Coherence-enhanced efficiency of feedback-driven quantum engines

TL;DR

This paper extends bounds on work extraction in feedback-driven quantum engines, showing that quantum coherence can be exploited to enhance efficiency beyond classical limits.

Contribution

It generalizes the energetic cost bound to periodic, finite-time quantum engines and demonstrates how quantum coherence can improve efficiency.

Findings

Optimal measurement observables often do not commute with the Hamiltonian.

Quantum feedback engines can utilize coherence to surpass classical efficiency limits.

The derived bounds define a natural measure for information-to-work conversion efficiency.

Abstract

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. 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 extend this bound to 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 operations. 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 enhance efficiency.