Faithful nonclassicality indicators and extremal quantum correlations in two-qubit states

TL;DR

This paper introduces a new, optimized measure of nonclassical correlations in two-qubit states that overcomes limitations of previous indicators, providing a more faithful and robust quantification of quantum correlations beyond entanglement.

Contribution

It proposes an ameliorated measurement-induced disturbance as a nonclassicality indicator, with a closed-form expression and analytical relation to quantum discord, identifying maximally quantum-correlated states.

Findings

The new indicator overcomes asymmetry and unfaithfulness of previous measures.

Closed-form expression derived for relevant two-qubit states.

Maximally quantum-correlated states are identified as most robust against noise.

Abstract

The state disturbance induced by locally measuring a quantum system yields a signature of nonclassical correlations beyond entanglement. Here we present a detailed study of such correlations for two-qubit mixed states. To overcome the asymmetry of quantum discord and the unfaithfulness of measurement-induced disturbance (severely overestimating quantum correlations), we propose an ameliorated measurement-induced disturbance as nonclassicality indicator, optimized over joint local measurements, and we derive its closed expression for relevant two-qubit states. We study its analytical relation with discord, and characterize the maximally quantum-correlated mixed states, that simultaneously extremize both quantifiers at given von Neumann entropy: among all two-qubit states, these states possess the most robust quantum correlations against noise.