Quantum interference effects in the Autler-Townes spontaneous spectrum

The quantum interference effects in the Autler-Townes spectrum associated with the spontaneous emission between two levels in an atomic system are studied. It is shown that when the upper level is coupled by a coherent field to a third level, then the interference between two processes of decay from two dressed upper levels to the same lower level results in a dark line in the spectrum. The presence of the dark line is independent of the Rabi frequency of the coherent field. By increasing the detuning of the coherent field with the atomic transition, one of the two peaks in the Autler-Townes doublet becomes narrower while its height remains constant. On the other hand, when the lower level is coupled by a coherent field to another level, there is no interference, and consequently no dark line is present. In this case, by increasing the detuning, the height of one of the two peaks is decreased while its width remains constant.