General bounds for quantum discord and discord distance

TL;DR

This paper derives bounds on quantum discord in bipartite states using the Koashi-Winter relation, confirming conjectures under certain conditions and establishing conditions for equality and subadditivity.

Contribution

It provides new upper bounds for purified quantum discord and discord distance, and confirms conjectures in quantum correlation theory when entanglement vanishes.

Findings

Luo et al.'s conjecture is true when entanglement of formation is zero.

The joint entropy bounds the discord distance if Lindblad conjecture holds.

Conditions for saturating bounds involve Araki-Lieb inequality and Lindblad conjecture.

Abstract

For any bipartite state, how strongly can one subsystem be quantum correlated with another? Using the Koashi-Winter relation, we study the upper bound of purified quantum discord, which is given by the sum of the von Neumann entropy of the unmeasured subsystem and the entanglement of formation shared between the unmeasured subsystem with the environment. In particular, we find that the Luo et al.'s conjecture on the quantum correlations and the Lindblad conjecture are all ture, when the entanglement of formation vanishes. Let the difference between the left discord and the right discord be captured by the discord distance. If the Lindblad conjecture is true, we show that the joint entropy is a tight upper bound for the discord distance. Further, we obtain a necessary and sufficient condition for saturating upper bounds of purified quantum discord and discord distance separately with the equality conditions for the Araki-Lieb inequality and the Lindblad conjecture. Furthermore, we show that the subadditive relation holds for any bipartite quantum discord.