Polarization Squeezing by Optical Faraday Rotation

Abstract

We show that it is possible to generate continuous-wave fields and pulses of polarization squeezed light by sending classical, linearly polarized laser light twice through an atomic sample which causes an optical Faraday rotation of the field polarization. We characterize the performance of the process, and we show that an appreciable degree of squeezing can be obtained under realistic physical assumptions.PACS numbers: 32.80.Qk; 42.50.Dv Highly squeezed states of light are valuable for numerous quantum information protocols and high precision metrology . Several techniques exist for the generation of squeezed states. In [1] the non-linear phase evolution due to the Kerr effect in an optical fiber was used to produce 5.1 dB of polarization squeezing. Cold atomic samples in high-finesse cavities cause similar Kerr-like effects, and 1.5 dB of quadrature squeezing [2] and ∼ 0.5 dB of polarization squeezing [3] has been observed. The most well established technique to date, however, is to use non-linear crystals in very good cavities (e.g. ∼ 7 dB in [4]). In this Letter we show, however, that by reflecting a light beam so that it interacts twice through the off-resonant Faraday-type interaction with an atomic sample, a simple, robust, and efficient source of strongly squeezed light is obtained, which may well outperform the above mentioned schemes.The optical Faraday rotation of light passing through a spin polarized atomic medium has been applied in a number of recent experiments to demonstrate entanglement and squeezing of atomic samples [5,6], atomic quantum memories for light [7], and teleportation of quantum states between light and matter [8]. In these experiments, the collective atomic population distribution on internal states is a quantum degree of freedom, and the measurement of the polarization of the light after the interaction alters the collective atomic quantum state. Measurements play an important role in the experiments reported in [5,6,7,8], but it has also been shown that multiple interactions between a light pulse and an atomic medium [9,10,11,12] suffice to effectively couple atoms and fields e.g. resulting in interspecies entanglement, atomic squeezing, or transfer of quantum states.Our proposed physical set-up is sketched in Figure 1. In part a) we depict an atomic gas, spin polarized perpendicular to the plane of the figure, through which a cw beam or a pulse of linearly polarized light is transmitted twice from different directions. In part b) of the figure we suggest an implementation with two oppositely polarized samples which are both traversed twice by the laser beam. As will become clear below, the desired dynamics arises solely in the limit in which the light field passes through the atoms from both directions simultaneously. Figure 1: Physical setup for production of squeezed light. In panel a), a coherent cw beam or pulse of linearly polarized light is transmitted through a spin polarized atomic gas, its polarization is rotated, and it is transmitted through the gas a second time...

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