  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 87Rb atoms and demonstrated fast
switching of the nonlinear absorption with a pulsed pump laser.
