Theory of light detection in the presence of feedback

The clarity of understanding associated with open-loop photodetection configurations does not extend to closed-loop systems in which there is a feedback path leading from the output of the detector to the light beam at the detector input. In this paper, the authors develop the quantum and semiclassical theories of light detection for closed-loop configurations. The authors demonstrate that the unmistakable signatures of nonclassical light associated with open-loop detection do not carry over to closed-loop systems. They use these results to provide quantum and semiclassical explanations for two recent closed-loop experiments in which sub-Poison photocount statistics were produced