Coherence and decoherence in quantum absorption refrigerators

Abstract: Absorption refrigerators transfer thermal energy from a cold reservoir to a hot reservoir using input energy from a third, so-called work reservoir. We examine the operation of quantum absorption refrigerators when coherences between eigenstates survive in the steady state limit. In our model, the working medium comprises a discrete, four-level system. We manifest that eigenbasis quantum coherences within this system generally suppress the cooling current in the refrigerator, while minimally affecting the coefficient of performance (cooling efficiency). We rationalize the behavior of the four-level refrigerator by studying two, three-level model systems for energy transport and refrigeration. Our calculations further illuminate the shortcomings of secular quantum master equations, and the necessity of employing dynamical equations of motion that retain couplings between population and coherences.