1. Polarisation squeezing

In a recent experiment, a linearly polarized beam interacts quasi-resonantly with the atoms. It follows the first experimental generation of a squeezed field state produced with cold atoms [1,2]. In this first experiment, a circularly polarized incident light interacted on the cesium D2 line (852nm). The atoms acting as a Kerr medium, the fluctuations of the field exiting the cavity were squeezed by about 40% below the Standard Quantum Limit ("shot noise").

In the new experiment, the linearly polarized incident light is affected by self-rotation effects when passing through the atoms, and polarization instabilities appear: the intracavity light polarization may switch from linear to circular [3]. This "polarization switching" phenomenon was first observed in hot sodium vapors [4].

More interesting in terms of quantum information is what happens to the outgoing field: not only the mean field mode is squeezed, but also the orthogonally polarized modes, which is therefore in a "squeezed vacuum state" [5].

This is equivalent to a "polarization squeezed" beam [6], for which one of the four quantum Stokes operators describing the polarization state of the beam is squeezed. The quantum properties of the polarization of light are currently intensively studied in connection with quantum information protocoles in the continuous variable regime.

[1] A. Lambrecht, T. Coudreau, A.M. Steimberg, E. Giacobino, Eur. Phys. Lett. 36, 93 (1996)

[2] A.Z. Khoury, T. Coudreau, C. Fabre, E. Giacobino, Phys. Rev. A 57, 4770 (1998)

[3] V. Josse, A. Dantan, A. Bramati, M. Pinard, E. Giacobino, J. Opt. B: Quantum and Semiclass. 5, S1 (2003) [quant-ph/0302142]

[4] E. Giacobino, Opt. Comm. 56, 249 (1985)

[5] V. Josse, A. Dantan, L. Vernac, A. Bramati, M. Pinard, E. Giacobino, Phys. Rev. Lett. 90, 103601 (2003) [quant-ph/0304022]

[6] N. Korolkova, G. Leuchs, R. Loudon, T.C. Ralph, C. Silberhorn, Phys. Rev. A 65, 052306 (2003) [quant-ph/0108098]