  We generalize the vacuum-Unruh effect to arbitrary excited states in the Fock
space and find that the Unruh mode at the horizon induces coherent excitation
on the canonical background ensemble measured by an accelerated observer. When
there is only one type of Unruh mode in the system, for example, the ones
outgoing from a black hole horizon, the mapping from an arbitrary density
matrix on the maximal foliation to a vector space spanned by the pseudo-thermal
density matrix on the partitioned spacetime wedge is one-to-one. Hence we
propose that the information of the particles that are inside a collapsing
shell, thus inside the asymptotic black hole horizon is at least partially
retrievable by measuring the deviation of the Hawking radiation from the black
body radiation spectrum. This work shows that the long-standing black hole
information confusion might come from overlooking the possibility that the
information could be preserved much better than we have expected in a seemingly
non-unitary process when the partitions of the system are strongly entangled.
