A Comparison of the Attempts of Quantum Discord and Quantum Entanglement to Capture Quantum Correlations

TL;DR

This paper compares quantum discord and entanglement in capturing quantum correlations, analyzing their relationship in bipartite systems, and identifying states with maximal discord for given entropy or entanglement.

Contribution

It provides general results on the relationship between discord, entanglement, and entropy, and clarifies how discord differs from entanglement in mixed states.

Findings

Discord and entanglement are identical for pure states.

Discord can be nonzero even when entanglement is zero.

Strong bounds on entanglement-discord and entropy-discord relations.

Abstract

Measurements of Quantum Systems disturb their states. To quantify this non-classical characteristic, Zurek and Ollivier introduced the quantum discord, a quantum correlation which can be nonzero even when entanglement in the system is zero. Discord has aroused great interest as a resource that is more robust against the effects of decoherence and offers exponential speed up of certain computational algorithms. Here, we study general two-level bipartite systems and give general results on the relationship between discord, entanglement, and linear entropy, and identify the states for which discord takes a maximal value for a given entropy or entanglement, thus placing strong bounds on entanglement-discord and entropy-discord relations. We find out that although discord and entanglement are identical for pure states, they differ when generalized to mixed states as a result of the difference in the method of generalization.