Entropic Accord: A new measure in the quantum correlation hierarchy

TL;DR

This paper introduces 'entropic accord,' a new quantum correlation measure that fits between entanglement and discord, revealing a strict hierarchy and offering a potentially more intuitive understanding of quantum correlations.

Contribution

The paper proposes a novel quantum correlation measure called entropic accord, establishing its position in the hierarchy between entanglement and discord, and analyzing its properties for two-qubit states.

Findings

Entropic accord fits between entanglement and discord in the quantum correlation hierarchy.

A strict hierarchy exists among entanglement, entropic accord, and discord for two-qubit states.

Entropic accord can reveal correlations distinct from both entanglement and discord.

Abstract

Quantum correlation often refers to correlations exhibited by two or more local subsystems under a suitable measurement. These correlations are beyond the framework of classical statistics and the associated classical probability distribution. Quantum entanglement is the most well known of such correlations and plays an important role in quantum information theory. However, there exist non-entangled states that still possess quantum correlations which cannot be described by classical statistics. One such measure that captures these nonclassical correlations is discord. Here we introduce a new measure of quantum correlations which we call entropic accord that fits between entanglement and discord. It is defined as the optimised minimax mutual information of the outcome of the projective measurements between two parties. We show a strict hierarchy exists between entanglement, entropic accord and discord for two-qubit states. We study two-qubit states which shows the relationship between the three entropic quantities. In addition to revealing a class of correlations that are distinct from discord and entanglement, the entropic accord measure can be inherently more intuitive in certain contexts.