PRECISION-MEASUREMENT OF PHASE-DEPENDENT RESONANCE FLUORESCENCE-SPECTRA

We measure phase-dependent fluorescence spectra for an elementary syst em: long-lived coherently driven two-level atoms in an atomic beam. Ph ase-dependent fluorescence spectra measure quadrature noise in the ato mic radiation field. These noise spectra are obtained using a hemodyne detection scheme that suppresses excess noise by subtracting signals from two identically prepared atomic samples. Noise spectra are obtain ed for atomic radiation which is in phase (0 degrees) and out of phase (90 degrees) with a resonant driving field, as well as +/-45 degrees out of phase with an off-resonant driving field. The measured phase-de pendent fluorescence spectra an much richer than ordinary fluorescence spectra, and exhibit many novel features. Particularly interesting ar e phase-dependent noise spectra for off-resonant excitation. These str ikingly exhibit direct manifestations of time ordering, which appear a s large differences between the measured 45 degrees and -45 degrees qu adrature noise spectra. The measured noise spectra are in excellent ag reement in magnitude and shape with the results of a quantum treatment using no free parameters. [S1050-2947(98)09002-7].