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.Comment: 18 pages, 10 figures, and 2 tables. Appendices included. Additional
references added. Comments welcome
