| 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 concrete example: I shall take as a source a three–level atom in the so called Λ configuration; although simple, such a system shows, when suitably stimulated by lasers, an interesting behaviour: the so called electron–shelving effect (or quantum jumps) [13, 9] |
