Two-time and two-frequency correlators

As in the LM, here it would be interesting to go as far as possible in the understanding of the correlators. In this case, cross correlators of the kind $\langle\ud{\sigma}_1(t)\sigma_2(t+\tau)\rangle$ and frequency resolved two-photon counting, $S^{(2)}(t_1,\omega_1;t_2,\omega_2)$ , or correlations, $S^{(2)}(\omega_1,\omega_2)$ , are easier to compute (through the recursion of the QRF) than in the LM, due to the finite Hilbert space.