  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
multipartite entanglement among the two atoms and two cavity fields. This
system provides a toy model in which a number of intriguing aspects of
entanglement can be studied, such as the relationship of entanglement to phase
transitions, entanglement of particles with different dimensionality, and the
connection between experimentally accessible local observables and entanglement
entropies.
