  A scheme for the detection of low-intensity optical coherent signals was
studied which uses a probabilistic amplifier operated in the non-heralded
version, as the underlying non-linear operation to improve the detection
efficiency. This approach allows us to improve the statistics by keeping track
of all possible outcomes of the amplification stage (including failures). When
compared with an optimized Kennedy receiver, the resulting discrimination
success probability we obtain presents a gain up to ~1.85% and it approaches
the Helstrom bound appreciably faster than the Dolinar receiver, when employed
in an adaptive strategy. We also notice that the advantages obtained can be
ultimately associated with the fact that, in the high gain limit, the
non-heralded version of the probabilistic amplifier induces a partial dephasing
which preserves quantum coherence among low energy eigenvectors while removing
it elsewhere. A proposal to realize such transformation based on an optical
cavity implementation is presented.
