  Bekenstein argued that black holes should have entropy proportional to their
areas to make black hole physics compatible with the second law of
thermodynamics. However, the heuristic picture for Hawking radiation, creation
of pairs of positive- and negative-energy particles, leads to an inconsistency
among the first law of black hole mechanics, Bekenstein's argument and quantum
mechanics. In this paper we propose an equation alternative to Bekenstein's
from the viewpoint of quantum information, rather than thermodynamics, to
resolve this inconsistency without changing Hawking's original proposal for the
radiation. This argues that the area of a black hole is proportional to the
coherent information, which is minus the conditional entropy, defined only in
the quantum regime, from the outside, to positive-energy particles inside it.
This hints that negative-energy particles inside a black hole behave as if they
have negative entropy. Our result suggests that the black holes store pure
quantum information, rather than classical information.
