Hellinger distance as a measure of Gaussian discord

Abstract

Abstract. Especially investigated in recent years, the Gaussian discord can tentatively 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.

Quantifying quantumness of correlations using Gaussian Rényi-2 entropy in optomechanical interfaces

“…Remarkably, the GR2D D m 2 and D op 2 are zero only for r = 0. This is a consequence of the fact that quantum discord vanishes only if the Gaussian state is a product state and therefore if and only if the determinant of the V i 3 matrices (see (17)) is zero [73]. Such a condition is well verified for r = 0, where c i ± = 0 and consequently det V i 3 = 0 (see Eqs.…”

Measures and applications of quantum correlations

“…Geometric measures of QCs based on the Hellinger distance have been discussed for Gaussian states in [131]. Operationally, the Hellinger distance admits an interpretation in terms of asymptotic state discrimination [132,56], as detailed in Section 4.2.2.…”

Geometric Measures of Quantum Correlations with Bures and Hellinger Distances

“…3.1). Further works have studied the geometric discords based on the more physically reliable Bures distance (see [2,87,81,82,1]), Hellinger distance (see [57,78]), and trace distance (see [63,68,24] and references therein). The discord D G B relative to subsystem B is defined by replacing C A by C B in (19).…”