LINEAR SUPERPOSITIONS OF STATES IN RADIATIVE PROCESSES

When an atom, in a linear superposition of states, absorbs or emits li ght, interference can occur between two different absorption or emissi on amplitudes, giving rise to observable physical effects, such as lev el crossing resonances, quantum beats, and dark states. Early experime ntal demonstrations of effects of this kind, using an optical detectio n of the atomic internal state, are briefly reviewed. In fact, such in terference effects are still playing a fundamental role in modern deve lopments, such as laser cooling and trapping. We present in this paper a few examples of such applications, including the mechanical Hanle e ffect, velocity selective resonances for an atom moving in a standing wave and subrecoil laser cooling by Velocity Selective Coherent Popula tion Trapping (VSCPT).