Angel Valle

Quantum key distribution (QKD) protocols leverage quantum mechanics to achieve information theoretically secure communication, yet real-world implementations must address experimental limitations, particularly phase correlations in weak coherent laser pulses (WCPs). High-speed gain-switching lasers, commonly used in QKD, can exhibit residual photons causing phase correlations between consecutive pulses, challenging the perfect phase randomization assumption crucial for the decoy-state BB84 protocol. Theoretical work has proposed security proofs that require knowledge of how closely each phase's probability distribution approximates uniformity, which is complex to estimate experimentally. In this study we introduce an experimental method to characterise phase correlations of any length under realistic conditions by modelling the phase generation process within the laser cavity. Additionally, we experimentally benchmark this practical routine for measuring second-order correlations using a double Michelson interferometer with tunable amplitude attenuators, allowing comprehensive characterisation of the phase generation process and accurate measurement of the phase probability distribution, thus enhancing the security of QKD systems.