Linear superpositions of states in radiative processes

Abstract

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).