When is a quantum heat engine quantum?

TL;DR

This paper investigates the quantum nature of a two-level Otto heat engine using the Leggett-Garg inequality, providing a phase diagram and transition temperature to distinguish quantum from classical regimes.

Contribution

It introduces a method to assess the quantumness of heat engines via Leggett-Garg inequality and maps out the phase diagram of the engine's quantum behavior.

Findings

Identifies three distinct phases of quantumness in the engine.

Provides a complete phase diagram as a function of engine parameters.

Derives an explicit expression for the transition temperature.

Abstract

Quantum thermodynamics studies quantum effects in thermal machines. But when is a heat engine, which cyclically interacts with external reservoirs that unavoidably destroy its quantum coherence, really quantum? We here use the Leggett-Garg inequality to assess the nonclassical properties of a single two-level Otto engine. We provide the complete phase diagram characterizing the quantumness of the engine as a function of its parameters and identify three distinct phases. We further derive an explicit expression for the transition temperature.