Heat currents in qubit systems

TL;DR

This paper investigates heat currents in qubit systems within quantum thermodynamics, demonstrating that a global Lindblad master equation approach aligns with the second law and analyzing steady-state coherences.

Contribution

It provides explicit heat current expressions for single and coupled qubits using a global master equation, clarifying thermodynamic consistency.

Findings

Heat currents obey the second law in the studied models.

Global Lindblad approach yields consistent heat flow expressions.

Steady-state coherences are discussed in the context of heat transport.

Abstract

There is a current interest in quantum thermodynamics in the context of open quantum systems. An important issue is the consistency of quantum thermodynamics, in particular the second law of thermodynamics, i.e., the flow of heat from a hot reservoir to a cold reservoir. Here recent emphasis has been on composite system and in particular the issue regarding the application of local or global master equations. In order to contribute to this discussion we discuss two cases, namely as an example a single qubit and as a simple composite system two coupled qubits driven by two heat reservoirs at different temperatures, respectively. Applying a global Lindblad master equation approach we present explicit expressions for the heat currents in agreement with the second law of thermodynamics. The analysis is carried out in the Born-Markov approximation. We also discuss issues regarding the possible presence of coherences in the steady state.