Phase-dependent Resonance Fluorescence Spectra For Two-level Atoms

Zhao Zhao, Lǔ, Thomas Thomas

H. Z. Zhao, Zhou Lu, A. M. Bacon +2 more

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 $(0\ifmmode^\circ\else\textdegree\fi{})$ and out of phase $(90\ifmmode^\circ\else\textdegree\fi{})$ with a resonant driving field, as well as $\ifmmode\pm\else\textpm\fi{}45\ifmmode^\circ\else\textdegree\fi{}$ 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 $45\ifmmode^\circ\else\textdegree\fi{}$ and $\ensuremath{-}45\ifmmode^\circ\else\textdegree\fi{}$ 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.

H. Z. Zhao, Zhou Lu, John E. Thomas

We measure phase-dependent fluorescence spectra of two-level atoms driven by a monochromatic laser field, for off-resonance excitation. Pairs of spectra with phases of opposite sign are found to display striking differences that arise entirely from time ordering.

H. Walther

Constantine Mavroyannis

Zhou Lu, Samir Bali, John E. Thomas

We measure squeezing spectra for phase-dependent fluorescence of coherently driven two-level atoms in free space. The measured spectra are in excellent agreement with our predictions for a single two-level atom. We find that maximum squeezing occurs for homodyne detection at a phase near ±45° relative to the exciting field, rather than in the in- or out-of-phase quadratures. A new physical picture of phase-dependent noise that incorporates quantum collapses into a Bloch vector model shows why this occurs.