Eur. Phys. J. D (2016) 70: 193
https://doi.org/10.1140/epjd/e2016-70210-x

Comment

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

Laboratoire de Physique des Lasers, Atomes et Molécules, CNRS et Université de Lille, 59655 Villeneuve d’ Ascq, France

^a e-mail: bernard.segard@univ-lille1.fr

Received: 22 March 2016
Published online: 22 September 2016

Abstract

In a recent theoretical article [S.H. Kazemi, S. Ghanbari, M. Mahmoudi, 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.