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Physical Review Letters

Observation of Squeezing in the Phase-Dependent Fluorescence Spectra of Two-Level Atoms

Zhou Lu, Samir Bali, John E. Thomas

Published Oct 26, 1998• 55 citations
Publisher

Abstract

We observe squeezing in the phase-dependent fluorescence spectra of two-level atoms that are coherently driven by a near-resonant laser field in free space. In contrast to previous predictions that emphasized the in- and out-of-phase quadratures, we find that maximum squeezing occurs for homodyne detection at a phase near $\ifmmode\pm\else\textpm\fi{}{45}^{\ensuremath{\circ}}$ relative to the exciting field. A new physical picture of phase-dependent noise is developed that incorporates quantum collapses into a Bloch vector model and yields a very simple form for the complete squeezing spectrum.

DOI
10.1103/physrevlett.81.3635

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arXiv:quant-ph/9812069•Physical Review Letters•Mar 22, 1999•45 citations
We observe squeezing in the phase-dependent fluorescence spectra of two-level atoms that are coherently driven by a near-resonant laser field in free space. In contrast to previous predictions that emphasized the in- and out-of-phase quadratures, we find that maximum squeezing occurs for homodyne detection at a phase near \ifmmode±\else\textpm\fi45\ensuremath∘\ifmmode\pm\else\textpm\fi{}{45}^{\ensuremath{\circ}}\ifmmode±\else\textpm\fi45\ensuremath∘ relative to the exciting field. A new physical picture of phase-dependent noise is developed that incorporates quantum collapses into a Bloch vector model and yields a very simple form for the complete squeezing spectrum.
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.
Phase-sensitive squeezing in the resonance fluorescence of two-level atoms, that are coherently driven by a near-resonant laser field in free space, was observed recently (Lu et al 1998 Phys. Rev .L ett. 81 3635). This was accomplished via homodyne detection at a phase near ±45 ◦ relative to the driving field for strong off-resonant excitation of ‘long-lived’ atoms (where th ea tomic lifetime far exceeded the laser–atom interaction time, meaning that relaxation effects could be ignored). On the other hand, traditional theoretical predictions of phase-sensitive squeezing in the resonance fluorescence from two-level atoms have emphasized in- and out-of-phase (i.e., 0 ◦ and 90 ◦ ) quadratures, and weak, on-resonant excitation of ‘short-lived’ atoms (where the observation time for laser–atom interaction fa re xceeded the natural atomic lifetime, meaning that relaxation effects dominate). Here, we calculate the probability of a delayed-coincidence detection in the interference field of a fluorescing dipole with a local oscillator (LO). We show that, despite the strikingly different conditions in which squeezing occurs in short- and long-lived atoms, squeezing in both cases can be shown to arise from a joint detection of two photons which are related by a quantum jump in the following way: the first photodetection precipitates a quantum jump of the atom to the ground state, and the second measures the mean amplitude of the fluorescent field subsequent to the quantum jump.
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.
The quadrature squeezing spectra produced in the resonance fluorescence of a two-level atom driven by a strong coherent field and a weak amplitude-fluctuating field are investigated. For far-off-resonance and strong excitation, the resonance fluorescence exhibits two-mode squeezing around the Rabi sideband frequencies. Thus we have a source of two mode squeezing that is very easily frequency tuned simply by adjusting the generalized Rabi frequency. The presence of the strength of the stochastic process reduces the degree of squeezing obtainable. The quadrature squeezing spectra are also effected by the relative phase of the two fields.
The squeezing spectrum of the fluorescent light is investigated for a laser-driven three-level atom of the $\ensuremath{\Lambda}$ configuration when quantum interference of the decay channels is accounted for. We show that when the two atomic transitions contribute to the detected fluorescence field, squeezing at certain frequency intervals is obtained in both the weak- and the high-Rabi-frequency regimes even for equally decay rates of the transitions. Unlike in two-level atoms in free space, squeezing can be obtained in both the in-phase and out-of-phase quadrature spectra although in different spectral regions. We also show that the squeezing spectrum can be controlled by an adequate selection of the Rabi frequencies and atomic detunings. Another remarkable effect is that squeezing can be achieved with proper relative phases of the driving fields. We provide an analytical description in the dressed basis which accounts for the main features of the squeezing spectra obtained from the numerical work.