Efficiency gain and bidirectional operation of quantum engines with decoupled internal levels

TL;DR

This paper introduces a quantum heat engine mechanism that leverages internal energy levels to enhance efficiency and enable bidirectional operation without quantum coherence, demonstrated through a quantum Otto cycle.

Contribution

It presents a novel method for efficiency improvement in quantum engines using internal levels, independent of quantum coherence or correlations.

Findings

Efficiency can be increased by channeling heat opposite to the second law.

The engine can operate in reverse to produce work.

Efficiency may increase as temperature difference decreases.

Abstract

We present a mechanism for efficiency increase in quantum heat engines containing internal energy levels that do not couple to the external work sink. The gain is achieved by using these levels to channel heat in a direction opposite to the one dictated by the second law. No quantum coherence, quantum correlations or ergotropy are required. A similar mechanism allows the engine to run in reverse and still produce useful work. We illustrate these ideas using a simple quantum Otto cycle in a coupled-spin system. We find this engine also exhibits other counterintuitive phenomenology. For example, its efficiency may increase as the temperature difference between the heat baths decreases. Conversely, it may cease to operate if the hotter bath becomes too hot or the colder bath too cold.