Thermal quantum and classical correlations in two qubit XX model in a nonuniform external magnetic field

TL;DR

This paper explores how thermal quantum discord and classical correlations in a two-qubit XX model are affected by temperature and nonuniform magnetic fields, revealing conditions where these correlations are equal and controllable.

Contribution

It identifies conditions under which quantum discord equals classical correlations in a two-qubit XX model with nonuniform magnetic fields, generalizing to other quantum systems.

Findings

Quantum discord equals classical correlations for nonuniform magnetic fields (B1 = -B2).

Thermal states belong to a class where QD and CC are equal under certain conditions.

Relative contributions of QD and CC can be controlled by magnetic field magnitudes.

Abstract

We investigate how thermal quantum discord (QD) and classical correlations (CC) of a two-qubit one-dimensional XX Heisenberg chain in thermal equilibrium depend on the temperature of the bath as well as on nonuniform external magnetic fields applied to two qubits and varied separately. We show that the behavior of QD differs in many unexpected ways from the thermal entanglement (EOF). For the nonuniform case (B1 = -B2), we find that QD and CC are equal for all values of (B1 = -B2) and for different temperatures. We show that, in this case, the thermal states of the system belong to a class of mixed states and satisfy certain conditions under which QD and CC are equal. The specification of this class and the corresponding conditions are completely general and apply to any quantum system in a state in this class satisfying these conditions. We further find that the relative contributions of QD and CC can be controlled easily by changing the relative magnitudes of B1 and B2. Finally, we connect our results with the monogamy relations between the EOF, CC and the QD of two qubits and the environment.