Seminar
Tuesday, 26 October, 2010
Group Seminar MPQ: Spontaneous Emission-Induced Frequency Shifts of Ground-State Quantum Beats
Tuesday, 26.10.2010 10 a.m. (s.t.) in Herbert-Walther-lecture room, MPQ Garching
David G. Norris, Department of Physics, University of Maryland and NIST
Quantum beats in spontaneous emission illustrate the fundamental role of interference in the quantum mechanics of atoms interacting with light. The beats occur when emission from the excited state proceeds along parallel paths, but only in the absence of "which-path" information. Thus, they occur commonly as excited-state beats, when two upper states decay simultaneously to a single lower state, but not as ground-state beats, when a single upper state decays to a superposition of two lower states. We demonstrate an atomic system in which ground-state beats do occur in the two-photon emission probability and are recovered by a conditional measurement of the intensity, which erases the "which-path" information. This system has allowed the measurement of a novel source of near-resonant light shifts, a coherent analogue of the “light shifts due to real transitions” observed by Cohen-Tannoudji, which have the striking effect of exactly reversing the sign of the normal AC-Stark shifts. We systematically characterize these shifts and relate them to a recently-published decoherence mechanism in Rayleigh scattering. Work supported by the National Science Foundation.