Signal velocity, causality, and quantum noise in superluminal light pulse propagation

We consider pulse propagation in a linear anomalously dispersive medium whe re the group velocity exceeds the speed of light in vacuum (c) or even beco mes negative. A signal velocity is defined operationally based on the optic al signal-to-noise ratio, and is computed for cases appropriate to the rece nt experiment where such a negative group velocity was observed. It is foun d that quantum fluctuations limit the signal velocity to values less than c .