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They mean faster than the slow light speed in the material not in the vacuum. 

PHYSICAL REVIEW  LETTERS PRL 104, 140403 (2010) 9 APRIL 2010


F. Belgiorno,1 S. L. Cacciatori,2 G. Ortenzi,3 V. G. Sala,2 and D. Faccio4 

"The interaction between the refractive-index perturbation and the quantum vacuum fluctuations of the electromagnetic field leads to the production of photon pairs. ... In the past 60 years a significant amount of research has been dedicated to pair production in external fields or in the presence of time-varying boundary conditions, from the renowned analysis by Schwinger to the Hawking effect [1,2]. These effects are still waiting experimental observation although advancing technologies and the increasing number of novel physical systems in which these effects are proposed leave hope for successful experiments in the near future. In this framework we shall focus our attention on pair creation of photons in a novel setting, i.e., in the presence of a moving refraction index perturbation.

Recent advances in different fields such as analogue gravity [3,4] and the generation of superluminal light pulses either in optical fibers or in free space using pulse shaping techniques demand a detailed quantum analysis of the pulse propagation characteristics in the presence of nonlinear media. In particular, intense light pulses will induce through the optical nonlinear Kerr effect a refractive-index perturbation that travels at the same speed v of the light pulse. Examples of superluminal light pulses are Bessel pulses [57] and filament pulses under appropriate operating conditions [8,9]. Therefore the refractive- index perturbation associated to such pulses will also travel superluminally. ... We therefore conclude that the physical effect we are describing is novel and distinct from other known, yet similar, effects. All of these, along with others such as Hawking radiation, may be somehow gathered under the same generic name of quantum friction [18], of which the mechanism proposed here represents a novel realization that holds promise for experimental detection. In this sense, similarly to other superluminal phenomena [10], we note that the emitted photons will drain energy from the moving refractive-index perturbation which in turn is fed by the pump laser pulse, thus either reducing its energy until the perturbation is switched off or, alternatively, slowing it down until the superluminality condition is no longer met within the transparency range of the medium. In any case we underline that the presence of dissipation does not modify the results: the evaluation of the emitted photon rate in the realistic experimental setting described above predicts a loss less than a photon per pulse. If we consider that the pump pulse will typically have an energy in the "J or mJ range, this implies that the quantum friction process is far too weak to significantly perturb the pump pulse propagation over short propagation distances.

In conclusion, we have described a novel photon production mechanism that has a twofold importance: on the one hand it represents a completely novel nonlinear optical effect by which an intense Gaussian pulse propagating in a Kerr medium emits correlated photons nearly isotropically within a limited cone angle and with a tunable spectral maximum in the midinfrared region. The precise correlation properties of the emitted photons are somewhat involved and will be presented in future work. On the other hand, the present effect has common features with a whole family of vacuum excitation mechanisms, generically referred to as ‘‘quantum friction’’ [17], that still await experimental observation."