Real-time phase-reference monitoring in a quasi-optimal coherent-state receiver

The Kennedy-like receiver is a quasi-optimal discrimination scheme employed in binary phase-shift-keyed communication schemes with coherent states. In its standard configuration, it is based on the interference of the two signals encoding the message with a reference local oscillator and on the measurement by means of ON/OFF detectors. Here we demonstrate that, without interrupting the communication, it is possible to monitor the relative phase between the signals and the local oscillator by applying a Bayesian processing to the very data sample used to discriminate the signals at any shot. We show, both numerically and experimentally, that the minimum uncertainty in phase estimation can be achieved both with ON/OFF and photon-number resolving detectors. The performances of our phase-estimation method in the presence of either uniform phase noise or phase diffusion are also investigated and discussed.