Causality and information transfer in simultaneously slow- and fast-light media

Abstract

Abstract:We demonstrate the simultaneous propagation of slow-and fast-light optical pulses in a four-wave mixing scheme using warm potassium vapor. We show that when the system is tuned such that the input probe pulses exhibit slow-light group velocities and the generated pulses propagate with negative group velocities, the information velocity in the medium is nonetheless constrained to propagate at, or less than, c. These results demonstrate that the transfer and copying of information on optical pulses to those with negative group velocities obeys information causality, in a manner that is reminiscent of a classical version of the no-cloning theorem. Additionally, these results support the fundamental concept that points of non-analyticity on optical pulses correspond to carriers of new information.

Faster light with competing absorption and gain

“…The fact that the distortion increases on either side, for both slow and fast light, is in agreement with the view that the mechanism for both advancement and delay is a pulse-reshaping (that is, a re-phasing) phenomenon. We note that even in the case of pulse advancement, the speed of information transfer is limited to the speed of light in vacuum [23].…”

“…Here, we show that the enhancement can occur in a nonlinear medium with regions of overlapping absorption and gain, without a modification to the lineshape. We demonstrate this effect using 4WM in warm potassium vapor, which has a smaller ground state hyperfine splitting than rubidium or cesium and generally leads to strong absorption of the probe pulse, even at room temperature [21][22][23]. Because the probe pulse is effectively positioned in the center of the absorption line, rather than on the wing [13,14], the shapes of the 4WM resonances remain symmetric in frequency.…”

“…Still, we find that our superluminal pulses can be less distorted than those demonstrated in a motivating experiment in rubidium [14], which could in part be ascribed to symmetric gain resonances. It is also worth mentioning that input pulse power [13] and spatial dispersion [14,23,27] contribute to both the advancement and distortion. So, future experiments might aim to quantify these effects and attempt to find the best methods of distortion management.…”

“…The experimental setup for generating pulsed 4WM in warm potassium vapor is based on previous 4WM experiments [13,[21][22][23][24] and shown schematically in Fig. 1.…”

Switching from normal dispersion to anomalous dispersion in a four-level atomic system

“…Bigelow et al have observed experimentally subluminal and superluminal light propagation in alexandrite crystal measuring group velocities as slow as 91 m s −1 to as fast as −800 m s −1 [14]. The simultaneous propagation of slow and fast light has been demonstrated in a four-wave mixing scheme [15]. Slow and fast light propagation has been discussed in a defect slab doped with polaritonic materials and nanoparticles [16].…”