  Based on negative entropy in entanglement, it is shown that a single-system
Copenhagen measurement protocol is equivalent to the two-system von Neumann
scheme with the memory filling up the system with negative information similar
to the Dirac sea of negative energy. After equating the two quantum measurement
protocols, we then apply this equivalence to the black hole radiation. That is,
the black hole evaporation corresponds to the quantum measurement process and
the two evaporation approaches, the observable-based single-system and the
two-system entanglement-based protocols, can be made equivalent using quantum
memory. In particular, the measurement choice, \theta, with the memory state
inside the horizon in the entanglement-based scheme is shown to correspond to
the observable of the measurement choice, \theta, outside the horizon in the
single-system protocol, that is, O_{\theta}^{out} = Q_{\theta}^{in}. This
indicates that the black hole as quantum memory is filling up with negative
information outside the horizon, and its entropy corresponds to the logarithm
of a number of equally probable measurement choices. This shows that the black
hole radiation is no different than ordinary quantum theory.
