Second law of thermodynamics and quantum feedback control: Maxwell’s demon with weak measurements

Recently Sagawa and Ueda Phys. Rev. Lett. 100, 080403 (2008) derived a bound on the work that can be extracted from a quantum system with the use of feedback control. For many quantum measurements their bound was not tight. We show that a tight version of this bound follows straightforwardly from recent work on Maxwell's demon by Alicki et al. Open Syst. Inf. Dyn. 11, 205 (2004), for both discrete and continuous feedback control. Our analysis also shows that bare, efficient measurements always do non-negative work on a system in equilibrium, but do not add heat.