Theoretical analysis on quantum interference effect in fast-light media

We make a systematic theoretical analysis on the quantum interference (QI) effects in various fast-light media (including gain-assisted $N$, gain-assisted ladder-I, and gain-assisted ladder-II atomic systems). We show that such fast-light media are capable of not only completely eliminating the absorption but also suppressing the gain of signal field, and hence provide the possibility to realize a stable propagation of the signal field with a superluminal velocity. We find that there is a destructive (constructive) QI effect in gain-assisted ladder-I (gain-assisted N) system, but no QI in the gain-assisted ladder-II system; furthermore, a crossover from destructive (constructive) QI to Autler-Townes splitting may occur for the gain-assisted ladder-I (gain-assisted N) system when the control field of the system is modulated. Our theoretical analysis can be applied to other multi-level systems, and the results obtained may have promising applications in optical and quantum information processing and transmission.