Observation of absorptive photon switching by quantum interference

We report an experimental demonstration of photon switching by quantum interference in a four-level atomic system proposed by Harris and Yamamoto [Phys. Rev. Lett. 81, 3611 (1998)]. Quantum interference inhibits single-photon absorption but enhances third-order, two-photon-type absorption in the four-level system. We have observed greatly enhanced nonlinear absorption in the four-level system realized with cold Rb-87 atoms and demonstrated fast switching of the nonlinear absorption with a pulsed pump laser.