Quantum limited detectors for weak classical signals

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Abstract
When a classical signal is very weak, the quantum features of its detector cannot be ignored. There appears then to be a conflict between the continuous nature of the classical signal and the discrete spectrum of a quantum device. Moreover, the final output cannot be read directly from a quantum system: The latter has to be ‘‘measured’’ by another device (the ‘‘meter’’) which then yields another classical signal—a real number. This paper examines the amount of distortion caused by the presence of a quantum interface between two classical signals. It is shown that the meter should have a moderate resolution, so as to lump together numerous levels of the detector. A finer resolution deteriorates the correspondence between the input and output signals. A perfect resolution, down to isolated eigenvalues, may completely lock the output signal (this is the quantum Zeno effect).