Gain-assisted superluminal light propagation

Abstract

Einstein's theory of special relativity and the principle of causality imply that the speed of any moving object cannot exceed that of light in a vacuum (c). Nevertheless, there exist various proposals for observing faster-than-c propagation of light pulses, using anomalous dispersion near an absorption line, nonlinear and linear gain lines, or tunnelling barriers. However, in all previous experimental demonstrations, the light pulses experienced either very large absorption or severe reshaping, 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 negative, while the shape of the pulse is preserved. We measure a group-velocity index of n(g) = -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.

Group velocity control in the ultraviolet domain via interacting dark-state resonances

“…In Ref. [13], gain-assisted superluminal light propagation was observed in a cesium vapor cell while in most other studies, superluminal light propagation is accompanied by considerable absorption. Sub-and superluminal light propagation together with nonlinear optical gain or losses were observed in [22].…”

“…The study of such pulse propagation phenomena has been triggered by a series of papers by Sommerfeld and Brillouin [2,3] and continues to be of much interest [4,5,6,7,8,9]. It is well known that the group velocity of a light pulse can be slowed down [10,11], can become faster than its value c in vacuum, or can even become negative [12,13]. Note that superluminal light propagation with group velocity larger than c cannot transmit information faster than the vacuum speed of light [14], such that it is not at odds with causality.…”

Narrowband Biphotons: Generation, Manipulation, and Applications

“…Now it has been well accepted that both phase velocity and group velocity of light in a dispersive medium can exceed c, the speed of light in vacuum [60,61]. Then, what is the information velocity of light?…”

Controllable Kerr Nonlinearity via Incoherent Pump Fields in Semiconductor Quantum Wells

“…For further discussion in linear optical properties of the medium, we introduce the group index n g = c v g , where c is the speed of light in vacuum and the group velocity is given by [49,50,51]:…”