  In this paper, a new quantum mechanical method to extract energy from black
holes with contracting horizons is proposed. The method is based on a gedanken
experiment on quantum energy teleportation (QET), which has been recently
proposed in quantum information theory. We consider this QET protocol for N
massless fields in near-horizon regions of large-mass black holes with
near-horizon geometry described by the Minkowski metric. For each field, a
two-level spin is strongly coupled with the local quantum fluctuation outside
the horizon during a short time period. After the measurement of N fields,
N-bit information is obtained. During the measurement, positive-energy wave
packets of the fields form and then fall into the black hole. The amount of
excitation energy is independent of the measurement result. After absorption of
the wave packets and increase of the black hole mass, a
measurement-result-dependent local operation of the N fields is performed
outside the horizon. Then, accompanying the extraction of positive energy from
the quantum fluctuation by the operation, negative-energy wave packets of the
fields form and then fall into the black hole, decreasing the black hole mass.
This implies that a part of the absorbed positive energy emitted from the
measurement devices is effectively retrieved from the black hole via the
measurement results.
