Discrimination of multiple nonorthogonal states beyond the quantum noise limit
Quantum mechanics sets fundamental limits on how well we can distinguish different states of a physical system with intrinsic quantum noise. This noise sets the ultimate sensitivity limit for conventional coherent communication with coherent states of light, referred to as the quantum-noise limit (QNL). Helstrom’s realization that quantum measurements can beat the conventional limits of detection for state discrimination  triggered many theoretical efforts to investigate feasible measurements for nonorthogonal coherent states surpassing the QNL and approaching the ultimate limits allowed by quantum mechanics. Such quantum measurements for states of light at the single-photon level have large potential for enhancing security in quantum communication, and can optimize information transfer in low-power classical communication. I will discuss our theoretical and experimental work in optimized discrimination strategies for receivers surpassing the QNL for multiple coherent states. These receivers are based on optimized adaptive measurements and single-photon counting, and achieve sensitivities surpassing the QNL at low input-power levels under realistic conditions with noise and loss. These discrimination measurements offer potential advantages for enhancing information transfer in communications beyond the limits of coherent detection.
 Helstrom, C. W. Quantum Detection and Estimation Theory, Mathematics in Science and Engineering 123 (Academic Press, 1976).