Hellinger distance as a measure of Gaussian discord

TL;DR

This paper introduces a new way to measure quantum correlations in Gaussian states using the Hellinger distance, providing a simple formula and analyzing specific classes of states.

Contribution

It develops an exact formula for Gaussian discord based on the Hellinger metric and applies it to symmetric, squeezed thermal, and mode-mixed thermal states.

Findings

The Hellinger-based Gaussian discord is consistent with traditional measures.

Derived a simple, exact formula for the Hellinger Gaussian discord.

Analyzed specific classes of states, confirming the measure's reliability.

Abstract

Especially investigated in recent years, the Gaussian discord can be quantified by a distance between a given two-mode Gaussian state and the set of all the zero-discord two-mode Gaussian states. However, as this set consists only of product states, such a distance captures all the correlations (quantum and classical) between modes. Therefore it is merely un upper bound for the geometric discord, no matter which is the employed distance. In this work we choose for this purpose the Hellinger metric that is known to have many beneficial properties recommending it as a good measure of quantum behaviour. In general, this metric is determined by affinity, a relative of the Uhlmann fidelity with which it shares many important features. As a first step of our work, the affinity of a pair of $n$-mode Gaussian states is written. Then, in the two-mode case, we succeeded in determining exactly the closest Gaussian product state and computed the Gaussian discord accordingly. The obtained general formula is remarkably simple and becomes still friendlier in the significant case of symmetric two-mode Gaussian states. We then analyze in detail two special classes of two-mode Gaussian states of theoretical and experimental interest as well: the squeezed thermal states and the mode-mixed thermal ones. The former are separable under a well-known threshold of squeezing, while the latter are always separable. It is worth stressing that for symmetric states belonging to either of these classes, we find consistency between their geometric Hellinger discord and the originally defined discord in the Gaussian approach. At the same time, the Gaussian Hellinger discord of such a state turns out to be a reliable measure of the total amount of its cross correlations.