Quantum correlations versus spin magnitude: Transition to the classical limit

TL;DR

This paper investigates how quantum correlations, including entanglement and discord, diminish as spin magnitude increases, revealing the transition from quantum to classical behavior in spin systems at thermal equilibrium.

Contribution

It provides new insights into the behavior of quantum correlations in large spin systems, showing how entanglement disappears and discord becomes unstable as the classical limit is approached.

Findings

Threshold temperature for entanglement decreases with increasing spin

Quantum discord can persist in large spins but is fragile to perturbations

Quantum correlations vanish in the classical limit as spin grows large

Abstract

Quantum-classical transitions have long attracted much attention. We study such transitions in quantum spin-($j$,1/2) systems at thermal equilibrium. Unlike the previous work [Phys. Rev. A 73, 064302 (2006)], it is found that the threshold temperature of quantum entanglement decreases with increasing spin $j$ and completely disappears in the limit $j\to\infty$. In the ground state of systems with highly symmetric interactions, the discord-type quantum correlations can exist even for arbitrarily large spin. Such correlations turn out to be unstable and are destroyed by small perturbations that violate the symmetry of the Hamiltonian. The stable quantum correlations gradually degrade as the spin $j$ grows and eventually vanish when the classical limit is reached.