Surpassing the Carnot Efficiency by extracting imperfect work

TL;DR

This paper demonstrates that by allowing a non-negligible entropy increase in the battery, small-scale quantum heat engines can surpass the Carnot efficiency, challenging traditional definitions of work in quantum thermodynamics.

Contribution

It introduces a new perspective on defining work in quantum systems, showing that imperfect work extraction can lead to efficiencies beyond Carnot limits.

Findings

Small-scale quantum heat engines can exceed Carnot efficiency with imperfect work.

Allowing entropy increase in the battery relaxes traditional thermodynamic constraints.

Finite-sized heat baths do not prevent surpassing Carnot efficiency under this framework.

Abstract

A suitable way of quantifying work for microscopic quantum systems has been constantly debated in the field of quantum thermodynamics. One natural approach is to measure the average increase in energy of an ancillary system, called the battery, after a work extraction protocol. The quality of energy extracted is usually argued to be good by quantifying higher moments of the energy distribution, or by restricting the amount of entropy to be low. This limits the amount of heat contribution to the energy extracted, but does not completely prevent it. We show that the definition of "work" is crucial. If one allows for a definition of work that tolerates a non-negligible entropy increase in the battery, then a small scale heat engine can possibly exceed the Carnot efficiency. This can be achieved even when one of the heat baths is finite in size.