAPANESE JOURNAL OF APPLIED PHYSICS 56 (9) 0021-4922 2017/09 [Refereed][Not invited]
     
    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
  • Shimpei Shimizu, Atsushi Okamoto, Fumiya Mizukawa, Kazuhisa Ogawa, Akihisa Tomita, Taketoshi Takahata, Satoshi Shinada, Naoya Wada
    Japanese Journal of Applied Physics Institute of Physics 56 (9S) 09NA05-1 - 09NA05-5 0021-4922 2017/08 [Refereed][Not invited]
     
    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  2017/07 [Refereed][Not invited]
  • Matsuoka Fumiaki, Tomita Akihisa, Shikano Yutaka
    Quantum Studies: Mathematics and Foundations 4 (2) 159 - 169 2196-5609 2017/06 [Refereed][Not invited]
  • Kensuke Nakata, Akihisa Tomita, Mikio Fujiwara, Ken-ichiro Yoshino, Akio Tajima, Atsushi Okamoto, Kazuhisa Ogawa
    OPTICS EXPRESS 25 (2) 622 - 634 1094-4087 2017/01 [Refereed][Not invited]
     
    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
  • Yuta Goto, Atsushi Okamoto, Yuta Wakayama, Kazuhisa Ogawa, Jin Nozawa, Akihisa Tomita, Takehiro Tsuritani
    OPTICS EXPRESS 24 (21) 24739 - 24749 1094-4087 2016/10 [Refereed][Not invited]
     
    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
  • Jin Nozawa, Atsushi Okamoto, Kazuhisa Ogawa, Akihisa Tomita
    OPTICS EXPRESS 24 (21) 24002 - 24017 1094-4087 2016/10 [Refereed][Not invited]
     
    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
  • Yuta Goto, Atsushi Okamoto, Masataka Toda, Yasuyuki Kuno, Jin Nozawa, Kazuhisa Ogawa, Akihisa Tomita
    Japanese Journal of Applied Physics 55 (8S3) 08RE04 - 08RE04 0021-4922 2016/07 [Refereed][Not invited]
  • Fumiaki Matsuoka, Akihisa Tomita, Atsushi Okamoto
    PHYSICAL REVIEW A 93 (3) 033308  2469-9926 2016/03 [Refereed][Not invited]
     
    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.
  • Jin Nozawa, Atsushi Okamoto, Masataka Toda, Yasuyuki Kuno, Akihisa Tomita
    PRACTICAL HOLOGRAPHY XXX: MATERIALS AND APPLICATIONS 9771 0277-786X 2016 [Refereed][Not invited]
     
    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.
  • Hiroki Sakuma, Atsushi Okamoto, Atsushi Shibukawa, Yuta Goto, Akihisa Tomita
    NEXT-GENERATION OPTICAL COMMUNICATION: COMPONENTS, SUB-SYSTEMS, AND SYSTEMS V 9774 0277-786X 2016 [Refereed][Not invited]
     
    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.
  • Koki Ishii, Atsushi Okamoto, Takehiro Tsuritani, Yuta Wakayama, Yuta Goto, Akihisa Tomita
    NEXT-GENERATION OPTICAL COMMUNICATION: COMPONENTS, SUB-SYSTEMS, AND SYSTEMS V 9774 0277-786X 2016 [Refereed][Not invited]
     
    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.
  • Jin Nozawa, Atsushi Okamoto, Atsushi Shibukawa, Masanori Takabayashi, Akihisa Tomita
    APPLIED OPTICS 54 (29) 8644 - 8652 1559-128X 2015/10 [Refereed][Not invited]
     
    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
  • Nozawa J, Okamoto A, Shibukawa A, Takabayashi M, Tomita A
    Applied optics 54 (29) 8644 - 8652 0003-6935 2015/10 [Refereed][Not invited]
  • Matsuoka F., Tomita A., Shikano Y.
    Meeting Abstracts of the Physical Society of Japan 一般社団法人日本物理学会 70 716 - 716 2189-079X 2015
  • Yuta Goto, Atsushi Okamoto, Atsushi Shibukawa, Akihisa Tomita, Kunihiro Sato
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 9319 1605-7422 2015 [Refereed][Not invited]
     
    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.
  • Jin Nozawa, Atsushi Okamoto, Masataka Toda, Yasuyuki Kuno, Akihisa Tomita
    2015 20TH MICROOPTICS CONFERENCE (MOC) 2015 [Refereed][Not invited]
     
    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.
  • Yanfeng Zhao, Atsushi Okamoto, Tomokazu Oda, Akihisa Tomita, Masatoshi Bunsen, Satoshi Honma
    2015 20TH MICROOPTICS CONFERENCE (MOC) 2015 [Refereed][Not invited]
     
    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.
  • Yuta Goto, Atsushi Okamoto, Atsushi Shibukawa, Akihisa Tomita, Masanori Takabayashi
    2015 20TH MICROOPTICS CONFERENCE (MOC) 2015 [Refereed][Not invited]
     
    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.
  • Yanfeng Zhao, Atsushi Okamoto, Tomohiro Maeda, Yuki Hirasaki, Akihisa Tomita, Masatoshi Bunsen
    NEXT-GENERATION OPTICAL COMMUNICATION: COMPONENTS, SUB-SYSTEMS, AND SYSTEMS IV 9389 0277-786X 2015 [Refereed][Not invited]
     
    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.
  • Yuta Goto, Atsushi Okamoto, Atsushi Shibukawa, Akihisa Tomita, Kunihiro Sato
    OPTICAL TOMOGRAPHY AND SPECTROSCOPY OF TISSUE XI 9319 0277-786X 2015 [Refereed][Not invited]
     
    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.
  • Yuta Goto, Atsushi Okamoto, Masataka Toda, Yasuyuki Kuno, Akihisa Tomita
    2015 20TH MICROOPTICS CONFERENCE (MOC) 2015 [Refereed][Not invited]
     
    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.
  • Jin Nozawa, Atsushi Okamoto, Masataka Toda, Yasuyuki Kuno, Akihisa Tomita
    2015 20TH MICROOPTICS CONFERENCE (MOC) 2015 [Refereed][Not invited]
     
    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.
  • Yuta Kan, Atsushi Okamoto, Akihisa Tomita, Atsushi Shibukawa, Hisatoshi Funakoshi
    PRACTICAL HOLOGRAPHY XXIX: MATERIALS AND APPLICATIONS 9386 0277-786X 2015 [Refereed][Not invited]
     
    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.
  • Toshiya Kobayashi, Akihisa Tomita, Atsushi Okamoto
    PHYSICAL REVIEW A 90 (3) 032320  1050-2947 2014/09 [Refereed][Not invited]
     
    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.
  • Kaori Nishimaki, Atsushi Okamoto, Atsushi Shibukawa, Akihisa Tomita, Yoshihisa Takayama, Masatoshi Bunsen
    JAPANESE JOURNAL OF APPLIED PHYSICS 53 (8) 0021-4922 2014/07 [Not refereed][Not invited]
     
    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
  • 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 1094-4087 2014/06 [Refereed][Not invited]
     
    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
  • Kaori Nishimaki, Atsushi Okamoto, Atsushi Shibukawa, Masanori Takabayashi, Akihisa Tomita, Yoshihisa Takayama
    OPTICAL REVIEW 21 (3) 415 - 424 1340-6000 2014/05 [Refereed][Not invited]
     
    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
  • Atsushi Shibukawa, Atsushi Okamoto, Yuta Goto, Satoshi Honma, Akihisa Tomita
    OPTICS EXPRESS 22 (10) 11918 - 11929 1094-4087 2014/05 [Refereed][Not invited]
     
    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
  • Atsushi Shibukawa, Atsushi Okamoto, Masanori Takabayashi, Akihisa Tomita
    Optics Express 22 (4) 3968 - 3982 1094-4087 2014/02/24 [Refereed][Not invited]
     
    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.
  • Atsushi Okamoto, Tomohiro Maeda, Yuki Hirasaki, Akihisa Tomita, Kunihiro Sato
    MICRO-OPTICS 2014 9130 0277-786X 2014 [Refereed][Not invited]
     
    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.
  • Hirofumi Hashimoto, Akihisa Tomita, Atsushi Okamoto
    ADVANCES IN PHOTONICS OF QUANTUM COMPUTING, MEMORY, AND COMMUNICATION VII 8997 0277-786X 2014 [Refereed][Not invited]
     
    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.
  • 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 2014 [Refereed][Not invited]
     
    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.
  • 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 - + 2014 [Refereed][Not invited]
     
    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.
  • Toshiya Kobayashi, Akihisa Tomita, Atsushi Okamoto
    ADVANCES IN PHOTONICS OF QUANTUM COMPUTING, MEMORY, AND COMMUNICATION VII 8997 0277-786X 2014 [Refereed][Not invited]
     
    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.
  • Kaori Nishimaki, Atsushi Okamoto, Akihisa Tomita, Yoshihisa Takayama, Masatoshi Bunsen
    Technical Digest of the 18th Microoptics Conference, MOC 2013 2013/12/01 [Not refereed][Not invited]
     
    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.
  • Atsushi Shibukawa, Atsushi Okamoto, Yuta Wakayama, Akihisa Tomita, Hisatoshi Funakoshi, Kunihiro Sato
    JAPANESE JOURNAL OF APPLIED PHYSICS 52 (9) 0021-4922 2013/09 [Refereed][Not invited]
     
    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
  • Keisuke Zukeran, Atsushi Okamoto, Masanori Takabayashi, Atsushi Shibukawa, Kunihiro Sato, Akihisa Tomita
    JAPANESE JOURNAL OF APPLIED PHYSICS 52 (9) 0021-4922 2013/09 [Refereed][Not invited]
     
    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
  • Fumiaki Matsuoka, Akihisa Tomita, Atsushi Okamoto
    Physical Review A - Atomic, Molecular, and Optical Physics 88 (2) 1050-2947 2013/08/12 [Refereed][Not invited]
     
    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.
  • Yuta Wakayama, Atsushi Okamoto, Kento Kawabata, Akihisa Tomita, Kunihiro Sato
    OPTICS EXPRESS 21 (10) 12920 - 12933 1094-4087 2013/05 [Refereed][Not invited]
     
    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
  • Kohei Aoki, Atsushi Okamoto, Yuta Wakayama, Akihisa Tomita, Satoshi Honma
    Optics Letters 38 (5) 769 - 771 0146-9592 2013/03/01 [Refereed][Not invited]
     
    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.
  • Tomohiro Maeda, Atsushi Okamoto, Akihisa Tomita, Yuki Hirasaki, Yuta Wakayama, Masatoshi Bunsen
    Pacific Rim Conference on Lasers and Electro-Optics, CLEO - Technical Digest 2013 [Refereed][Not invited]
     
    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.
  • K. Yoshino, T. Ochi, M. Fujiwara, A. Tomita, M. Sasaki, A. Tajima
    2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS PACIFIC RIM (CLEO-PR) 2013 [Refereed][Not invited]
     
    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.
  • Kento Kawabata, Atsushi Okamoto, Satoshi Honma, Yuta Wakayama, Kunihiro Sato, Akihisa Tomita
    NEXT-GENERATION OPTICAL COMMUNICATION: COMPONENTS, SUB-SYSTEMS, AND SYSTEMS II 8647 0277-786X 2013 [Refereed][Not invited]
     
    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%.
  • 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 0018-9197 2012/04 [Refereed][Not invited]
     
    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.
  • 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 0146-9592 2012/01 [Refereed][Not invited]
     
    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
  • Yoshito Miyamoto, Kenji Tsujino, Jun Kataoka, Akihisa Tomita
    European Conference and Exhibition on Optical Communication, ECEOC 2012 2012 [Refereed][Not invited]
     
    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.
  • 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 [Refereed][Not invited]
     
    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.
  • Yuta Wakayama, Atsushi Okamoto, Akihisa Tomita
    2012 International Conference on Photonics in Switching (PS) 2155-8515 2012 [Refereed][Not invited]
     
    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.
  • 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 [Refereed][Not invited]
  • 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 0733-8724 2011/11 [Refereed][Not invited]
     
    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.
  • Y. Nambu, S. Takahashi, K. Yoshino, A. Tanaka, M. Fujiwara, M. Sasaki, A. Tajima, S. Yorozu, A. Tomita
    OPTICS EXPRESS 19 (21) 20531 - 20541 1094-4087 2011/10 [Refereed][Not invited]
     
    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
  • 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 1094-4087 2011/09 [Refereed][Not invited]
     
    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
  • Atsushi Shibukawa, Atsushi Okamoto, Akihisa Tomita, Masanori Takabayashi, Kunihiro Sato
    JAPANESE JOURNAL OF APPLIED PHYSICS 50 (9) 0021-4922 2011/09 [Refereed][Not invited]
     
    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
  • Masanori Takabayashi, Atsushi Okamoto, Akihisa Tomita, Masatoshi Bunsen
    JAPANESE JOURNAL OF APPLIED PHYSICS 50 (9) 0021-4922 2011/09 [Refereed][Not invited]
     
    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
  • 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 2011/08 [Refereed]
  • Atsushi Okamoto, Keisuke Kunori, Masanori Takabayashi, Akihisa Tomita, Kunihiro Sato
    OPTICS EXPRESS 19 (14) 13436 - 13444 1094-4087 2011/07 [Refereed][Not invited]
     
    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
  • 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 1094-4087 2011/05 [Refereed][Not invited]
     
    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
  • 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 2158-3234 2011 [Refereed][Not invited]
     
    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.
  • Kenji Tsujino, Akihisa Tomita
    2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 2011 [Refereed][Not invited]
     
    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.
  • 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 [Refereed][Not invited]
     
    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.
  • Takuya Oda, Atsushi Okamoto, Daiki Soma, Akihisa Tomita, Yuta Wakayama
    BROADBAND ACCESS COMMUNICATION TECHNOLOGIES V 7958 0277-786X 2011 [Refereed][Not invited]
     
    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.
  • Atsushi Okamoto, Kazutaka Hira, Alexander A. Grabar, Hisatoshi Funakoshi, Yuta Wakayama, Akihisa Tomita
    OPTOELECTRONIC INTERCONNECTS AND COMPONENT INTEGRATION XI 7944 0277-786X 2011 [Refereed][Not invited]
     
    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.
  • 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) 2160-9020 2011 [Refereed][Not invited]
     
    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. 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) 2011 [Refereed][Not invited]
     
    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.
  • 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) 2011 [Refereed][Not invited]
     
    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.
  • Yuta Wakayama, Atsushi Okamoto, Akihisa Tomita, Kunihiro Sato
    BROADBAND ACCESS COMMUNICATION TECHNOLOGIES V 7958 0277-786X 2011 [Refereed][Not invited]
     
    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.
  • Kenji Tsujino, Yoshito Miyamoto, Jun Kataoka, Akihisa Tomita
    QUANTUM COMMUNICATION, MEASUREMENT AND COMPUTING (QCMC): THE TENTH INTERNATIONAL CONFERENCE 1363 0094-243X 2011 [Refereed][Not invited]
     
    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.
  • Kaori Nishimaki, Atsushi Okamoto, Tomohiro Fujita, Alexander A. Grabar, Masanori Takabayashi, Jun Uozumi, Akihisa Tomita, Yoshihisa Takayama
    FREE-SPACE LASER COMMUNICATION TECHNOLOGIES XXIII 7923 0277-786X 2011 [Refereed][Not invited]
     
    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)
  • Akihisa Tomita
    IEICE TRANSACTIONS ON FUNDAMENTALS OF ELECTRONICS COMMUNICATIONS AND COMPUTER SCIENCES E93A (5) 871 - 871 0916-8508 2010/05 [Refereed][Not invited]
  • 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 1068-5200 2010/01 [Refereed][Invited]
     
    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.
  • 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 2010 [Refereed][Not invited]
     
    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
  • 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) 1882-0778 2010 [Refereed][Not invited]
     
    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
  • M. Fujiwara, M. Toyoshima, M. Sasaki, K. Yoshino, Y. Nambu, A. Tomita
    APPLIED PHYSICS LETTERS 95 (26) 261103  0003-6951 2009/12 [Refereed][Not invited]
     
    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.
  • Akihiro Tanaka, Wakako Maeda, Seigo Takahashi, Akio Tajima, Akihisa Tomita
    IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 15 (6) 1622 - 1629 1077-260X 2009/11 [Refereed][Not invited]
     
    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.
  • Wakako Maeda, Akihiro Tanaka, Seigo Takahashi, Akio Tajima, Akihisa Tomita
    IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 15 (6) 1591 - 1601 1077-260X 2009/11 [Refereed][Invited]
     
    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.
  • 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 2009/05/27 [Not refereed][Not invited]
     
    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...
  • Kunihiro Kojima, Akihisa Tomita
    JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS 26 (4) 836 - 848 0740-3224 2009/04 [Refereed][Not invited]
     
    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
  • Yuta Okubo, Francesco Buscemi, Akihisa Tomita
    QUANTUM COMMUNICATION, MEASUREMENT AND COMPUTING (QCMC) 1110 355 - + 0094-243X 2009 [Refereed][Not invited]
     
    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.
  • Akihisa Tomita
    QUANTUM COMMUNICATIONS REALIZED II 7236 0277-786X 2009 [Refereed][Not invited]
     
    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.
  • A. Tanaka, A. Tomita, A. Tajima
    2009 35TH EUROPEAN CONFERENCE ON OPTICAL COMMUNICATION (ECOC) 2009 [Refereed][Not invited]
     
    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.
  • 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 1094-4087 2008/07 [Refereed][Not invited]
     
    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.
  • Masahito Hayashi, Akihisa Tomita, Keiji Matsumoto
    NEW JOURNAL OF PHYSICS 10 1367-2630 2008/04 [Refereed][Not invited]
     
    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.
  • Yuta Okubo, Xiang-Bin Wang, Yun-Kun Jiang, Seiichiro Tani, Akihisa Tomita
    PHYSICAL REVIEW A 77 (3) 032343  1050-2947 2008/03 [Refereed][Not invited]
     
    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.
  • Yoshihiro Nambu, Ken'ichiro Yoshino, Akihisa Tomita
    JOURNAL OF MODERN OPTICS 55 (12) 1953 - 1970 0950-0340 2008 [Refereed][Not invited]
     
    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.
  • W. Maeda, A. Tanaka, S. Takahashi, A. Tajima, A. Tomita
    2008 34TH EUROPEAN CONFERENCE ON OPTICAL COMMUNICATION (ECOC) 2008 [Refereed][Not invited]
     
    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.
  • 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) 2008 [Refereed][Not invited]
     
    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.
  • 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 - + 2008 [Refereed][Not invited]
     
    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.
  • Bao-Sen Shi, Chang Zhai, Guang-Can Guo, Yun-Kun Jiang, Akihisa Tomita
    OPTICS COMMUNICATIONS 278 (2) 363 - 367 0030-4018 2007/10 [Refereed][Not invited]
     
    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.
  • Xiang-Bin Wang, Tohya Hiroshima, Akihisa Tomita, Masahito Hayashi
    PHYSICS REPORTS-REVIEW SECTION OF PHYSICS LETTERS 448 (1-4) 1 - 111 0370-1573 2007/08 [Refereed][Invited]
     
    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.
  • Akio Tajima, Akihiro Tanaka, Wakako Maeda, Seigo Takahashi, Akihisa Tomita
    IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 13 (4) 1031 - 1038 1077-260X 2007/07 [Refereed][Not invited]
     
    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.
  • Mark A. Itzler, Sergio Cova, Michael Wahl, Akihisa Tomita
    IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 13 (4) 849 - 851 1077-260X 2007/07 [Refereed][Not invited]
  • Masayuki Shirane, Shunsuke Kono, Jun Ushida, Shunsuke Ohkouchi, Naoki Ikeda, Yoshimasa Sugimoto, Akihisa Tomita
    JOURNAL OF APPLIED PHYSICS 101 (7) 073107  0021-8979 2007/04 [Refereed][Not invited]
     
    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.
  • Jian Li, Yun-Kun Jiang, Akihisa Tomita
    PHYSICS LETTERS A 362 (4) 269 - 272 0375-9601 2007/03 [Refereed][Not invited]
     
    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.
  • Kunihiro Kojima, Akihisa Tomita
    PHYSICAL REVIEW A 75 (3) 032320  1050-2947 2007/03 [Refereed][Not invited]
     
    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.
  • Shunsuke Kono, Akihisa Tomita, Junichi Fujikata, Hideaki Saito, Kenichi Nishi
    Optics InfoBase Conference Papers 2007
  • Shunsuke Kono, Akihisa Tomita, Junichi Fujikata, Hideaki Saito, Kenichi Nishi
    Conference on Lasers and Electro-Optics Europe - Technical Digest 2007
  • Yun-Kun Jiang, Akihisa Tomita
    JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 40 (2) 437 - 443 0953-4075 2007/01 [Refereed][Not invited]
     
    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.
  • Masahito Hayashi, Bao-Sen Shi, Akihisa Tomita, Keiji Matsumoto, Yoshiyuki Tsuda, Yun-Kun Jiang
    PHYSICAL REVIEW A 74 (6) 1050-2947 2006/12 [Refereed][Not invited]
     
    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.
  • Yun-Kun Jiang, Akihisa Tomita
    OPTICS COMMUNICATIONS 267 (1) 278 - 281 0030-4018 2006/11 [Refereed][Not invited]
     
    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.
  • HASEGAWA TOSHIO, NISHIOKA TSUYOSHI, ISHIZUKA HIROKAZU, NAMBU YOSHIHIRO, TOMITA AKIHISA, TAJIMA AKIO
    情報通信研究機構季報 情報通信研究機構 52 (3) 55-62 - 96 1349-3191 2006/09/29 [Not refereed][Not invited]
  • Akihiro Kirihara, Shunsuke Kono, Akihisa Tomita, Kazuo Nakamura
    OPTICAL REVIEW 13 (4) 279 - 282 1340-6000 2006/07 [Refereed][Not invited]
     
    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.
  • Yoshihiro Nambu, Ken'ichiro Yoshino, Akihisa Tomita
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 45 (6A) 5344 - 5348 0021-4922 2006/06 [Refereed][Not invited]
     
    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.
  • K Kojima, A Tomita
    PHYSICAL REVIEW B 73 (19) 195312  1098-0121 2006/05 [Refereed][Not invited]
     
    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.
  • BS Shi, A Tomita
    JOURNAL OF MODERN OPTICS 53 (7) 1003 - 1009 0950-0340 2006/05 [Refereed][Not invited]
     
    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.
  • BS Shi, A Tomita
    NEW JOURNAL OF PHYSICS 8 38  1367-2630 2006/03 [Refereed][Not invited]
     
    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.
  • 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 0021-4922 2006/03 [Refereed][Not invited]
     
    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.
  • Akihisa Tomita, Bao-Sen Shi
    QUANTUM COMPUTATION AND INFORMATION: FROM THEORY TO EXPERIMENT 102 243 - 275 0303-4216 2006 [Refereed][Not invited]
     
    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.
  • YK Jiang, XB Wang, BS Shi, A Tomita
    OPTICS EXPRESS 13 (23) 9415 - 9421 1094-4087 2005/11 [Refereed][Not invited]
     
    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.
  • S Kono, A Kirihara, A Tomita, K Nakamura, J Fujikata, K Ohashi, H Saito, K Nishi
    PHYSICAL REVIEW B 72 (15) 1098-0121 2005/10 [Refereed][Not invited]
     
    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.
  • A Tomita
    OPTICS AND SPECTROSCOPY 99 (2) 204 - 210 0030-400X 2005/08 [Refereed][Not invited]
     
    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.
  • BS Shi, A Tomita
    JOURNAL OF MODERN OPTICS 52 (5) 755 - 761 0950-0340 2005/03 [Refereed][Not invited]
     
    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.
  • 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 [Refereed][Not invited]
     
    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.
  • AV Gopal, A Tomita, H Yamada, S Lan
    OPTICS EXPRESS 13 (2) 460 - 467 1094-4087 2005/01 [Refereed][Not invited]
     
    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.
  • BS Shi, A Tomita
    JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS 21 (12) 2081 - 2084 0740-3224 2004/12 [Refereed][Not invited]
     
    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.
  • BS Shi, A Tomita
    JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS 21 (12) 2081 - 2084 0740-3224 2004/12 [Refereed][Not invited]
     
    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.
  • 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 0021-4922 2004/09 [Refereed][Not invited]
     
    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.
  • BS Shi, A Tomita
    OPTICS COMMUNICATIONS 235 (4-6) 247 - 252 0030-4018 2004/05 [Refereed][Not invited]
     
    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.
  • K Usami, A Tomita, K Nakamura
    INTERNATIONAL JOURNAL OF QUANTUM INFORMATION 2 (1) 101 - 117 0219-7499 2004/03 [Refereed][Invited]
     
    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.
  • A Tomita, K Nakamura
    INTERNATIONAL JOURNAL OF QUANTUM INFORMATION 2 (1) 119 - 131 0219-7499 2004/03 [Refereed][Not invited]
     
    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.
  • K Usami, Y Nambu, BS Shi, A Tomita, K Nakamura
    PHYSICAL REVIEW LETTERS 92 (11) 113601  0031-9007 2004/03 [Refereed][Not invited]
     
    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.
  • A Tomita, K Nakamura
    INTERNATIONAL JOURNAL OF QUANTUM INFORMATION 2 (1) 119 - 131 0219-7499 2004/03 [Refereed][Not invited]
     
    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.
  • K Usami, Y Nambu, BS Shi, A Tomita, K Nakamura
    PHYSICAL REVIEW LETTERS 92 (11) 0031-9007 2004/03 [Refereed][Not invited]
     
    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.
  • XB Wang, BS Shi, A Tomita, K Matsumoto
    PHYSICAL REVIEW A 69 (1) 014303  1050-2947 2004/01 [Refereed][Not invited]
     
    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.
  • Bao-Sen Shi, Akihisa Tomita
    Physical Review A - Atomic, Molecular, and Optical Physics 69 (1) 4  1094-1622 2004 [Refereed][Not invited]
     
    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.
  • A Tomita, K Nakamura
    Quantum Informatics 2004 5833 239 - 250 0277-786X 2004 [Refereed][Not invited]
     
    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.
  • BS Shi, A Tomita
    PHYSICAL REVIEW A 69 (1) 2469-9926 2004/01 [Refereed][Not invited]
     
    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.
  • H Kosaka, A Tomita, Y Nambu, T Kimura, K Nakamura
    ELECTRONICS LETTERS 39 (16) 1199 - 1201 0013-5194 2003/08 [Refereed][Not invited]
     
    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.
  • A Tomita
    NEC RESEARCH & DEVELOPMENT 44 (3) 290 - 293 0547-051X 2003/07 [Refereed][Not invited]
     
    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.
  • A Tomita
    NEC RESEARCH & DEVELOPMENT 44 (3) 282 - 284 0547-051X 2003/07 [Refereed][Not invited]
     
    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.
  • BS Shi, A Tomita
    PHYSICAL REVIEW A 67 (4) 2469-9926 2003/04 [Refereed][Not invited]
     
    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.
  • Bao-Sen Shi, Akihisa Tomita
    Physical Review A - Atomic, Molecular, and Optical Physics 67 (4) 4  1094-1622 2003 [Refereed][Not invited]
     
    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.
  • W Xiang-Bin, BS Shi, A Tomita, K Matsumoto
    QUANTUM INFORMATION AND COMPUTATION 5105 59 - 67 0277-786X 2003 [Refereed][Not invited]
     
    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.
  • BS Shi, A Tomita
    JOURNAL OF OPTICS B-QUANTUM AND SEMICLASSICAL OPTICS 4 (6) 380 - 382 1464-4266 2002/12 [Refereed][Not invited]
     
    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.
  • A Tomita, K Nakamura
    OPTICS LETTERS 27 (20) 1827 - 1829 0146-9592 2002/10 [Refereed][Not invited]
     
    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.
  • BS Shi, A Tomita
    PHYSICS LETTERS A 296 (4-5) 161 - 164 0375-9601 2002/04 [Refereed][Not invited]
     
    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.
  • XB Wang, M Keiji, T Akihisa
    PHYSICAL REVIEW LETTERS 87 (13) 137903  0031-9007 2001/09 [Refereed][Not invited]
     
    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.
  • Akihisa Tomita
    Physics Letters, Section A: General, Atomic and Solid State Physics 282 (6) 331 - 335 0375-9601 2001/04/30 [Not refereed][Not invited]
     
    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.
  • A Tomita, O Hirota
    JOURNAL OF OPTICS B-QUANTUM AND SEMICLASSICAL OPTICS 2 (6) 705 - 710 1464-4266 2000/12 [Refereed][Not invited]
     
    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.
  • Y Nambu, A Tomita, Y Chiba-Kohno, K Nakamura
    PHYSICAL REVIEW A 62 (1) 012312  2469-9926 2000/07 [Refereed][Not invited]
     
    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.
  • 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 0021-4922 2000/07 [Refereed][Not invited]
     
    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.
  • H Kosaka, A Tomita, T Kawashima, T Sato, S Kawakami
    PHYSICAL REVIEW B 62 (3) 1477 - 1480 1098-0121 2000/07 [Refereed][Not invited]
     
    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.
  • M Tokushima, H Kosaka, A Tomita, H Yamada
    APPLIED PHYSICS LETTERS 76 (8) 952 - 954 0003-6951 2000/02 [Refereed][Not invited]
     
    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].
  • H Kosaka, T Kawashima, A Tomita, T Sato, S Kawakami
    APPLIED PHYSICS LETTERS 76 (3) 268 - 270 0003-6951 2000/01 [Refereed][Not invited]
     
    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].
  • Hideo Kosaka, A. Tomita, T. Kawashima, T. Sato, S. Kawakami
    Phys. Rev. B 62 (3) 1477 - 1480 0163-1829 2000 [Refereed][Not invited]
  • Yoshihiro Nambu, Akihisa Tomita, Yoshie Chiba-Kohno, Kazuo Nakamura
    Physical Review A - Atomic, Molecular, and Optical Physics 62 (1) 11  1094-1622 2000 [Refereed][Not invited]
     
    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.
  • H Kosaka, T Kawashima, A Tomita, M Notomi, T Tamamura, T Sato, S Kawakami
    JOURNAL OF LIGHTWAVE TECHNOLOGY 17 (11) 2032 - 2038 0733-8724 1999/11 [Refereed][Not invited]
     
    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.
  • 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 0021-4922 1999/09 [Refereed][Not invited]
     
    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.
  • H Kosaka, T Kawashima, A Tomita, M Notomi, T Tamamura, T Sato, S Kawakami
    APPLIED PHYSICS LETTERS 74 (10) 1370 - 1372 0003-6951 1999/03 [Refereed][Not invited]
     
    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].
  • H Kosaka, T Kawashima, A Tomita, M Notomi, T Tamamura, T Sato, S Kawakami
    APPLIED PHYSICS LETTERS 74 (9) 1212 - 1214 0003-6951 1999/03 [Refereed][Not invited]
     
    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].
  • H Kosaka, T Kawashima, A Tomita, M Notomi, T Tamamura, T Sato, S Kawakami
    PHYSICAL REVIEW B 58 (16) 10096 - 10099 2469-9950 1998/10 [Refereed][Not invited]
     
    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].
  • 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 0021-4922 1998/07 [Refereed][Not invited]
     
    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.
  • KOSAKA H.
    Phys. Rev., B 58 (16) R10096 - R10099 0163-1829 1998 [Not refereed][Not invited]
  • H Yamazaki, A Tomita, M Yamaguchi, Y Sasaki
    APPLIED PHYSICS LETTERS 71 (6) 767 - 769 0003-6951 1997/08 [Refereed][Not invited]
     
    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.
  • A Tomita
    PHYSICS LETTERS A 233 (1-2) 115 - 120 0375-9601 1997/08 [Refereed][Not invited]
     
    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.
  • A Tomita, J Shah, RS Knox
    PHYSICAL REVIEW B 53 (16) 10793 - 10803 1098-0121 1996/04 [Refereed][Not invited]
     
    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.
  • A Tomita
    PHYSICAL REVIEW B 54 (8) 5609 - 5619 0163-1829 1996 [Not refereed][Not invited]
     
    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.
  • A TOMITA, J SHAH, JE CUNNINGHAM, SM GOODNICK, P LUGLI, SL CHUANG
    PHYSICAL REVIEW B 52 (7) 5445 - 5445 1098-0121 1995/08 [Refereed][Not invited]
  • A TOMITA
    JOURNAL OF APPLIED PHYSICS 77 (5) 2029 - 2031 0021-8979 1995/03 [Refereed][Not invited]
  • A TOMITA
    IEEE JOURNAL OF QUANTUM ELECTRONICS 30 (12) 2798 - 2802 0018-9197 1994/12 [Refereed][Not invited]
     
    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.
  • Y NAMBU, A TOMITA
    IEEE JOURNAL OF QUANTUM ELECTRONICS 30 (9) 1981 - 1994 0018-9197 1994/09 [Refereed][Not invited]
     
    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.
  • A TOMITA, J SHAH, JE CUNNINGHAM, SM GOODNICK, P LUGLI, SL CHUANG
    SEMICONDUCTOR SCIENCE AND TECHNOLOGY 9 (5) 449 - 452 0268-1242 1994/05 [Refereed][Not invited]
     
    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.
  • A TOMITA, J SHAH, JE CUNNINGHAM, SM GOODNICK, P LUGLI, SL CHUANG
    PHYSICAL REVIEW B 48 (8) 5708 - 5711 0163-1829 1993/08 [Not refereed][Not invited]
     
    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.
  • JI SHIMIZU, A KIMURA, K NANIWAE, M NIDO, S MURATA, A TOMITA, A SUZUKI
    ELECTRONICS LETTERS 29 (7) 579 - 581 0013-5194 1993/04 [Refereed][Not invited]
     
    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.
  • S MURATA, A TOMITA, A SUZUKI
    IEEE PHOTONICS TECHNOLOGY LETTERS 5 (1) 16 - 19 1041-1135 1993/01 [Refereed][Not invited]
     
    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.
  • S MURATA, K NANIWAE, J SHIMIZU, M NIDO, A TOMITA, A SUZUKI
    ELECTRONICS LETTERS 28 (15) 1456 - 1457 0013-5194 1992/07 [Refereed][Not invited]
     
    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.
  • J SHIMIZU, H YAMADA, S MURATA, A TOMITA, M KITAMURA, A SUZUKI
    IEEE TRANSACTIONS ON ELECTRON DEVICES 38 (12) 2698 - 2698 0018-9383 1991/12 [Refereed][Not invited]
  • S MURATA, A TOMITA, J SHIMIZU, A SUZUKI
    IEEE PHOTONICS TECHNOLOGY LETTERS 3 (11) 1021 - 1023 1041-1135 1991/11 [Refereed][Not invited]
     
    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.
  • J SHIMIZU, H YAMADA, S MURATA, A TOMITA, M KITAMURA, A SUZUKI
    IEEE PHOTONICS TECHNOLOGY LETTERS 3 (9) 773 - 776 1041-1135 1991/09 [Refereed][Not invited]
     
    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 TOMITA, A SUZUKI
    IEEE JOURNAL OF QUANTUM ELECTRONICS 27 (6) 1630 - 1641 0018-9197 1991/06 [Refereed][Not invited]
     
    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.
  • S MURATA, A TOMITA, J SHIMIZU, M KITAMURA, A SUZUKI
    APPLIED PHYSICS LETTERS 58 (14) 1458 - 1460 0003-6951 1991/04 [Refereed][Not invited]
     
    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.
  • S MURATA, A TOMITA, J SHIMIZU, A SUZUKI
    IOOC-ECOC 91, PTS 1-4 85 - 88 1991 [Refereed][Not invited]
  • A TOMITA, Y KOHGA, A SUZUKI, T TERAKADO, A AJISAWA
    APPLIED PHYSICS LETTERS 55 (18) 1817 - 1819 0003-6951 1989/10 [Refereed][Not invited]
  • A TOMITA, A SUZUKI, K KOMATSU
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 28 (8) 1523 - 1524 0021-4922 1989/08 [Refereed][Not invited]
  • A TOMITA, A SUZUKI
    IEEE JOURNAL OF QUANTUM ELECTRONICS 23 (7) 1155 - 1159 0018-9197 1987/07 [Refereed][Not invited]
  • A TOMITA, A SUZUKI
    OPTICAL AND QUANTUM ELECTRONICS 19 S75 - S82 0306-8919 1987/07 [Refereed][Not invited]
  • A TOMITA, T TERAKADO, A SUZUKI
    JOURNAL OF APPLIED PHYSICS 59 (6) 1839 - 1842 0021-8979 1986/03 [Refereed][Not invited]

Books etc

  • 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 (Contributor第2章第6節)
    情報機構 2019/02
  • 量子情報工学
    富田 章久 (Single work)
    森北出版 2017/02
  • 基礎からの量子光学
    富田 章久 (Joint work量子暗号鍵配布の安全性理論)
    オプトロニクス社 2009
  • 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 (Joint work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
  • Quantum Computation and Information - From Theory to Experiment,; Topics in Applied Physics, Vol. 102
    TOMITA Akihisa (Joint workRealization of Quantum Information System)
    Springer Verlag 2006
  • 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 (Joint work第2編第8章 量子暗号)
    オーム社 2004

Conference Activities & Talks

MISC

Industrial Property Rights

  • 特願2016-176364: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  2016年/09/09
    吉野 健一郎, 藤原 幹生, 佐々木 雅英, 富田 章久
  • 特願2016-173063: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  2016年/09/05
    富田 章久, 中田 賢祐
  • 特願2016-019552: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  2016年/02/04
    富田 章久, 中田 賢祐
  • US8509446 B2:Time-bin polarization format exchange technique for entangled optical source    2013/08/13
    Mikio Fujiwara, Morio Toyoshima, Masahide Sasaki, Akihisa Tomita, Yoshihiro Nambu, Ken-ichiro Yoshino
  • 富田 章久  日本電気株式会社  201103060240174690
  • 富田 章久  日本電気株式会社  201103007133642631
  • 富田 章久  日本電気株式会社  201103008312629253
  • 富田 章久  日本電気株式会社  201103039566009770
  • 富田 章久  日本電気株式会社  201103005292204391
  • 富田 章久  日本電気株式会社  201103014597701840
  • 富田 章久  日本電気株式会社  200903006245770115
  • 富田 章久  日本電気株式会社  201103069385240558
  • 富田 章久  光技術研究開発株式会社  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
    特許4132860号
  • 量子回路および量子計算機
    特許4030838号
  • 量子回路
    特許第3981969号
  • 単一光子発生装置
    特許第3422482号
  • 光変調器
    特許第2513265号
  • アバランシエフオトダイオート
    特許第2100015号
  • 光ファイバ分散補償方法
    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
    特許第2976919号

Research Grants & Projects

  • Ministry of Internal Affairs and Communications：ICT重点技術の研究開発プロジェクト
    Date (from‐to) : 2020/09 -2025/03 
    Author : Akihisa Tomita
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C)
    Date (from‐to) : 2021/04 -2024/03 
    Author : 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.
  • Cabinet office of Japan：Cross-ministerial Strategic Innovation Promotion Program
    Date (from‐to) : 2018/09 -2023/03 
    Author : Akihisa Tomita
  • 日本学術振興会：科学研究費助成事業（基盤研究（S))
    Date (from‐to) : 2018/07 -2023/03 
    Author : 富田 章久
  • Cabinet Office, Government of Japan：ImPACT
    Date (from‐to) : 2015/04 -2019/03 
    Author : TOMITA Akihisa
  • 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
    NICT：contract research project
    Date (from‐to) : 2010 -2015 
    Author : Tamaki Kiyoshi
  • 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）
    Date (from‐to) : 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
  • Single Photon detection, Generation, Measurement, Control of quantum states, Optical quantum gates
    JST Basic Research Programs (Exploratory Research for Advanced Technology :ERATO)

Educational Activities

Teaching Experience

  • Advanced Optoelectronics
    開講年度 : 2021
    課程区分 : 修士課程
    開講学部 : 情報科学院
    キーワード : 半導体レーザ、光検出、光通信、量子通信、非線形光学、高調波発生、パラメリック発振、光誘起屈折率効果
  • Advanced Optoelectronic Devices
    開講年度 : 2021
    課程区分 : 修士課程
    開講学部 : 情報科学研究科
    キーワード : 半導体レーザ、光検出、光通信、量子通信、非線形光学、高調波発生、パラメリック発振、光誘起屈折率効果
  • Advanced Optoelectronics
    開講年度 : 2021
    課程区分 : 博士後期課程
    開講学部 : 情報科学院
    キーワード : 半導体レーザ、光検出、光通信、量子通信、非線形光学、高調波発生、パラメリック発振、光誘起屈折率効果
  • Advanced Optoelectronic Devices
    開講年度 : 2021
    課程区分 : 博士後期課程
    開講学部 : 情報科学研究科
    キーワード : 半導体レーザ、光検出、光通信、量子通信、非線形光学、高調波発生、パラメリック発振、光誘起屈折率効果
  • Environment and People
    開講年度 : 2021
    課程区分 : 学士課程
    開講学部 : 全学教育
    キーワード : 未来社会，情報科学，エレクトロニクス，情報セキュリティ，コンピュータ，コミュニケーション，知識と知能，人と医療，仮想世界，エネルギー，微小世界
  • Quantum Mechanics
    開講年度 : 2021
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 重ね合わせ，測定，物理量，確率解釈，2準位系の量子力学，シュレーディンガー方程式，波動関数，固有値と固有関数，不確定関係，1次元ポテンシャル問題，調和振動子
  • Exercise in Electrical and Electronic Engineering II
    開講年度 : 2021
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 演算子，期待値，交換関係，測定，角運動量，スピン

Campus Position History

  • 2020年4月1日 
    - 
    2022年3月31日 
    大学院情報科学研究院副研究院長

Position History

  • 2020年4月1日 
    - 
    2022年3月31日 
    大学院情報科学研究院副研究院長

Committee Membership

  • 2021/04 - Today   NICT   革新的情報通信技術研究開発委託研究評価委員
  • 2019 -2021   OSA   Advanced Photonics Congress Program Comitee

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