  We experimentally investigate the collective decay of a single Rydberg
superatom, formed by an ensemble of thousands of individual atoms supporting
only a single excitation due to the Rydberg blockade. Instead of observing a
constant decay rate determined by the collective coupling strength to the
driving field, we show that the enhanced emission of the single stored photon
into the forward direction of the coupled optical mode depends on the dynamics
of the superatom before the decay. We find that the observed decay rates are
reproduced by an expanded model of the superatom which includes coherent
coupling between the collective bright state and subradiant states.
