Superluminal light propagation via quantum interference in decay channels

We examine the propagation of a weak probe light through a coherently driven $Y$-type system. Under the condition that the excited atomic levels decay via same vacuum modes, the effects of quantum interference in decay channels are considered. It is found that the interference in decay channels results in a lossless anomalous dispersion between two gain peaks. We demonstrate that the probe pulse propagation can in principle be switched from subluminal to superluminal due to the decay-induced interference. We also show that the system exhibits a high index of refraction with negligible absorption for the driving fields. A dressed-state picture of the atom-light interaction is described to explain the numerical results.