Operational modes of a Raman-coupled two-qubit quantum thermal machine

TL;DR

This paper explores a two-qubit quantum thermal machine with Raman coupling, analyzing its operation across various thermodynamic cycles and identifying conditions for different modes like engine or refrigerator, with high efficiency potential.

Contribution

It introduces a Raman-coupled two-qubit system analyzed within multiple thermodynamic cycles, revealing controllable operational modes and high efficiency regimes through frequency tuning.

Findings

Sharp transitions between engine and refrigerator in Carnot cycle

Rich operational mode coexistence in Otto cycle

Near-ideal efficiencies with regenerator in Stirling cycle

Abstract

We investigate a quantum thermal machine composed of two qubits coupled through a Raman-induced exchange interaction and driven by inhomogeneous transition frequencies. The system is analyzed within Carnot, Otto, and Stirling thermodynamic cycles, including the Stirling cycle with and without regeneration. We identify the conditions under which the device operates as a heat engine, refrigerator, thermal accelerator, or heater. Efficiency maps and operational-mode diagrams reveal well-defined boundaries in parameter space, governed by the frequency ratio $r=\bar{\omega}/\omega$, the coupling strength $g$, and the thermal gradient between reservoirs. The Carnot cycle exhibits sharp transitions between engine and refrigerator regimes, while the Otto cycle displays a richer structure with the coexistence of all operational modes. The Stirling cycle shows enhanced versatility and performance, particularly when assisted by a regenerator, where near-ideal efficiencies are achieved. Overall, the Raman-type interaction introduces a controllable left-right asymmetry that enables nontrivial manipulation of thermodynamic behavior through frequency tuning.