  We consider a driven single mode Dicke-Hamiltonian coupled to a dissipative
zero-temperature bath. We derive the cumulant generating function for emitted
photons of this quantum-critical system by using a $P$-representation of the
master equation in the thermodynamic limit. This cumulant generating function
is shown to consist of two parts: a macroscopic component, which is Poissonian
in nature with characteristic rate proportional to the order parameter of the
system; and a part describing fluctuations which is non-trivial in form and
divergent around the quantum phase transition.
