János Bergou

The discrimination of coherent states is a crucial component in quantum communication with continuous variables, especially in quantum key distribution protocols, which rely on the ability to distinguish among different coherent states to establish a shared secret key between two parties. In this work, we propose and analyze a novel strategy for distinguishing among N phase-symmetric coherent states, which optimally takes unambiguous discrimination (UD) to the deterministic regime, at the inevitable cost of having nonzero probability of error. Despite the disturbance introduced by the UD process, we show that for N > 2, the “failure” states of UD still retain residual information about the original input states, which can be further used for discrimination. Rather than discarding inconclusive outcomes, we show that the “failure” states of UD can be optimally recycled by performing a sequential minimum-error (ME) measurement. This strategy, which we call information recycling, combines benefits from both ME and optimal UD: It always provides conclusive results while allowing for a subset of those results to be error-free, which are identifiable by an ancillary system. Our results broaden the possibilities of sequential discrimination strategies for continuous-variable quantum states and motivate the investigation of applications to quantum communication protocols.