  Coherent states of light, and methods for distinguishing between them, are
central to all applications of laser light. We obtain the ultimate quantum
limit on the error probability exponent for discriminating among any M
multimode coherent-state waveforms via the quantum Chernoff exponent in M-ary
multi-copy state discrimination. A receiver, i.e., a concrete realization of a
quantum measurement, called the Sequential Waveform Nulling (SWN) receiver, is
proposed for discriminating an arbitrary coherent-state ensemble using only
auxiliary coherent-state fields, beam splitters, and non-number-resolving
single photon detectors. An explicit error probability analysis of the SWN
receiver is used to show that it achieves the quantum limit on the error
probability exponent, which is shown to be a factor of four greater than the
error probability exponent of an ideal heterodyne-detection receiver on the
same ensemble. We generalize the philosophy of the SWN receiver, which is
itself adapted from some existing coherent-state receivers, and propose a
receiver -- the Sequential Testing (ST) receiver-- for discriminating n copies
of M pure quantum states from an arbitrary Hilbert space. The ST receiver is
shown to achieve the quantum Chernoff exponent in the limit of a large number
of copies, and is remarkable in requiring only local operations and classical
communication (LOCC) to do so. In particular, it performs adaptive copy-by-copy
binary projective measurements. Apart from being of fundamental interest, these
results are relevant to communication, sensing, and imaging systems that use
laser light and to photonic implementations of quantum information processing
protocols in general.
