Differential-phase-shift quantum key distribution experiment

Toshimori Honjo and Kyo Inoue
Optical Science Laboratory

 Quantum key distribution (QKD) has been studied as a way to realize unconditionally secure communications. We proposed a new QKD scheme called a differential-phase-shift QKD (DPS-QKD). This scheme has several advantages including suitability for fiber transmission. This study demonstrates a feasibility of DPS-QKD by using a glass waveguide interferometer.
 Fig. 1 shows the experimental setup. The signal source was an external-cavity laser diode(λ=1551 nm). Cw light from the laser was converted into a pulse stream by an intensity modulator, and then was randomly phase-modulated for each pulse by {0,π}. The pulse width was 125 ps and the repetition rate was 1GHz. The light power was attenuated to be 0.1 photons per pulse. After 20-km fiber transmission (4.46-dB propagation loss), the light passed through a glass waveguide Mach-Zehnder interferometer packaged with fiber ports. The path-length difference was 20 cm, which introduced one-bit delay of 1 ns at 1 Gbit/s. The waveguide is made of silica glass by using PLC (planar lightwave circuit) technology, and the excess loss of the interferometer was only 2.64dB. The polarization dependence was small, such that the extinction ratio of the interferometer ranged from 0.27% to 0.46% when the input polarization state was changed. The two outputs from the interferometer were received by APDs gated at 5 MHz. After the photon transmission, Bob told Alice the photon arriving time, from which a key was created. Since APDs had time jitter, the arrival time of photons vacillated and errors could be induced. To reduce this timing error, Bob took data within a time window, at the expense of the key generation rate. Fig. 2 shows the raw key generation rate and quantum bit error rate (QBER) for several time windows, where the polarization state was set to give the worst QBER. A sufficient QBER was obtained to create a secret key after error correction and privacy amplification.
 In summary, a differential-phase-shift QKD experiment was carried out using a glass waveguide Mach-Zehnder interferometer. Stable polarization-insensitive operation was demonstrated in 20-km fiber transmission. The key creation rate was 3076bit/s with a 5.0% QBER.
[1] K. Inoue, E. Waks and Y. Yamamoto, Phys. Rev. A 67, (2003) 022317.
[2] T. Honjo, K. Inoue and H. Takahashi, Opt. Lett. 29, (2004) 2797.
Fig. 1. Experimental setup
Fig. 2. Experimental results

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