Coherence-enhanced efficiency of feedback-driven quantum engines

Abstract

A genuine feature of projective quantum measurements is that they inevitably\nalter the mean energy of the observed system if the measured quantity does not\ncommute with the Hamiltonian. Compared to the classical case, Jacobs proved\nthat this additional energetic cost leads to a stronger bound on the work\nextractable after a single measurement from a system initially in thermal\nequilibrium [Phys. Rev. A 80, 012322 (2009)]. Here, we extend this bound to a\nlarge class of feedback-driven quantum engines operating periodically and in\nfinite time. The bound thus implies a natural definition for the efficiency of\ninformation to work conversion in such devices. For a simple model consisting\nof a laser-driven two-level system, we maximize the efficiency with respect to\nthe observable whose measurement is used to control the feedback operations. We\nfind that the optimal observable typically does not commute with the\nHamiltonian and hence would not be available in a classical two level system.\nThis result reveals that periodic feedback engines operating in the quantum\nrealm can exploit quantum coherences to enhance efficiency.\n