Introduction 682 1. Interference transitions between states of nonradiating systems 682 2. Effect of radiation on interference transitions 684 3. Beats 687 4. Experimental applications of beats 689 5. Interference effects in level crossing 693 6. Modulation 696 References 698
When an atom, in a linear superposition of states, absorbs or emits light, interference can occur between two different absorption or emission amplitudes, giving rise to observable physical effects, such as level crossing resonances, quantum beats, and dark states. Early experimental demonstrations of effects of this kind, using an optical detection of the atomic internal state, are briefly reviewed. In fact, such interference effects are still playing a fundamental role in modern developments, such as laser cooling and trapping. We present in this paper a few examples of such applications, including the mechanical Hanle effect, velocity selective resonances for an atom moving in a standing wave and subrecoil laser cooling by Velocity Selective Coherent Population Trapping (VSCPT).
We study the effect of quantum interference on the structure and properties of spontaneous and stimulated transitions in a degenerate V-type three-level atom with an arbitrary total momentum of each state. Explicit expressions for the factors in the terms of the relaxation operator and stimulated transition operator with account of quantum interference effects are obtained. It has been demonstrated that the condition for the dipole transition moments to be parallel is insufficient and for conventional atoms the interference cross terms are zeros for both operators. The conditions when quantum interference influences the properties of the relaxation operators have been analysed. For the stimulated transitions this condition is shown to be anisotropy of the photon field interacting with the atoms. This case is minutely studied for the D-line of alkali metals.
Decoherence is the main process behind the quantum to classical transition. It is a purely quantum mechanical effect by which the system loses its ability to exhibit coherent behavior. The recent experimental observation of diffraction and interference patterns for large molecules raises some interesting questions. In this context, we identify possible agents of decoherence to take into account when modeling these experiments, and study their visible (or not) effects on the interference pattern. We thereby present an analysis of matter wave interferometry in the presence of a dynamic quantum environment, and study how much the visibility fringe is reduced and in which timescale the decoherence effects destroy the interference of massive objects. Finally, we apply our results to the experimental data reported on fullerenes and cold neutrons.