  We consider a bipartite entangled system half of which falls through the
event horizon of an evaporating black hole, while the other half remains
coherently accessible to experiments in the exterior region. Beyond complete
evaporation, the evolution of the quantum state past the Cauchy horizon cannot
remain unitary, raising the questions: How can this evolution be described as a
quantum map, and how is causality preserved? The answers are subtle, and are
linked in unexpected ways to the fundamental laws of quantum mechanics. We show
that terrestrial experiments can be designed to constrain exactly how these
laws might be altered by evaporation.
