ARE THERE QUANTUM BEATS FROM VACUUM-INDUCED COHERENCE

In a recent paper by Hegerfeldt and Plenio [1] it has been argued that for a certain class of V-level systems quantum beats could be observed even if the system was not in a coherent superposition of the upper levels. The beat frequency would be different to the upper level splitting. In their analysis, this was an effect of the quantum vacuum field which generated a coherent superposition of the upper levels in the course of the interaction. Their derivation is reanalyzed here using standard techniques which lead to a master-equation. Calculating the weak probe absorption spectrum, one finds that - as expected - the experimentally accessible, physical levels are the eigenstates of the combined system consisting of the atom and the vacuum which by definition do not couple. Thus, a coherent superposition will not be generated starting from a true, renormalized upper state and therefore quantum beating will only occur if an initial coherent superposition of renormalized upper levels is provided.