Ergodiscord: An Operational and Distinct Notion of Quantumness of Correlations

TL;DR

This paper introduces ergodiscord, an operational measure of nonclassical correlations in quantum systems, revealing phenomena like nonlocal energy locking and surpassing entanglement in energy storage potential.

Contribution

The paper proposes ergodiscord as a new, operational quantifier of nonclassicality that differs from quantum discord and uncovers nonlocal energy locking phenomena.

Findings

Ergodiscord accurately captures nonclassicality in bipartite states.

Nonclassical states can lock more work than maximally entangled states.

Discovery of nonlocal energy locking phenomenon.

Abstract

Nonclassicality in composite quantum systems depicts several puzzling manifestations, with Einstein-Podolsky-Rosen entanglement, Schr\"odinger steering, and Bell nonlocality being the most celebrated ones. In addition to those, an unentangled quantum state can also exhibit nonclassicality, as evidenced from notions such as quantum discord and work deficit. In this work, we present a general framework for exploring quantumness of correlations in multipartite quantum states. By exploiting the different signatures reflected on observable quantities depending on whether subsystems of a composite systems are probed jointly or independently, we introduce an operational quantifier of nonclassicality, termed {\em ergodiscord}. As we show, this newly proposed quantifier faithfully captures nonclassicality in any bipartite quantum state, while being fundamentally distinct from the original quantum discord. Moreover, ergodiscord uncovers an intriguing phenomenon called `nonlocal energy locking', where a useful form of energy (i.e. work) gets locked in correlations of nonclassical states. We also show that a mixed nonclassical state can lock more work than the maximally entangled state of the corresponding system, establishing an interesting super-additivity phenomenon of nonlocal energy locking. The present work may inspire novel designs of quantum energy storage devices by utilizing nonclassical correlations in composite quantum systems.