Fast Causal Information Transmission in a Medium With a Slow Group Velocity

It is widely believed that the velocity of information υᵢ encoded on an optical pulse is equal to the group velocity υg, at least when υg is less than the speed of light in vacuum c. On the other hand, several authors suggest that υᵢ=c, although the size of the signal traveling at this velocity may be small, thereby making it difficult to measure. Here, we measure υᵢ for pulses propagating through a resonant ``slow-light'' medium where υg≈0.006c. We find υᵢ=1.03c_-0.25c+0.49c, or that υᵢ≈168υg, clearly demonstrating that the speed of information cannot be generally described by υg, but is characterized by its own velocity.