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

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We report an experimental observation of third-order nonlinear absorption by quantum interference in (87)Rb atoms cooled and confined in a magneto-optic trap. A coupling laser creates electromagnetically induced transparency (EIT) in a multilevel Rb system in which the third-order nonlinear absorption is enhanced by constructive quantum interference while the linear absorption is inhibited by destructive interference. Our experiment demonstrates the EIT system proposed by Harris and Yamamoto [Phys. Rev. Lett. 81, 3611 (1998)], which absorbs two photons but not one photon in the dressed-state picture.

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