  We present analytical solutions for the evolution of collective states of N
atoms. On the one hand is a (timed) Dicke state prepared by absorption of a
single photon and exhibiting superradiant decay. This is in strong contrast to
evolution of a symmetric Dicke state which is trapped for large atomic clouds.
We show that virtual processes yield a small effect on the evolution of the
rapidly decaying timed Dicke state. However, they change the long time dynamics
from exponential decay into a power-law behavior which can be observed
experimentally. For trapped states virtual processes are much more important
and provide new decay channels resulting in a slow decay of the otherwise
trapped state.
