Field Trial of Differential-phase-shift Quantum Key Distribution Using Polarization Independent Frequency Up-conversion Detectors


Toshimori Honjo1, Shuto Yamamoto2, Takashi Yamamoto2,
Hidehiko Kamada1, Yoshiki Nishida3, Osamu Tadanaga3, Masaki Asobe3, and Kyo Inoue4
1Optical Science Laboratory, 2NTT Network Innovation Laboratories,
3NTT Photonics Laboratories, 4Osaka University/NTT Research Professor

  Quantum key distribution (QKD) has been studied as a way to realize unconditionally secure communications. We had been intensively working on differential-phase-shift QKD (DPS-QKD) experiment where randomly phase-modulated coherent pulse stream was used [1, 2]. Recently, we performed a field trial of DPS-QKD using polarization independent frequency up-conversion detectors to show the feasibility of our QKD scheme [3].
  In this experiment, a sender generated a 1-GHz coherent pulse stream, and each pulse was randomly phase-modulated by 0,π. The pulse was attenuated to 0.2 photons per pulse and then transmitted to a receiver's site over the 17.6-km installed fiber. At the receiver's site, the pulse stream was input into a planar lightwave circuit Mach-Zehnder interferometer. The output ports of the interferometer were connected to the polarization independent up-conversion detectors. Figure 1 shows the setup of our detector. A signal pulse (photon) was input into a polarization beam splitter (PBS), which split the polarization of the incoming photon into horizontally and vertically polarized pulses. The horizontally polarized pulse was directly input into a 50:50 coupler. The vertically polarized pulse was input into a fiber delay line as a horizontally polarized pulse. After 50:50 coupler, horizontally polarized pulses were combined with a strong pump light, and injected into a periodically poled lithium niobate (PPLN) waveguide. In the PPLN waveguide, a 600 nm photon was generated via the sum frequency generation (SFG) process. After suppressing the pump, the SFG photon was detected with a single photon counting module (SPCM) based on a Si-APD. The detected signals were input into a time interval analyzer to record the photon detection events.
  With this setup, we performed a long-term stability test. We successfully demonstrated stable operation for 6 hours and achieved a sifted key generation rate of 120 kbps and an average quantum bit error rate of 3.14 %, which revealed the feasibility of our QKD scheme.
  This research was supported in part by National Institute of Information and Communications Technology of Japan.

[1] K. Inoue, et al., Phys. Rev. A 68 (2003) 022317.
[2] H. Takesue, et al., New J. Phys. 7 (2005) 232.
[3] T. Honjo, et al., Opt. Express 15 (2007) 15920.

Fig. 1. Polarization independent frequency up-conversion detector.

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