Entangled Quantum Fields near the Event Horizon and Entropy
  By fully exploiting the existence of the unitarily inequivalent representations of quantum fields, we exhibit the entanglement between inner and outer particles, with respect to the event horizon of a black hole. We compute the entanglement entropy and we find that the nonunitarity of the mapping, between the vacua in the flat and the curved frames, makes the entanglement very robust. 
A. Iorio, G. Lambiase, G. Vitiello