Enhancing the performance of coupled quantum Otto thermal machines without entanglement and quantum correlations

TL;DR

This paper investigates how coupled spin systems can enhance quantum Otto engine performance without relying on entanglement or quantum correlations, emphasizing the role of energy level structure and system size.

Contribution

It demonstrates that efficiency and work improvements are due to energy level structure, not quantum correlations, and explores effects of system size and interaction type on performance.

Findings

Efficiency, work, and COP increase with system size, especially for odd numbers of spins.

Entanglement and quantum correlations are not necessary for performance enhancements.

Energy level structure determines the thermodynamic performance of coupled spin systems.

Abstract

We start with a revision study of two coupled spin-$1/2$ under the influence of Kaplan-Shekhtman-Entin-Wohlman-Aharony (KSEA) interaction and a magnetic field. We first show the role of idle levels, i.e., levels that do not couple to the external magnetic field, when the system is working as a heat engine as well as when it is a refrigerator. Then we extend the results reported in [PRE. 92, (2015) 022142] by showing that it is not necessary to change both the magnetic field as well as the coupling parameters to break the extensive property of the work extracted globally from two coupled spin-$1/2$ as has been demonstrated there. Then we study the role of increasing the number of coupled spins on efficiency, extractable work, and coefficient of performance (COP). First, we consider two- and three-coupled spin-$1/2$ Heisenberg $\mathrm{XXX}$-chain. We prove that the latter can outperform the former in terms of efficiency, extractable work, and COP. Then we consider the Ising model, where the number of interacting spins ranges from two to six. We show that only when the number of interacting spins is odd the system can work as a heat engine in the strong coupling regime. The enhancements in efficiency and COP are explored in detail. Finally, this model confirms the idea that entanglement and quantum correlations are not the reasons behind the enhancements observed in efficiency, extracatable work, and COP, but only due to the structure of the energy levels of the Hamiltonian of the working substance. In addition to this, the extensive property of global work as well, is not affected by entanglement and quantum correlations.