Quantum information cannot be completely hidden in correlations:   implications for the black-hole information paradox
  The black-hole information paradox has fueled a fascinating effort to reconcile the predictions of general relativity and those of quantum mechanics. Gravitational considerations teach us that black holes must trap everything that falls into them. Quantum mechanically the mass of a black hole leaks away as featureless (Hawking) radiation, but if the black hole vanishes, where is the information about the matter that made it? We treat the states of the in-fallen matter quantum mechanically and show that the black-hole information paradox becomes more severe. Our formulation of the paradox rules out one of the most conservative resolutions: that the state of the in-falling matter might be hidden in correlations between semi-classical Hawking radiation and the internal states of the black hole. As a consequence, either unitarity or Hawking's semi-classical predictions must break down. Any resolution of the black-hole information crisis must elucidate one of these possibilities. 
Samuel L. Braunstein and Arun K. Pati