  Superradiance in an ensemble of atoms leads to the collective enhancement of
radiation in a particular mode shared by the atoms in their spontaneous decay
from an excited state. The quantum aspects of this phenomenon are highlighted
when such collective enhancement is observed in the emission of a single
quantum of light. Here we report a further step in exploring experimentally the
nonclassical features of superradiance by implementing the process not only
with single excitations, but also in a two-excitations state. Particularly we
measure and theoretically model the wave-packets corresponding to superradiance
in both the single-photon and two-photons regimes. Such progress opens the way
to the study and future control of the interaction of nonclassical light modes
with collective quantum memories at higher photon numbers.
