  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 Einstein-Podolsky-Rosen type, it 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.
