  We found that the Wigner rotation of a particle with spin 1/2, which is
unitary in Minkowski spacetime, becomes non-unitary as the particle is falling
onto the black hole. This implies that, from the quantum information processing
point of view, any quantum information encoded in spins will be dissipated near
the black hole. The gravitational field around the black hole thus acts like a
dissipative quantum channel, especially the bit flip channel.
