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

TLDR

The study reports a long‑term field trial of 1‑GHz clock differential phase‑shift QKD integrated into Tokyo’s metropolitan optical fiber network. The experiment used a 90‑km loop‑back fiber link with 30‑dB loss, superconducting single‑photon detectors, and detailed bit‑error analysis to evaluate performance. Stable, maintenance‑free operation was achieved for 25 days, yielding an average sifted key rate of 11 kb/s (QBER 2.6 %) and a secure key rate of 1.1 kb/s (±0.5 kb/s), with performance variations linked to meteorological conditions.

Abstract

This paper reports experimental results for a long-term field trial of 1-GHz clock differential phase shift quantum key distribution that was incorporated in the test-bed optical fiber network installed in the Tokyo metropolitan area. A photon transmitter and a photon receiver were connected to a 90-km loop-back optical link with a 30-dB loss. Superconducting single photon detectors were employed to detect photons with a low dark count rate. Stable maintenance-free operation was achieved for 25 days, where the average sifted key generation rate and the quantum bit error rate were 11 kb/s and 2.6%, respectively. The average generation rate of a secure key after error correction and privacy amplification was 1.1 kb/s with a fluctuation of ± 0.5 kb/s. The achieved performance was examined using detailed bit error analysis. A comparison with meteorological data revealed the way in which the field environment influenced the key distribution performance.