Quantum and classical thermal correlations in the XY spin-1/2 chain

TL;DR

This paper analyzes quantum and classical correlations in the XY spin-1/2 chain, revealing how quantum discord characterizes phase transitions and how correlations behave with temperature and model parameters.

Contribution

It provides analytical expressions for quantum and classical correlations at any distance and temperature, highlighting their roles in quantum phase transitions.

Findings

Quantum discord detects phase transitions beyond entanglement.

Thermal quantum correlations can increase with temperature under magnetic fields.

Quantum discord dominates in the XX limit, classical correlation in the Ising limit.

Abstract

We investigate pairwise quantum correlation as measured by the quantum discord as well as its classical counterpart in the thermodynamic limit of anisotropic XY spin-1/2 chains in a transverse magnetic field for both zero and finite temperatures. Analytical expressions for both classical and quantum correlations are obtained for spin pairs at any distance. In the case of zero temperature, it is shown that the quantum discord for spin pairs farther than second-neighbors is able to characterize a quantum phase transition, even though pairwise entanglement is absent for such distances. For finite temperatures, we show that quantum correlations can be increased with temperature in the presence of a magnetic field. Moreover, in the XX limit, the thermal quantum discord is found to be dominant over classical correlation while the opposite scenario takes place for the transverse field Ising model limit.