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Physical Review A

Conditions for anomalous resonance fluorescence in a squeezed vacuum

S. Swain, Peng Zhou

Published Dec 1, 1995• 32 citations
Publisher

Abstract

We discuss the conditions which give rise to anomalous features in the resonance fluorescence of a two-level atom interacting with a resonant classical field and a squeezed vacuum. A simple expression is obtained, which is shown to coincide closely with the conditions for amplification of a probe beam and with the collapse of the atomic system into a pure state. We thereby illustrate striking observable consequences of the evolution of a driven system into a pure state. This situation also provides a method for preparing an atom in a pure state whose nature depends on the squeezing phase and Rabi frequency. The conditions for hole burning are obtained analytically, and the sensitivity of these effects to the two-photon correlations produced by the squeezed vacuum is demonstrated.

DOI
10.1103/physreva.52.4845

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We discuss the conditions which give rise to anomalous features in the resonance fluorescence of a two-level atom interacting with a resonant classical field and a squeezed vacuum. A simple expression is obtained, which is shown to coincide closely with the conditions for amplification of a probe beam and with the collapse of the atomic system into a pure state. We thereby illustrate striking observable consequences of the evolution of a driven system into a pure state. This situation also provides a method for preparing an atom in a pure state whose nature depends on the squeezing phase and Rabi frequency. The conditions for hole burning are obtained analytically, and the sensitivity of these effects to the two-photon correlations produced by the squeezed vacuum is demonstrated.
As a realistic system for observing the distinctive features of the interaction of squeezed light with atoms, we consider a single, coherently driven, two-level atom in a resonant optical cavity coupled to a broadband squeezed vacuum field. In the bad cavity limit we derive equations of motion for the atomic operators that are identical to those for the free-space situation, except for having modified parameters. We focus our attention on the resonance fluorescence from this system and show that, in the above limit, many of the unusual spectral features previously reported for the free atom situation persist. We also confirm the link between these anomalous spectra and the collapse of an atom into a pure state. The cavity parameter values needed to verify some of these interesting effects appear to be within the range of present day technology. \textcopyright{} 1996 The American Physical Society.
The squeezing properties of the fluorescence field emitted by a two-level atom driven by a coherent laser field in a squeezed vacuum are calculated. We show that in the region of the anomalous resonance fluorescence the emitted field exhibits squeezing that is much larger than that in the input squeezed vacuum. The squeezing spectrum attains a minimum value that corresponds to 75% squeezing. We also find that, in the total fluorescence field, squeezing attains an optimum achievable value in the fluorescence field emitted by a two-level atom. The optimum squeezing is associated with the collapse of the system into a pure state.
It is shown that 100100%100 squeezed output can be produced in the resonance fluorescence from a coherently driven two-level atom interacting with a squeezed vacuum. This is only possible for N=1/8N=1/8N=1/8 squeezed input, and is associated with a pure atomic state, i.e., a completely polarized state. The quadrature for which optimal squeezing occurs depends on the squeezing phase $\ensuremath{\Phi}$, the Rabi frequency $\ensuremath{\Omega}$, and the atomic detuning $\ensuremath{\Delta}$. Pure states are described for arbitrary $\ensuremath{\Phi}$, not just $\ensuremath{\Phi}=0$ or $\ensuremath{\pi}$ as in previous work. For small values of N, there may be a greater degree of squeezing in the output field than the input, i.e., we have squeezing amplification.
We show that the resonance fluorescence of a two-level atom interacting with a squeezed vacuum can show a wide variety of anomalous features (including hole burning at line center) over a continuous but very narrow range of parameter values. The spectra have a variety of profiles, all of which are qualitatively different from those observed in normal resonance fluorescence and are a distinctive manifestation of the presence of the nonclassical field. The anomalous features persist in a cavity environment, which provides the best means for their experimental observation.
The steady-state resonance fluorescence spectrum of a two-level atom driven by a bichromatic field in a broadband squeezed vacuum is studied. When the carrier frequency of the squeezed vacuum is tuned to the frequency of the central spectral line, anomalous spectral features, such as hole burning and dispersive profiles, can occur at the central line. We show that these features appear for wider, and experimentally more convenient, ranges of the parameters than in the case of monochromatic excitation. The absence of a coherent spectral component at the central line makes any experimental attempt to observe these features much easier. We also discuss the general features of the spectrum. When the carrier frequency of the squeezed vacuum is tuned to the first odd or even sidebands, the spectrum is asymmetric and only the sidebands are sensitive to phase. For appropriate choices of the phase the linewidths of only the odd or even sidebands can be reduced. A dressed-state interpretation is provided.
We report unusual profiles in the resonance fluorescence spectrum of a two-level atom interacting with a resonant classical field of intermediate intensity and a squeezed vacuum. These have a dispersive profile, quite unlike any previously described, and are a distinctive feature of the interaction of atomic systems with nonclassical fields. An approximate analytic expression is obtained which explains these features, and it is shown that they arise from atomic correlations not probed in normal resonance fluorescence.