  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.
