Advancing Quantum Otto Engine Performance via Additional Magnetic Field and Effective Negative Temperature

TL;DR

This paper proposes a four-stroke quantum Otto engine protocol utilizing additional magnetic fields and effective negative temperatures, demonstrating enhanced efficiency and operational range through precise magnetic control and coherence management.

Contribution

It introduces a novel magnetic field protocol that improves quantum Otto engine performance and extends operational domains beyond previous limitations.

Findings

Enhanced engine efficiency with additional magnetic field control.

Extended operational temperature range including negative temperatures.

Robustness of efficiency gains against magnetic field impurities.

Abstract

We formulate a protocol for a four-stroke quantum Otto engine that is capable of achieving superior performance when operating between two thermal reservoirs: one at a positive spin temperature and the other at an effective negative spin temperature. We adopt a protocol that encompasses a rotating magnetic field in the (x, y)-plane, as well as an additional magnetic field in the (z)-direction that possesses distinct strengths. Consequently, we acquire the capability to manipulate the strength of the magnetic field autonomously in both directions during dynamics. We report that by precisely adjusting the strength and the direction of the magnetic field in the (z)-direction and manipulating other relevant system parameters, we can effectively enhance the transition probability and hence the efficiency of the engine as well which can not be achieved without the additional magnetic field, although the impact is not ubiquitous. Additionally, another important significance of our model is that these engines operate within an extended operational domain, reaching into temperature ranges where the effective negative temperature-based quantum Otto engines operating only on the rotational magnetic field in the (x,y) plane, are unable to function. Specifically, we identify a threshold value for the magnetic field, dependent on the driving time, at which an improvement in efficiency is observed. We propose that this advantage may arise from the system exhibiting greater coherence with respect to the driving time, which we evaluate using the l1-norm coherence measure. Another noteworthy aspect is that the advantage in efficiency gained from the additional magnetic field continues to surpass that of the protocol without the field, even in the presence of impurities in the magnetic field having a specific range of disorder strengths.