Evaluation of the phase randomness of the light source in quantum key distribution systems with an attenuated laser

TL;DR

This study examines the phase randomness of laser sources in high-speed quantum key distribution systems, demonstrating that proper laser operation ensures security at 10 GHz despite potential phase correlation issues.

Contribution

It provides experimental and theoretical analysis confirming secure laser operation conditions for high-speed QKD systems at 10 GHz.

Findings

Low interference visibility when laser is driven far below threshold

Interference emerges near the laser threshold

Secure decoy BB84 implementation is feasible at 10 GHz with proper laser conditions

Abstract

The phase randomized light is one of the key assumptions in the security proof of Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol implemented with an attenuated laser. Though the assumption has been believed to be satisfied for conventional systems, it should be reexamined for current high speed QKD systems. The phase correlation may be induced by the overlap of the optical pulses, the interval of which decreases as the clock frequency. The phase randomness was investigated experimentally by measuring the visibility of interference. An asymmetric Mach-Zehnder interferometer was used to observe the interference between adjacent pulses from a gain-switched distributed feedback laser diode driven at 10 GHz. Low visibility was observed when the minimum drive current was set far below the threshold, while the interference emerged when the minimum drive current was close to the threshold. Theoretical evaluation on the impact of the imperfect phase randomization provides target values for the visibility to guarantee the phase randomness. The experimental and theoretical results show that secure implementation of decoy BB84 protocol is achievable even for the 10-GHz clock frequency, by using the laser diode under proper operating conditions.