Anomalous dispersion and negative group velocity in a coherence-free cold atomic medium

We have observed the propagation of an approximately 35 ns long light pulse with a negative group velocity through a laser-cooled 85 Rb atomic medium. The anomalous dispersion results from linear atom–light interaction and is unrelated to long-lived ground-state coherences often associated with fast light in atomic media. The observed negative group velocity (− c /360) in the Rb magneto-optical trap for a pulse attenuated by less than 50% is in good agreement with the value of dispersion measured independently by a rf heterodyne method. The spectral region of anomalous dispersion is between 15 and 40 MHz, which is an order of magnitude wider than that typically associated with ground-state coherences.