Quasi-unambiguous state discrimination for coherent states with phase fluctuation

This paper describes unambiguous state discrimination (USD) for a sequence of coherent states with fluctuating phase, while considering eavesdropping against differential-phase-shift quantum key distribution systems. A measurement system extracts a fraction of the incoming signal of coherent states, estimates the carrier phase from that fraction, and then conducts a USD measurement on the main signal based on the estimated carrier phase. In such a system, the estimated carrier phase can be different from a true value because of the phase fluctuation, and an error can occur in the USD measurement because of the error in the estimated phase. The USD performance in such a situation is formalized and simulated.