Quantum thermal machines in coupled spin systems

Abstract

Classical thermodynamics deals with systems that contain many particles as long as they remain in equilibrium. Because of the existence of many particles, heat and work fluctuations are negligible with respect to their mean values. On the other hand, in the recent few years, people have been interested in studying the thermodynamics of quantum systems, even at the level of a single spin or an atom, with technological advances in the control and measurement of quantum systems and recent research interest in quantum devices. Also, apart from the academic need, novel quantum technologies can be created by harnessing quantum thermodynamic features. The study in quantum thermodynamics aims to develop a novel thermodynamic framework, that goes beyond conventional thermodynamics, the inclusion of non-equilibrium dynamics, and accounts for finite-size effects and explores the possible advantages of non-classical features, namely entanglement and quantum coherence, of the system. A major study in quantum thermodynamics involves quantum thermal machines (QTMs), primarily whether these machines can outperform their classical counterparts by exploiting quantum properties or other quantum mechanical behaviour in QTMs’ performance. In a so-called reciprocating heat-work cycle of a standard QTM, the coupling between the system and the heat baths is switched off and on, during the work extraction stage and the heating or cooling stage, respectively. In our work, we have focused on two different types of QTMs, namely, the Stirling engine and the Otto engine, and studied how their various stages can be implemented using a few-spin chain. We explore how spin-spin interaction can affect the operation and performance of these QTMs. We investigate how the cycle behaves as different thermal machines depending on the cycle parameters. We study the performance of quantum Stirling machines near a quantum critical point in a two-spin working system, in which the nearest neighbour interaction of the Heisenberg-XX type couples the spins. We show how this system can exhibit a QPT which is examined by the measure of entanglement and correlation. Weshowthat at the QCP, the engine efficiency and the coefficient of performance of the refrigerator attain corresponding values of their Carnot counterparts, along with maximum work output. We analyze how such enhancement can be attributed to the non-analytic behaviour of spin-spin correlation and entanglement near the QCP. Westudy quantum Otto thermal engines with a two-spin working system coupled by anisotropic interaction. Also, we consider two scenarios of fueling the engines, one by a heat bath and another by non-selective quantum measurements. We investigate how a measurement-based QOE behaves differently from a standard QOE in finite time. We introduce the case of a QOE operating with a local spin working system. We discuss different thermodynamic figures of merit of local QOE operation. We aim to find the role of anisotropy in the performance of various HEs operating in various timeframes. We discuss the effect of quantum internal friction that arises due to finite-time unitary time evolution processes. We show that for anisotropic interaction, the efficiency of a measurement-based and local spin engine oscillates in f inite times. Therefore, for a suitable choice of timing of the unitary processes in the short time regime, the engine can have a higher work output and less heat absorption, such that it works more efficiently than a quasi-static engine. We analytically show that this oscillation comes into the picture through an interference-like effect between two probability amplitudes. Finally, we discuss the case of an always-on heat bath.