Large negative and positive delay of optical pulses in coherently   prepared dense Rb vapor with buffer gas
  We experimentally study the group time delay for a light pulse propagating through hot Rb vapor in the presence of a strong coupling field in a $\Lambda$ configuration. We demonstrate that the ultra-slow pulse propagation is transformed into superluminal propagation as the one-photon detuning of the light increases due to the change in the transmission resonance lineshape. Negative group velocity as low as -c/10^6=-80 m/s is recorded. We also find that the advance time in the regime of the superluminal propagation grows linearly with increasing laser field power. 
Eugeniy E. Mikhailov, Vladimir A. Sautenkov, Irina Novikova, George R.   Welch