  We study an array of coupled optical cavities in presence of two-photon
driving and dissipation. The system displays a critical behavior similar to
that of a quantum Ising model at finite temperature. Using the corner-space
renormalization method, we compute the steady-state properties of finite
lattices of varying size, both in one- and two-dimensions. From a finite-size
scaling of the average of the photon number parity, we highlight the emergence
of a critical point in regimes of small dissipations, belonging to the quantum
Ising universality class. For increasing photon loss rates, a departure from
this universal behavior signals the onset of a quantum critical regime, where
classical fluctuations induced by losses compete with long-range quantum
correlations.
