Ground-state entanglement in a coupled-cavity model

Bipartite entanglement entropies are calculated for the ground state of the two-excitation subspace in a two-site coupled cavity model. Each region in the phase diagram (atomic insulator, polaritonic insulator, photonic superfluid, and polaritonic superfluid) is found to be characterized by unique entanglement properties. In particular, the polaritonic superfluid region exhibits genuine 4-partite entanglement among the two atoms and two-cavity fields. The difference in entanglement properties between the small- and large-hopping limits provides further evidence that the transitions in the two limits are qualitatively different and that the large-hopping limit cannot be described by a simple Bose-Hubbard-type model.