Abstract
Anomalous dispersion cannot occur in a transparent, passive medium where electromagnetic radiation is being absorbed at all frequencies, as pointed out by Landau and Lifshitz. Here we show, using gain-assisted linear anomalous dispersion, that superluminal pulse propagation can occur in a transparent medium when gain is present. Superluminal light pulse propagation is observed at a negative group velocity of −c/315 through an atomic medium with little pulse distortion. Experimentally, a pulse propagating through the atomic vapor cell has its peak reaching the exit side before entering it, resulting in a negative transit time. This counterintuitive effect is a direct result of the rephasing process owing to the wave nature of light and is not at odds with causality.
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