Quantum discord in finite XY chains

TL;DR

This paper investigates quantum discord in finite XY spin chains, revealing a universal finite limit near the factorizing field, with detailed analysis of mixed states and coherence effects in small samples.

Contribution

It provides an exact analytical characterization of quantum discord behavior in finite XY chains, including the effects of coupling range and state mixtures.

Findings

Quantum discord approaches a universal finite limit near the factorizing field.

Analytic expressions for discord in finite chains are derived.

Coherence effects influence discord and parity splitting in small samples.

Abstract

We examine the quantum discord between two spins in the exact ground state of finite spin 1/2 arrays with anisotropic XY couplings in a transverse field B. It is shown that in the vicinity of the factorizing field B_s, the discord approaches a common finite non-negligible limit which is independent of the pair separation and the coupling range. An analytic expression of this limit is provided. The discord of a mixture of aligned pairs in two different directions, crucial for the previous results, is analyzed in detail, including the evaluation of coherence effects, relevant in small samples and responsible for a parity splitting at B_s. Exact results for finite chains with first neighbor and full range couplings and their interpretation in terms of such mixtures are provided.