Superluminal group velocities and information transfer

Four-fold reduction in the speed of light at practical power levels using Brillouin scattering in a 2-m Bismuth-oxide fiber

“…Many authors have chosen to use the peak of the pulses for this purpose. However, Brillouin-induced slow-light generation may be accompanied by distortion of the pulse shape because of the narrow bandwidth of the Brillouin gain profile, and it has been mathematically demonstrated that in superluminal propagation there is no casual relationship between the peak position at the input and at the output of the system [8]. We have therefore chosen to use the temporal centre of gravity of the main lobe of the pulse as a better definition of the pulse position throughout our measurements (this parameter will coincide with the position of the peak in symmetric pulses when there is no distortion, as is the case in our experiments).…”

Light Propagation in Optically Dressed Media

“…It is very important to stress that superluminality does not violate causality. It has been shown that neither energy [31] nor information [32] are transferred more quickly than light in vacuum. The proof is based on noticing that the energy of the pulses created at the end of the sample is in some sense "borrowed" from the energy stored inside the medium and not from the energy of the incoming pulse.…”

Bounds and enhancements for negative scattering time delays

“…Many of these works have discussed the relativistic or "Einstein causality" principle, i.e., the limiting role of the velocity of light in the transmission of signals [13,14,15,16,17,18,19], which must be applicable to relativistic wave equations; the influence of the different wavepacket regions (rear, front) in the transmitted signal, also in the non-relativistic case [20]; the attainability of a sensible signal to noise ratio in superluminal experiments with a small number of photons [21,22,23]; or the role of the frequency band limitation of the signals [24,25,26,27,28]. Much less attention has been paid to the consequences of the more primitive and general causality principle stating that "the effect cannot precede the cause".…”