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The question is studied as to which state a radiating atom has to be reset to if a photon is detected and absorbed. This is important for applications of a recently proposed approach to photon counts from a radiating atom which is based upon repeated gedanken measurements and associated reductions as a technical tool. The general reset density matrix is determined. It may depend on the atomic state shortly before the photon detection, and may thus be time dependent. It may also have off-diagonal terms which may lead to interesting coherence effects. It is shown that the repeated measurements together with the reset matrix lead to the optical Bloch equations for a general N-level atom. The general photon-counting distribution for an N-level radiating atom is derived and the axiomatic continuous-measurement theory of Srinivas and Davies Opt. Acta 28, 981 (1981); 29, 235 (1982) for quantum-counting processes is shown to follow in the present framework from ordinary quantum mechanics.
Gerhard C. Hegerfeldt (Fri,) studied this question.