Evaluation of pairwise entanglement in translationally invariant systems with the random phase approximation

TL;DR

This paper presents a mean field plus RPA method to evaluate pairwise entanglement in translationally invariant spin chains, accurately predicting entanglement properties across various conditions and interaction ranges.

Contribution

It introduces a simple local RPA-based approach for analyzing entanglement in translationally invariant systems, providing accurate analytic predictions for concurrence.

Findings

Accurately predicts entanglement range and separability field.

Method's accuracy improves with increasing interaction range.

Provides analytic description of entanglement in spin chains.

Abstract

We discuss a general mean field plus random phase approximation (RPA) for describing composite systems at zero and finite temperature. We analyze in particular its implementation in finite systems invariant under translations, where for uniform mean fields it requires just the solution of simple local-type RPA equations. As test and application, we use the method for evaluating the entanglement between two spins in cyclic spin 1/2 chains with both long and short range anisotropic XY-type couplings in a uniform transverse magnetic field. The approach is shown to provide an accurate analytic description of the concurrence for strong fields, for any coupling range, pair separation or chain size, where it predicts an entanglement range which can be at most twice that of interaction. It also correctly predicts the existence of a separability field together with full entanglement range in its vicinity. The general accuracy of the approach improves as the range of the interaction increases.