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arXiv:2202.05050 · Physical review. E

Quantum correlations and ergotropy

Gianluca Francica

Published May 17, 2022• 51 citations
arXivHTMLPDF

Abstract

Understanding the role of classical and quantum correlations in work extraction is a problem of fundamental importance in thermodynamics. We approach this problem by considering that, in closed quantum systems, the maximum cyclic work extractable is equal to the ergotropy. Thus, we aim to identify and investigate the contributions to the ergotropy coming from different kinds of initial correlations (total, classical, discord, and entanglement correlations). By doing so, we have introduced and studied quantifiers of correlations which are based on ergotropy. In particular, our results suggest that only discord correlations always give a positive contribution to work extraction, whereas total, classical, and entanglement correlations can reduce the work extraction.

DOI
10.1103/physreve.105.l052101

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Understanding the role of classical and quantum correlations in work extraction is a problem of fundamental importance in thermodynamics. We approach this problem by considering that, in closed quantum systems, the maximum cyclic work extractable is equal to the ergotropy. Thus, we aim to identify and investigate the contributions to the ergotropy coming from different kinds of initial correlations (total, classical, discord, and entanglement correlations). By doing so, we have introduced and studied quantifiers of correlations which are based on ergotropy. In particular, our results suggest that only discord correlations always give a positive contribution to work extraction, whereas total, classical, and entanglement correlations can reduce the work extraction.
Abstract We investigate how the presence of quantum correlations can influence work extraction in closed quantum systems, establishing a new link between the field of quantum non-equilibrium thermodynamics and the one of quantum information theory. We consider a bipartite quantum system and we show that it is possible to optimize the process of work extraction, thanks to the correlations between the two parts of the system, by using an appropriate feedback protocol based on the concept of ergotropy. We prove that the maximum gain in the extracted work is related to the existence of quantum correlations between the two parts, quantified by either quantum discord or, for pure states, entanglement. We then illustrate our general findings on a simple physical situation consisting of a qubit system.
Nonlocality is a defining feature of quantum mechanics and has long served as a key indicator of quantum resources since the formulation of Bell's inequalities. Identifying the contribution of nonlocality to extractable work remains a central problem in quantum thermodynamics. We address this by introducing a quantifier of nonlocal contributions to extractable work in bipartite systems. It is shown that closed form expressions can be calculated for our quantity in terms of the Schmidt coefficients. Further for strictly non-interacting Hamiltonian, the direct relationship between ergotropy and correlations is established. Our results reveal that nonlocal resources invariably enhance extractable work under non-interacting Hamiltonians, while in the presence of interactions, their contribution can either increase or diminish depending on the structure of the state and the Hamiltonian.
We investigate the dynamics of quantum ergotropy in a multipartite system of two-level atoms interacting with a quantized field. The role of multipartite quantum mutual information and quantum relative entropy is analyzed to understand how correlations and the distinguishability between the passive state and the product of local states affect the extractable work. Numerical results for systems containing two to five atoms show that the maximum ergotropy does not increase linearly with the system size, while the intervals of zero ergotropy are gradually modified and suppressed for larger systems. We also study the effect of the average thermal photon number and different initial atomic states. Increasing the thermal photon number mainly reduces the relative-entropy contribution, whereas the initial state strongly influences both the magnitude and persistence of ergotropy. Among the considered states, the partially entangled state provides the largest and more persistent extractable work, while the mixed GHZ state gives the smallest ergotropy. These results highlight the importance of multipartite correlations, thermal effects, and state preparation in controlling quantum work extraction and may be useful for future studies of quantum batteries and quantum power.
Abstract In many standard analyses of quantum thermodynamics, the system and its environment are assumed to be initially independent and free of any correlations. However, in many physical scenarios, initial system-environment correlations are unavoidable. In this work, we investigate the role of initial system-environment correlations in work extraction from open quantum systems. Furthermore, we examine the effects of these correlations and the nature of environmental dynamics on the evolution of coherence, coherence-based ergotropy, and total ergotropy. In both Markovian and non-Markovian regimes, we show that initial correlations directly influence the amount of ergotropy. We also demonstrate that in the non-Markovian regime, reducing the initial correlation enables optimal utilization of memory effects and maintains work extraction efficiency over longer time intervals. In contrast, in the Markovian regime, such correlations only accelerate the decay process without producing a revival of quantum resources; therefore, work extraction must be carried out within very short time intervals.
The paradigm of extracting work from an isolated quantum system through a cyclic Hamiltonian process is a topic of immense research interest. The optimal work extracted under such a process is termed ergotropy [Europhys. Lett. 67, 565 (2004)]. Here, in a multiparty scenario, we consider only a class of such cyclic processes that can be implemented locally, giving rise to the concept of local ergotropy. Eventually, the presence of quantum correlations results in a nonvanishing thermodynamic quantity called an ergotropic gap, measured by the difference between global and local ergotropy. However, the converse does not hold in general, i.e., its nonzero value does not necessarily imply the presence of quantum correlations. For arbitrary multiparty states, we quantify this gap. We also evaluate the difference between maximum global and local extractable work for arbitrary states when the system is no longer isolated but put in contact with a bath of the same local temperature.
Abstract It is an established fact that quantum coherences have thermodynamic value. The natural question arises, whether other genuine quantum properties such as entanglement can also be exploited to extract thermodynamic work. In the present analysis, we show that the ergotropy can be expressed as a function of the quantum mutual information, which demonstrates the contributions to the extractable work from classical and quantum correlations. More specifically, we analyze bipartite quantum systems with locally thermal states, such that the only contribution to the ergotropy originates in the correlations. Our findings are illustrated for a two-qubit system collectively coupled to a thermal bath.