Long-Term Performance of Quantum Key Distribution over 90-km Optical Links Installed in a Field Environment of Tokyo Metropolitan Area

Kaoru Shimizu, Kiyoshi Tamaki, and Toshimori Honjo*
Optical Science Laboratory, *NTT Secure Platform Laboratories

　The long-term operation performance, robustness and feasibility, of a differential phase shift (DPS) quantum key distribution (QKD) system to provide a long-distance key distribution service over optical fiber networks was examined by a series of field experiments that were carried out using a test-bed optical network in the Tokyo metropolitan area: Tokyo QKD network (Fig. 1) [1]. In spite of a 30-dB transmission loss and 90-km propagation along an optical link installed in a field environment, the stable generation of a secure key was demonstrated for 25 days in a completely free-run operation with a 1-GHz clock DPS-QKD system employing a weak coherent light pulse and superconducting single photon detectors (SSPDs) [2]. The average sifted key generation rate and the quantum bit error rate (QBER) were 11 kbps and 2.6 % (Fig. 2(a)), respectively. The average generation rate of secure key after error correction and privacy amplification was 1.1 kbps with a fluctuation of ±0.5 kbps (Fig. 2(b)). Here, we defined the secure key as the final key which was proven to be secure against general individual attacks.
　A comparison with meteorological data revealed the way in which the field environment influenced the key distribution performance. The secure key generation was considerably stable unless a strong wind disturbed the optical link which was composed of both aerial and underground regions with the approximately same total length. With the current system, some imperfections in photonic devices still limited the achievable performance levels, for example, the temporal bit error deterioration and the finite operation period. Some technical solutions were also provided to cope with these remaining problems.
　This work was carried out in fruitful collaboration with the National Institute of Information and Communications Technology (NICT).


Fig. 1. Experimental configuration, (a) Setup of 90-km optical link and (b) Setup of DPS-QKD system.	Fig. 2. Experimental (a) QBER and (b) secure key generation rate for 25 days / 1 hour average.