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Density-matrix equations of motion derived from Heisenberg picture source-field quantum electrodynamics contain damping constants and frequency shifts which are not associated with a single radiative transition but which instead couple two separate transitions together. In the absence of degeneracies in the atomic levels, these coupling terms are normally neglected when the rotating-wave approximation is invoked. These terms cannot be neglected when there are degeneracies or near degeneracies. In this paper, we interpret the generalized frequency shifts, show how they can be renormalized and included in shifts of atomic energy levels and altered damping constants, and discuss their experimental significance.
Cardimona et al. (Sun,) studied this question.
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