  Laser-driven Bose-Einstein condensate of ultracold atoms loaded into a lossy
high-finesse optical resonator exhibits critical behavior and, in the
thermodynamic limit, a phase transition between stationary states of different
symmetries. The system realizes an open-system variant of the celebrated
Dicke-model. We study the transition for a finite number of atoms by means of a
Hartree-Fock-Bogoliubov method adapted to a damped-driven open system. The
finite-size scaling exponents are determined and a clear distinction between
the non-equilibrium and the equilibrium quantum criticality is found.
