Thermodynamics of quantum measurements

Quantum measurement of a system can change its mean energy, as well as entropy. A selective measurement (classical or quantum) can be used as a "Maxwell's demon" to power a single-temperature heat engine, by decreasing the entropy. Quantum mechanically, so can a non-selective measurement, despite increasing the entropy of a thermal state. The maximal amount of work extractable following the measurement is given by the change in free energy: W_max^(non-)sel.=ΔE_meas-T_BathΔS_meas^(non-)sel.. This follows from the "generalized 2nd law for nonequilibrium initial state" [Hasegawa et. al, PLA (2010)], of which an elementary reduction to the standard law is given here. It is shown that W_max^sel.-W_max^non-sel. equals the work required to reset the memory of the measuring device, and that no such resetting is needed in the non-selective case. Consequently, a single-bath engine powered by either kind of measurement works at a net loss of T_BathΔS_meas^non-sel per cycle. By replacing the measurement by a reversible "premeasurement" and allowing a work source to couple to the system and memory, the cycle can be made completely reversible.