Better Late than Never: Information Retrieval from Black Holes

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We show that, in order to preserve the equivalence principle until late times in unitarily evaporating black holes, the thermodynamic entropy of a black hole must be primarily entropy of entanglement across the event horizon.

• For such black holes, we show that the information entering a black hole becomes encoded in correlations within a tripartite quantum state, the quantum analogue of a one-time pad, and is only decoded into the outgoing radiation very late in the evaporation.
• This behavior generically describes the unitary evaporation of highly entangled black holes and requires no specially designed evolution.
• Our work suggests the existence of a matter-field sum rule for any fundamental theory.