Berlin 2015 – scientific programme

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DY: Fachverband Dynamik und Statistische Physik

DY 56: Poster - Statistical Physics

DY 56.12: Poster

Thursday, March 19, 2015, 16:00–18:00, Poster A

Second law-like inequality for periodic, feedback-driven quantum engines — •Michael Bauer, Kay Brandner, Michael Schmid, and Udo Seifert — II. Institut für Theoretische Physik, Universität Stuttgart, 70550 Stuttgart, Germany

A genuine feature of projective quantum measurements is that they inevitably alter the mean energy of the observed system if the measured quantity does not commute with the Hamiltonian of the observed system. Compared to the classical case, Jacobs proved that this additional energetic cost leads to a stronger bound on the work extractable after a single measurement from a system initially in thermal equilibrium [Phys. Rev. A 80, 012322 (2009)]. Here, we show that the same bound holds for a large class of feedback-driven quantum engines operating periodically and in finite time. The bound thus implies a natural definition for the efficiency of information to work conversion in such devices.

For a simple model consisting of a laser-driven two level system, we maximize the efficiency with respect to the observable whose measurement is used to control the feedback protocol. We find that the optimal observable typically does not commute with the Hamiltonian and hence would not be available in a classical two level system. This result reveals that periodic feedback engines operating in the quantum realm can exploit quantum coherences to reach high efficiency.