Quantum limitations on superluminal propagation
  Unstable systems such as media with inverted atomic population have been shown to allow the propagation of analytic wavepackets with group velocity faster than that of light, without violating causality. We illuminate the important role played by unstable modes in this propagation, and show that the quantum fluctuations of these modes, and their unitary time evolution, impose severe restrictions on the observation of superluminal phenomena. 
Yakir Aharonov (Tel-Aviv and South Carolina) Benni Reznik (Los Alamos)   and Ady Stern (Weizmann)