Phase diagram for the one-way quantum deficit of two-qubit X states

Abstract

The one-way quantum deficit, a measure of quantum correlation, can exhibit for X quantum states the regions (subdomains) with the phases ${\Delta }_0$ and ${\Delta }_{\pi /2}$ which are characterized by constant (i.e., universal) optimal measurement angles, correspondingly, zero and $\pi /2$ with respect to the $z$-axis and a third phase ${\Delta }_{\vartheta }$ with the variable (state-dependent) optimal measurement angle $\vartheta$. We build the complete phase diagram of one-way quantum deficit for the XXZ subclass of symmetric X states. In contrast to the quantum discord where the region for the phase with variable optimal measurement angle is very tiny (more exactly, it is a very thin layer), the similar region ${\Delta }_{\vartheta }$ is large and achieves the sizes comparable to those of regions ${\Delta }_0$ and ${\Delta }_{\pi /2}$. This instils hope to detect the mysterious fraction of quantum correlation with the variable optimal measurement angle experimentally.

20 pages, 10 figures (20 eps files)