  We study the dynamic sensitivity of an atomic ensemble dressed by a
single-mode cavity field (called a photon-dressed atomic ensemble), which is
described by the Dicke model near the quantum critical point. It is shown that
when an extra atom in a pure initial state passes through the cavity, the
photon-dressed atomic ensemble will experience a quantum phase transition,
showing an explicit sudden change in its dynamics characterized by the
Loschmidt echo of this quantum critical system. With such dynamic sensitivity,
the Dicke model can resemble to the cloud chamber for detecting a flying
particle by the enhanced trajectory due to the classical phase transition.
