Strong system-bath coupling reshapes characteristics of quantum thermal machines

TL;DR

This paper investigates how strong system-bath couplings fundamentally alter the performance and operational window of quantum thermal machines, revealing complex effects beyond traditional weak-coupling assumptions.

Contribution

It introduces a reaction coordinate quantum master equation approach to analyze strong couplings, showing reshaped performance windows and new transport pathways in quantum refrigerators.

Findings

Strong couplings reshape the cooling performance window.

Parameter renormalization dominates at strong couplings.

Direct transport pathways emerge, affecting efficiency.

Abstract

We study the performance of quantum absorption refrigerators, paradigmatic autonomous quantum thermal machines, and reveal central impacts of strong couplings between the working system and the thermal baths. Using the reaction coordinate quantum master equation method, which treats system-bath interactions beyond weak coupling, we discover that reshaping of the window of performance is a central outcome of strong system-bath couplings. This alteration of the cooling window stems from the dominant role of parameter renormalization at strong couplings. We further show that strong coupling admits direct transport pathways between the thermal reservoirs. Such beyond-second-order transport mechanisms are typically detrimental to the performance of quantum thermal machines. Our study reveals that it is inadequate to claim for either a suppression or an enhancement of the cooling performance at strong coupling when analyzed against a single parameter and in a limited domain. Rather, a comprehensive approach should be adopted so as to uncover the reshaping of the operation window.