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arXiv:quant-ph/0302142 · Journal of Optics B Quantum and Semiclassical Optics

Polarization squeezing in a four-level system

Vincent Josse, Aurélien Dantan, Alberto Bramati, M. Pinard, E. Giacobino

Published Aug 1, 2003• 13 citations
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Abstract

We present a theoretical study of an ensemble of X-like four-level atoms placed in an optical cavity driven by a linearly polarized field. We show that the self-rotation (SR) process leads to polarization switching (PS). Below the PS threshold, both the mean field mode and the orthogonal vacuum mode are squeezed. We provide a simple analysis of the phenomena responsible for the squeezing and trace the origin of vacuum squeezing not to SR, but to a crossed Kerr effect induced by the mean field. Last, we show that this vacuum squeezing can be interpreted as polarization squeezing .

DOI
10.1088/1464-4266/5/4/358

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We present a theoretical study of an ensemble of X-like four-level atoms placed in an optical cavity driven by a linearly polarized field. We show that the self-rotation (SR) process leads to polarization switching (PS). Below the PS threshold, both the mean field mode and the orthogonal vacuum mode are squeezed. We provide a simple analysis of the phenomena responsible for the squeezing and trace the origin of vacuum squeezing not to SR, but to a crossed Kerr effect induced by the mean field. Last, we show that this vacuum squeezing can be interpreted as polarization squeezing .
We study the interaction of a nearly resonant linearly polarized laser beam with a cloud of cold cesium atoms in a high finesse optical cavity. We show theoretically and experimentally that the cross-Kerr effect due to the saturation of the optical transition produces quadrature squeezing on both the mean field and the orthogonally polarized vacuum mode. An interpretation of this vacuum squeezing as polarization squeezing is given and a method for measuring quantum Stokes parameters for weak beams via a local oscillator is developed.
We theoretically study quadrature and polarization squeezing in dispersive optical bistability through a vectorial Kerr cavity model describing a nonlinear cavity filled with an isotropic ${\ensuremath{\chi}}^{(3)}$ medium in which self-phase- and cross-phase-modulation, as well as four-wave mixing, occur. We derive expressions for the quantum fluctuations of the output field quadratures as a function of which we express the spectrum of fluctuations of the output-field Stokes parameters. We pay particular attention to study how the bifurcations affecting the non-null linearly polarized output mode squeezes the orthogonally polarized vacuum mode, and show how this produces polarization squeezing.
Summary form only given. We suggest a promising way to obtain the squeezing of vacuum fluctuations. We study the interaction of linearly polarized light with atoms in a double-/spl Lambda/ configuration, where the two lower levels are Zeeman sublevels of the atomic ground state and the upper levels are the hyperfine components of the atomic excited state. In such systems, Kerr nonlinearity associated with the ac-Stark effect causes self-rotation of the elliptical polarization. At the same time when the linearly polarized light traverses such a medium, a squeezed vacuum is produced in the orthogonal polarization.
Polarization squeezing [1] is attracting much attention in the last few years because of its potential applications in quantum information. We reconsider here the analysis of quantum fluctuations in dispersive optical bistability by generalizing the standard model in order to take into account fluctuations in the two orthogonal polarization components of the intracavity field, and not only in the pumped one as it is customary. In other words, we consider a relatively simple model for a vectorial Kerr cavity: The Kerr nonlinearity is the responsible for dispersive bistability, and two extra nonlinear terms, describing cross-phase modulation and four-wave mixing, couple the intense (pumped) intracavity field (with polarization parallel to that of the field injected in the cavity) with the orthogonally polarized mode. The weight of the new nonlinearities is controlled by the Maker-Terhune coefficients, which verily A + B!2 = 1 for an isotropic material, which we consider. This system exhibits a very rich variety of bifurcations [2], In this communication we describe the simplest results.
When linearly polarized light propagates through a medium in which elliptically polarized light would undergo self-rotation, squeezed vacuum can appear in the orthogonal polarization. A simple relationship between self-rotation and the degree of vacuum squeezing is developed. Taking into account absorption, we find the optimum conditions for squeezing in any medium that can produce self-rotation. We then find analytic expressions for the amount of vacuum squeezing produced by an atomic vapor when light is near-resonant with a transition between various low-angular-momentum states. Finally, we consider a gas of multilevel Rb atoms, and analyze squeezing for light tuned near the D lines under realistic conditions.
We present experimental and theoretical analysis of quantum fluctuation in a vacuum field in the presence of orthogonal linearly polarized pump field propagating through a Rb vapor cell. Previously reported theoretical and experimental studies provided somewhat contradictory conclusions regarding the possibility to observe the "squeezed vacuum" -- the reduction of vacuum fluctuations below standard quantum limit -- in this system. Here, using the D1 transitions of Rb in a cell without buffer as as an example, we demonstrate that vacuum squeezing is corrupted by incoherent processes (such as spontaneous emission, elastic scattering, etc.), and its observation is only possible in a specific small region of the experimental parameter space. Numerical simulations, in good agreement with the experiment, demonstrate that the two excited state hyperfine levels play a crucial role in the squeezing and excess noise production. The significant influence of far-detuned atoms on the field fluctuations at low noise frequencies imposes the explicit consideration of the full velocity distribution of the atomic vapor.