Thermal rectification through a nonlinear quantum resonator

TL;DR

This paper systematically investigates thermal rectification in a nonlinear quantum resonator, especially a qubit, exploring conditions, bounds, and effects of strong coupling and many-level participation on rectification efficiency.

Contribution

It derives bounds on rectification in weak coupling, explores enhancement strategies, and compares methods for systems with multiple levels, advancing understanding of quantum thermal transport.

Findings

Strong coupling can surpass weak-coupling bounds.

Lamb shift can enhance rectification.

Different methods show varying rectification predictions.

Abstract

We present a comprehensive and systematic study of thermal rectification in a prototypical low-dimensional quantum system -- a non-linear resonator: we identify necessary conditions to observe thermal rectification and we discuss strategies to maximize it. We focus, in particular, on the case where anharmonicity is very strong and the system reduces to a qubit. In the latter case, we derive general upper bounds on rectification which hold in the weak system-bath coupling regime, and we show how the Lamb shift can be exploited to enhance rectification. We then go beyond the weak-coupling regime by employing different methods: i) including co-tunneling processes, ii) using the non-equilibrium Green's function formalism and iii) using the Feynman-Vernon path integral approach. We find that the strong coupling regime allows us to violate the bounds derived in the weak-coupling regime, providing us with clear signatures of high order coherent processes visible in the thermal rectification. In the general case, where many levels participate to the system dynamics, we compare the heat rectification calculated with the equation of motion method and with a mean-field approximation. We find that the former method predicts, for a small or intermediate anharmonicity, a larger rectification coefficient.