Photon Correlation Signals in Coupled-Cavity Polaritons Created by Entangled Light

We study theoretically a photon-coincidence counting technique aimed at measuring the entanglement between two optical cavities connected by an optical waveguide. Delocalized doubly excited polariton states can be selectively probed by two-photon absorption of an entangled photon pair generated by spontaneous parametric down-conversion with a narrowband pump. Deviations from unity of the second-order correlation function g((2))(tau) of photons emitted from remote cavities reveal intercavity correlations. The coincidence counting signal in the frequency domain reveals the transition frequencies between single-polariton states and between single polaritons and tripolaritons. High-frequency spectral features arise from the counter-rotating terms in the cavity-molecule coupling, rendering them a direct signature for the ultrastrong coupling regime in cavity quantum electrodynamics.