Quantum Key Distribution over 200 km of Fiber

Quantum Key Distribution over 200 km of Fiber
　
Hiroki Takesue¹, Sae Woo Nam², Qiang Zhang³, Robert H. Hadfield²,
Toshimori Honjo¹, Kiyoshi Tamaki¹, and Yoshihisa Yamamoto³
¹Optical Science Laboratory, ²NIST, ³Stanford University
　　It is important to establish the technologies to increase key distribution distance and key rate for realizing practical quantum key distribution (QKD) systems. We have recently demonstrated a 200-km QKD, which set the world record of key distribution distance, using a 10-GHz clock system and superconducting single photon detectors (SSPD) [1].
　　Figure 1 shows the system configuration, in which the differential phase shift (DPS) protocol is implemented [2]. At Alice’s site, a continuous light is modulated into 10-GHz clock pulses using a high-speed intensity modulator, and the phase of each pulse is randomly modulated by {0, π}. Then, the pulses are attenuated so that the average photon number per pulse becomes 0.2, and sent to Bob through an optical fiber. Bob inputs the pulse train into a 1-bit delayed interferometer with which he can measure the phase differences of adjacent pulses: if the phase difference is 0 (π), the photon output from port 1 (2) and is detected by SSPD1 (2). Then, Bob informs Alice the time instances in which he observed photons through a conventional communication line. As a result, Alice and Bob share the phase difference information at those time slots, which can be converted to "keys" for one-time pad cryptography.
　　The working principle of the SSPD is explained as follows. When a photon hits a current-biased superconducting NbN nanowire, it "breaks" the superconductivity, and a macroscopic voltage pulse is generated. By discriminating the voltage pulse, we can detect the arrival of the photon. Although the current quantum efficiency is relatively small (about 1 %), the SSPD has very low dark count rate (about 10 Hz), and so is suitable for long-distance QKD. In addition, the SSPD can detect the 10-GHz clock signal without suffering from errors due to inter-bit interference, thanks to its good timing resolution (60 ps).
　　Figure 2 shows the obtained secure key rate as a function of the fiber length. Here, the secure key rate was calculated based on a security model considering general individual attacks [3]. We successfully distributed secure keys over 200 km of fiber. In addition, we obtained 17-kbit/s secure key rate at 105 km of fiber, which is two orders of magnitude larger than the previous bit-rate record at 100 km of fiber.
　　This research was supported in part by CREST program of Japan Science and Technology Agency and National Institute of Information and Communications Technology of Japan.
[1] H. Takesue, et al., Nature Photonics 1 (2007) 343.
[2] K. Inoue, E. Waks, and Y. Yamamoto, Phys. Rev. Lett. 89 (2002) 037902.
[3] E. Waks, H. Takesue, and Y. Yamamoto, Phys. Rev. A. 73 (2006) 012344.

Fig. 1. System configuration.

Fig. 2. Experimental result.

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