Ergodiscord: An operational and distinct notion of quantumness of correlations

Nonclassicality in composite quantum systems depicts several puzzling manifestations, with Einstein-Podolsky-Rosen entanglement, Schrödinger 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 paper, 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 system are probed jointly or independently, we introduce an operational quantifier of nonclassicality, termed . As we show, this 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 , 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 superadditivity phenomenon of nonlocal energy locking. This paper may inspire designs of quantum energy storage devices by utilizing nonclassical correlations in composite quantum systems.