Phase-controlled quantum interference in two-color atomic photoionization

We report evidence of an interference process between two bound-free atomic transitions induced by laser fields. Sodium atoms, initially prepared in the 3P1/2 excited state, are ionized by a nonresonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The experiment has provided a fringe visibility as high as 42%.