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

TL;DR

This paper demonstrates that in a four-wave mixing setup with warm potassium vapor, the transfer of information in slow- and fast-light media respects causality, with information velocity not exceeding the speed of light, despite negative group velocities.

Contribution

It provides experimental evidence that information transfer in slow- and fast-light media obeys causality, even when pulses exhibit negative group velocities, supporting fundamental principles of information theory.

Findings

Information velocity is limited to c, even with negative group velocities.

Transfer and copying of information obeys causality, akin to a classical no-cloning principle.

Points of non-analyticity on pulses carry new information.

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.