Observations of absorptive photon switching and suppression of two-photon absorption in cold atoms

Atomic coherence and interference in an atomic medium exhibiting electromagnetically induced transparency may lead to enhancement or suppression of nonlinear susceptibilities. Absorptive photon switching has been observed by constructive quantum interference, which is based on the enhanced third-order, nonlinear absorption in a four-level system. In a different four-level system, suppression of the two-photon absorption by destructive quantum interference has been observed. Experiments were carried out on ⁸⁷Rb atoms cooled and confined in a magneto-optical trap and the experimental results agree with theoretical calculations of simple four-level model systems.