Characterising higher-order phase correlations in gain-switched laser sources with application to quantum key distribution

TL;DR

This paper investigates phase correlations in gain-switched laser sources used in quantum key distribution, proposing experimental methods to assess their security implications due to multi-photon emission correlations.

Contribution

It introduces experimental schemes to characterize phase correlations in gain-switched lasers and links these correlations to security assessments in QKD implementations.

Findings

Residual phase correlations affect security in high-rate gain-switched lasers

Interferometric measures can quantify higher-order phase correlations

Optimizing these measures helps determine the security level of QKD systems

Abstract

Multi-photon emissions in laser sources represent a serious threat for the security of quantum key distribution (QKD). While the decoy-state technique allows to solve this problem, it requires uniform phase randomisation of the emitted pulses. However, gain-switched lasers operating at high repetition rates do not fully satisfy this requirement, as residual photons in the laser cavity introduce correlations between the phases of consecutive pulses. Here, we introduce experimental schemes to characterise the phase probability distribution of the emitted pulses, and demonstrate that an optimisation task over interferometric measures suffices in determining the impact of arbitrary order correlations, which ultimately establishes the security level of the implementation according to recent security proofs. We expect that our findings may find usages beyond QKD as well.