Limits of phase recovery in CV-QKD systems: an experimental analysis

Phase recovery is a key enabling process in continuous-variable quantum key distribution (CV-QKD) systems operating at ultra-low optical power, where phase noise from the finite linewidths of the transmitter and local oscillator lasers critically impacts system performance. Here, we investigate the performance of a phase recovery scheme for a prepare-and-measure CV-QKD setup using the Viterbi–Viterbi carrier phase estimation algorithm. We experimentally evaluate the scheme by varying the power of a binary phase shift keying (BPSK)-modulated pilot tone, using laser sources with nominal linewidths of 1 kHz and 25 kHz. Our results reveal a well-defined operational regime in which stable phase recovery is achieved, as well as a critical threshold below which pilot tone power becomes insufficient, leading to signal distortion and phase recovery failure. As expected, narrower-linewidth lasers exhibit improved phase stability and reduced variance, confirming the combined impact of pilot tone power and laser linewidth on phase recovery performance. These findings provide practical guidelines for CV-QKD receiver synchronization.