  There have been several experiments which hint at evidence for superluminal
transport of electromagnetic energy through a material slab. On the theoretical
side, it has appeared evident that acausal signals are indeed possible in
quantum electrodynamics. However, it is unlikely that superluminal signals can
be understood on the basis of a purely classical electrodynamic signals passing
through a material. The classical and quantum theories represent quite
different views, and it is the quantum view which may lead to violations of
Einstein causality.

Quantum Physics