Constructive and destructive features of multiphoton quantum interferences in atomic systems

One-photon coupling of the upper level of an atomic transition to a metastable state can generate a dark line in the atom’s emission spectrum. We show in this paper that if such a coupling is replaced by a dominant two-photon coupling, the spectrum is dynamically narrowed instead. The dominant two-photon coupling is achieved here by coupling these two levels to a common broad intermediate state using two laser fields. We also show that if one exchanges the metastable state with the broad one in this system, the one-photon coupling of all coupled states becomes dominant, causing the generation of a dynamically damped dark line effect. We explain these effects in terms of constructive and destructive quantum interferences between a direct and a three-photon Ramam path in the former case, and between a direct and a two-photon Raman path in the latter case. Additionally, we show how these interfering paths provide elements of coherent control of the atomic emission spectra, in general.