Stationary entanglement of photons and atoms in a high-finesse resonator

We predict that the collective excitations of an atomic array become entangled with the light of a high-finesse cavity mode when they are suitably coupled. This entanglement is of the Einstein-Podolsky-Rosen type, is robust against cavity losses, and is a stationary property of the coupled system. It is generated when the atomic array is aligned along the cavity axis and driven transversally by a laser, when coherent scattering of photons into the cavity mode is suppressed because of phase mismatching. We identify the parameter regimes under which entanglement is found and show that these are compatible with existing experimental setups.