Comment on "Gain-assisted superluminal light propagation through a Bose-Einstein condensate cavity system"

TL;DR

This paper critically examines a previous claim of superluminal light in a Bose-Einstein condensate system, demonstrating that the system's behavior is actually subluminal and highlighting issues with the original analysis.

Contribution

It clarifies that the previous claim of superluminal propagation was incorrect and shows the system is not minimum-phase-shift, affecting the interpretation of group delay.

Findings

The superluminal propagation claim is invalid; the system exhibits subluminal behavior.

The system is not minimum-phase-shift, affecting the use of Kramers-Kronig relations.

Group delay and phase shift can have opposite signs in this system.

Abstract

In a recent theoretical article [Eur. Phys. J. D 70, 1 (2016)], Kazemi et al. claim to have demonstrated superluminal light transmission in an optomechanical system where a Bose-Einstein condensate serves as the mechanical oscillator. In fact the superluminal propagation is only inferred from the existence of a minimum of transmission of the system at the probe frequency. This condition is not sufficient and we show that, in all the cases where superluminal propagation is claimed by Kazemi et al., the propagation is in reality subluminal. Moreover, we point out that the system under consideration is not minimum-phase-shift. The Kramers-Kronig relations then only fix a lower limit to the group delay and we show that these two quantities have sometimes opposite signs.