Slow Light amplification in a non-inverted gain medium

– We investigate the propagation of a coherent probe light pulse through a threelevel atomic medium (in the Λ–configuration) in the presence of a pump laser under the conditions for gain without inversion. When the carrier frequency of the probe pulse and the pump laser are in a Raman configuration, we show that it is possible to amplify a slow propagating pulse. We also analyze the regime in which the probe pulse is slightly detuned from resonance where we observe anomalous light propagation. The intriguing physics of atomic three–level systems, for the first time observed in Pisa in 1976 by the group of A. Gozzini [1], is at the basis of all the recent experiments on slow light propagation [2] and speculations about possible realizations of quantum memories [3], quantum phase gates [4] and photon–counters with unprecedented efficiency [5, 6]. Many other phenomena involving three-level systems have attracted much interest; among them the possibility of observing amplification without inversion (AWI) [7] and anomalous propagation [8, 9]. Anomalous propagation but in an inverted two–level system has already been studied by the group of R. Chiao [10] and more recently in a three–level configuration with Raman scheme by the group of Dogariu [11]. On the other hand subluminal propagation inside the gain band of a xenon discharge has been observed early in the seventies [12]. In this letter we extend the treatment of [8] to the case of a pumped Λ–configuration under the conditions for gain without inversion. This offers the possibility to observe gain while at the same time reducing the propagation speed of a pulse. Furthermore, it is possible to reduce absorption in the anomalous propagation regime. Indeed, in the latter regime, even though the pulse center of mass is advanced with respect to a pulse propagating in vacuum, the pulse edge never goes faster than the speed of light in vacuum, unless population inversion is present. In order to observe gain without inversion it is necessary to populate both the ground states of the system without populating the excited state. We show how this can be achieved starting from the thermal population (equal population in the ground states and