Propagation of a squeezed optical field in a medium with superluminal group velocity

We investigated the propagation of a squeezed optical field, generated via the polarization self-rotation (PSR) effect, with a sinusoidally-modulated degree of squeezing through an atomic medium with anomalous dispersion. We observed the advancement of the signal propagating through a resonant Rb vapor compared to the reference signal, propagating in air. The measured advancement time grew linearly with atomic density, reaching a maximum of $11 \pm 1~\mu$s, which corresponded to a negative group velocity of $v_g\approx - 7,000~$m/s. We also confirmed that the increasing advancement was accompanied by a reduction of output squeezing levels due to optical losses, in good agreement with theoretical predictions.