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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.
Yan et al. (Wed,) studied this question.