On the Quantum Theory of Direct Detection

Abstract

By using the theory of measurements continuous in time in quantum mechanics [1][8], a photon detection theory has been formulated [9]– [12]; see Refs. [10]– [12] and [8] for detailed references. A quantum source as an atom, an ion or a more complicated system, eventually placed inside an optical cavity, is stimulated by lasers or by a thermal bath. The emitted light is detected by photon counters (direct detection), possibly after interference with a reference laser beam (heterodyne and homodyne detection). Just to illustrate detection theory, in this paper I shall present only counting processes [1], [3]– [12] (direct detection). Moreover, I shall consider only a Λ concrete example: I shall take as a source a three-level atom in the so called A configuration; although simple, such a system shows, when suitably stimulated by lasers, an interesting behaviour: the so called electron-shelving effect (or quantum jumps) [9,13].