Einstein's theory of special relativity and the principle of causality1,2,3,4 imply that the speed of any moving object cannot exceed that of light in a vacuum (c). Nevertheless, there exist various proposals5,6,7,8,9,10,11,12,13,14,15,16,17,18 for observing faster-than- c propagation of light pulses, using anomalous dispersion near an absorption line4,6,7,8, nonlinear9 and linear gain lines10,11,12,13,14,15,16,17,18, or tunnelling barriers19. However, in all previous experimental demonstrations, the light pulses experienced either very large absorption7 or severe reshaping9,19, resulting in controversies over the interpretation. Here we use gain-assisted linear anomalous dispersion to demonstrate superluminal light propagation in atomic caesium gas. The group velocity of a laser pulse in this region exceeds c and can even become negative16,17, while the shape of the pulse is preserved. We measure a group-velocity index of ng = -310(±5); in practice, this means that a light pulse propagating through the atomic vapour cell appears at the exit side so much earlier than if it had propagated the same distance in a vacuum that the peak of the pulse appears to leave the cell before entering it. The observed superluminal light pulse propagation is not at odds with causality, being a direct consequence of classical interference between its different frequency components in an anomalous dispersion region.
We thank R. A. Linke for several stimulating discussions. We thank D. K. Walter, W. Happer, J. A. Giordmaine, D. J. Chadi, S. A. Solin, R. Y. Chiao, S. E. Harris and E. S. Polzik for helpful discussions. We thank E. B. Alexandrov and N. P. Bigelow for the use of the paraffin-coated caesium cells.