Multiphoton quantum interference in precision spectroscopic experiments

Modern resonance spectroscopic experiments on the measurement of transition frequencies in atoms have reached a level where a meticulous description of all aspects of the studied processes has become obligatory. The precision achieved has led to the fact that the determination of the transition frequency on the basis of measured data is substantially refined by theoretical treatment of the observed spectral line profile. Thus, a large impact of effects arising beyond the resonance approximation, in particular due to the effect of quantum interference, is found experimentally. We show that the picture becomes even more complicated when the observed spectral line profile is "identified" with one of the processes—emission or absorption. An accurate determination of the transition frequency requires a description of the absorption line profile inseparable from the emission process, and vice versa. The theoretical aspects discussed in this paper create prerequisites for more accurate experiments.