Abstract
The one-way quantum deficit, a measure of quantum correlation, can exhibit for X quantum states the regions (subdomains) with the phases Δ₀ and Δ_π/2 which are characterized by constant (i.e., universal) optimal measurement angles, correspondingly, zero and π/2 with respect to the z-axis and a third phase Δ_ϑ with the variable (state-dependent) optimal measurement angle ϑ. 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 Δ_ϑ is large and achieves the sizes comparable to those of regions Δ₀ and Δ_π/2. This instils hope to detect the mysterious fraction of quantum correlation with the variable optimal measurement angle experimentally.