When is a quantum heat engine quantum?

Department of Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg - D-91058 Erlangen, Germany

Received: 21 September 2017
Accepted: 20 November 2017

Abstract

Quantum thermodynamics studies quantum effects in thermal machines. Both quantum coherence and quantum correlations have been theoretically shown to be a physical resource able to boost their performance. But when is a heat engine, which cyclically interacts with external reservoirs that unavoidably destroy its phase coherence, really quantum? We here use the Leggett-Garg inequality to assess the nonclassical properties of a paradigmatic two-level Otto engine with quantum friction. We provide the complete phase diagram characterizing the quantumness of the engine as a function of its parameters and distinguish three different phases: a quantum phase separated from a classical phase by a transition regime. We further derive an analytical expression for the quantum-to-classical transition temperature.