Quantum correlations close to a square pattern forming instability

We analyze the quantum fluctuations of the degenerate optical parametric oscillator close to an instability for the formation of a square pattern. While strong correlations between the
fluctuations of the signal modes emitted at the critical wave number and with opposite wave vector are present both below and above threshold, no features signalling the square character of the pattern forming above threshold have been identified below threshold in the spatio-temporal second-order coherence.