Fast causal information transmission in a medium with a slow group velocity

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