Minimal two-body quantum absorption refrigerator

TL;DR

This paper investigates minimal quantum absorption refrigerators using coupled qubits and resonators, revealing how different interactions affect cooling efficiency and the impact of higher-order processes.

Contribution

It compares qubit-based and resonator-based setups, showing the detrimental effect of XX interactions and the advantage of multi-level systems for better cooling.

Findings

XX interactions hinder cooling in qubit systems

Multi-level systems outperform qubits at certain non-linearity levels

Higher order processes reduce cooling efficiency compared to master equation predictions

Abstract

We study the phenomenon of absorption refrigeration, where refrigeration is achieved by heating instead of work, in two different setups: a minimal set up based on coupled qubits, and two non-linearly coupled resonators. Considering ZZ interaction between the two qubits, we outline the basic ingredients required to achieve cooling. Using local as well as global master equations, we observe that inclusion of XX type term in the qubit-qubit coupling is detrimental to cooling. We compare the cooling effect obtained in the qubit case with that of non-linearly coupled resonators (multi-level system) where the ZZ interaction translates to a Kerr-type non-linearity. For small to intermediate strengths of non-linearity, we observe that multi-level quantum systems, for example qutrits, give better cooling effect compared to the qubits. Using Keldysh non-equilibrium Green's function formalism, we go beyond first order sequential tunneling processes and study the effect of higher order processes on refrigeration. We find reduced cooling effect compared to the master equation calculations.