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

TL;DR

This paper reports the observation of negative group velocity in a cold atomic medium caused by linear atom-light interaction, independent of ground state coherences, with potential implications for light manipulation.

Contribution

It demonstrates negative group velocity in a cold atomic medium without relying on long-lived ground state coherences, expanding understanding of fast light phenomena.

Findings

Negative group velocity of -c/360 observed

Anomalous dispersion spans 15-40 MHz, wider than typical

Good agreement between measured dispersion and theoretical predictions

Abstract

We have observed the propagation of an approximately 35 ns long light pulse with a negative group velocity through a laser-cooled 85Rb 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 an 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.