Why should we care about quantum discord?

TL;DR

This paper explores quantum discord as a measure of quantum correlations in mixed states, highlighting its advantages over entanglement in various physical scenarios and quantum information protocols.

Contribution

It discusses the operational relevance of quantum discord in settings where entanglement is insufficient to describe correlations.

Findings

Quantum discord captures correlations missed by entanglement.

In various physical scenarios, discord provides a better measure of quantum correlations.

Entanglement may fail to describe all relevant features of correlated quantum systems.

Abstract

Entanglement is a central feature of quantum theory. Mathematical properties and physical applications of pure state entanglement make it a template to study quantum correlations. However, an extension of entanglement measures to mixed states in terms of separability does not always correspond to all the operational aspects. Quantum discord measures allow an alternative way to extend the idea of quantum correlations to mixed states. In many cases these extensions are motivated by physical scenarios and quantum information protocols. In this chapter we discuss several settings involving correlated quantum systems, ranging from distributed gates to detectors testing quantum fields. In each setting we show how entanglement fails to capture the relevant features of the correlated system, and discuss the role of discord as a possible alternative.