Generalized second law of black hole thermodynamics and quantum information theory - art. no. 104008

We propose a quantum version of a gedanken experiment which supports the ge neralized second law of black hole thermodynamics. A quantum measurement of particles in the region outside of the event horizon decreases the entropy of the outside matter due to the entanglement of the inside and outside pa rticle states. This decrease is compensated, however, by the increase in th e detector entropy. If the detector is conditionally dropped into the black hole, depending on the experimental outcome, the decrease of the matter en tropy is more than compensated for by the increase of the black hole entrop y via the increase of the black hole mass, which is ultimately attributed t o the work done by the measurement.