Linear optics and photodetection achieve near-optimal unambiguous coherent state discrimination

Coherent states of the quantum electromagnetic field, the quantum description of ideal laser light, are prime candidates as information carriers for optical communications. A large body of literature exists on their quantum limited estimation and discrimination. However, very little is known about the practical realizations of receivers for unambiguous state discrimination (USD) of coherent states. Here we fill this gap and outline a theory of USD with receivers that are allowed to employ: passive multimode linear optics, phase space displacements, auxiliary vacuum modes, and on-off photon detection. Our results indicate that, in some regimes, these currently available optical components are typically sufficient to achieve near-optimal unambiguous discrimination of multiple, multimode coherent states.