Quantum limitations on the detection of coherent and incoherent signals

In order to discover what effect the quantum nature of radiation has on the detection of signals at optical frequencies, the detectability of coherent and, in the threshold limit, incoherent signals is calculated on the basis of an idealized quantum-mechanical model of a receiver. The detectabilities reduce at low frequencies to those found by ordinary signal-detection theory; at high frequencies they depend on Planck's constant. For incoherent signals of high frequency the threshold detector is equivalent to a system that focuses the radiation on a photoelectric surface of unit quantum efficiency and optimally processes the emitted electrons.