  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.
