Abstract:
We measure phase-dependent fluorescence spectra for an elementary system: long-lived coherently driven two-level atoms in an atomic beam. Phase-dependent fluorescence spectra measure quadrature noise in the atomic radiation field. These noise spectra are obtained using a homodyne detection scheme that suppresses excess noise by subtracting signals from two identically prepared atomic samples. Noise spectra are obtained for atomic radiation which is in phase [Formula Presented] and out of phase [Formula Presented] with a resonant driving field, as well as [Formula Presented] out of phase with an off-resonant driving field. The measured phase-dependent fluorescence spectra are much richer than ordinary fluorescence spectra, and exhibit many novel features. Particularly interesting are phase-dependent noise spectra for off-resonant excitation. These strikingly exhibit direct manifestations of time ordering, which appear as large differences between the measured [Formula Presented] and [Formula Presented] quadrature noise spectra. The measured noise spectra are in excellent agreement in magnitude and shape with the results of a quantum treatment using no free parameters. © 1998 The American Physical Society.
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