Quantum relaxation processes spontaneous emission

The rapid development of lasers has led to the publication of increasing numbers of papers concerned this year with such subjects as superfluorescence and co-operative radiation processes,451 the thermodynamics of co-operative luminescence,452 saturation, collisional dephasing, and quenching of fluorescence of organic vapours in intense laser excitation studies,453 a theoretical model for fluorescence in gases subjected to continuous i.r. excitation,454 a quantum treatment of spontaneous emission from strongly driven two-level atoms,455 the development of site-selection spectroscopy,45 and measurements of relaxation times 457 using laser excitation. 

Quantum-state decay to a continuum or changes in its population via coupling to a thermal bath is known as amplitude noise (AN). It characterizes decoherence processes in many quantum systems, for example, spontaneous emission of photons by excited atoms [35], vibrational and collisional relaxation of trapped ions [36] and the relaxation of current-biased Josephson junctions [37], Another source of decoherence in the same systems is proper dephasing or phase noise (PN) [38], which does not affect the populations of quantum states but randomizes their energies or phases. 

So far we have neglected the fact that the levels a) and b) are not only coupled by transitions induced by the external field but may also decay by spontaneous emission or by other relaxation processes such as collision-induced transitions. We can include these decay phenomena in our formulas by adding phenomenological decay terms to (2.68), which can be expressed by the decay constant ya and yt (Fig. 2.17). A rigorous treatment requires quantum electrodynamics [2.23]. 

Equation (5.7) gives a general expression for the radiation force on an atom moving in a laser field. From a quantum mechanical point of view, the radiation force (5.7) arises as a result of the quantum mechanical momentum exchange between the atom and the laser field in the presence of spontaneous relaxation. The change in the atomic momentum comes from the elementary processes of photon absorption and emission stimulated absorption, stimulated emission, and spontaneous emission. The radiation force (5.7) is a function of the coordinates and velocity of the center of mass of the atom. 

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