  The quantum phase transition of the Dicke-model has been observed recently in
a system formed by motional excitations of a laser-driven Bose--Einstein
condensate coupled to an optical cavity [1]. The cavity-based system is
intrinsically open: photons can leak out of the cavity where they are detected.
Even at zero temperature, the continuous weak measurement of the photon number
leads to an irreversible dynamics towards a steady-state which exhibits a
dynamical quantum phase transition. However, whereas the critical point and the
mean field is only slightly modified with respect to the phase transition in
the ground state, the entanglement and the critical exponents of the singular
quantum correlations are significantly different in the two cases.
