Refrigeration beyond weak internal coupling

TL;DR

This paper models a three-spin quantum absorption refrigerator across all coupling strengths, revealing optimal performance at moderate coupling and identifying effects that hinder cooling at strong coupling.

Contribution

It introduces a refined open quantum system model that accurately describes the refrigerator's behavior beyond weak coupling approximations.

Findings

Optimal refrigeration occurs at moderate coupling strength.

Strong coupling introduces additional heat channels that impair cooling.

High-temperature work reservoirs can prevent refrigeration in the strong coupling regime.

Abstract

We investigate the performance of a three-spin quantum absorption refrigerator using a refined open quantum system model valid across all inter-spin coupling strengths. It describes the transition between previous approximate models for the weak and the ultrastrong coupling limit, and it predicts optimal refrigeration for moderately strong coupling, where both approximations are inaccurate. Two effects impede a more effective cooling: the coupling between the spins no longer reduces to a simple resonant energy exchange (the rotating wave approximation fails), and the interactions with the thermal baths become sensitive to the level splitting, thus opening additional heat channels between the reservoirs. We identify the modified conditions of refrigeration as a function of the inter-spin coupling strength, and we show that, contrary to intuition, a high-temperature work reservoir thwarts refrigeration in the strong coupling regime.