The Second Law of Thermodynamics and Quantum Feedback Control: Maxwell's Demon with Weak Measurements

TL;DR

This paper demonstrates that the quantum work bound derived by Sagawa and Ueda can be achieved with any measurement, and clarifies the thermodynamic role of efficient measurements in quantum feedback control.

Contribution

It proves that the work bound is attainable for all measurements and connects recent Maxwell's demon work to quantum measurement thermodynamics.

Findings

The work bound can be achieved for every measurement.

Bare, efficient measurements do non-negative work on equilibrium systems.

Efficient measurements do not add heat to the system.

Abstract

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. They left open the question of whether this bound could be achieved for every measurement that could be made by the controller. We show that it can, and that this follows straightforwardly from recent work on Maxwell's demon by Alicki et al. [Open Syst. Inform. Dynam. 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.