Thermal and Magnetic Quantum Discord in Heisenberg models

TL;DR

This paper explores how quantum discord in a two-qubit Heisenberg chain varies with temperature and magnetic field, revealing behaviors distinct from entanglement and indicating quantum phase transitions at finite temperatures.

Contribution

It demonstrates that quantum discord exhibits unique thermal behaviors, including increase with temperature and regrowth, differing from entanglement, and signals quantum phase transitions in Heisenberg models.

Findings

Quantum discord can increase with temperature even when entanglement decreases.

Quantum discord signals quantum phase transitions at finite temperature.

Proper tuning of magnetic field or interactions can sustain non-zero quantum discord at any temperature.

Abstract

We investigate how the quantum correlations (quantum discord) of a two-qubit one dimensional XYZ Heisenberg chain in thermal equilibrium depends on the temperature (T) of the bath and also on an external magnetic field B. We show that the behavior of the thermal quantum discord (QD) differs in many unexpected ways from the thermal entanglement. For example, we show situations where QD increases with T when entanglement decreases, cases where QD increases with T even in regions with zero entanglement, and that QD signals a quantum phase transition even at finite T. We also show that by properly tuning B or the interaction between the qubits we get non-zero QD for any T and we present a new effect not seen for entanglement, the regrowth of thermal QD.