Negative group velocity of a light pulse in cesium vapour

Electromagnetically induced absorption in cesium vapour causes an extremely strong anomalous dispersion, leading to the propagation of radiation at a low negative group velocity V_g. As a result, a resonance light pulse appears from the medium before than a nonresonance pulse. The advance time measured in the experiment corresponds to the group velocity V_g ≈ -c/3600, which is an order of magnitude lower than that reported by Wang L.J., et al. Nature, 406, 277 (2000).