  We demonstrate collectively enhanced vacuum-induced quantum beat dynamics
from a three-level V-type atomic system. Exciting a dilute atomic gas of
magneto-optically trapped $^{85}$Rb atoms with a weak drive resonant on one of
the transitions, we observe the forward-scattered field after a sudden shut-off
of the laser. The subsequent radiative dynamics, measured for various optical
depths of the atomic cloud, exhibits superradiant decay rates, as well as
collectively enhanced quantum beats. Our work is also the first experimental
illustration of quantum beats arising from atoms initially prepared in a single
excited level as a result of the vacuum-induced coupling between excited
levels.
