Two-spin subsystem entanglement in spin 1/2 rings with long range interactions

TL;DR

This study investigates how two-spin entanglement in a spin 1/2 ring varies with interaction range, revealing unique features at the solvable Haldane-Shastry point and energy-dependent entanglement patterns for other interaction ranges.

Contribution

It demonstrates that two-spin entanglement patterns are highly sensitive to the interaction parameter  and distinguishes the special case =2 from other ranges, highlighting energy-dependent entanglement behaviors.

Findings

No entanglement beyond nearest neighbors at =2.

Selective entanglement at any distance for high-energy eigenstates when 2.

Nearest neighbor entanglement remains consistent regardless of interaction range.

Abstract

We consider the two-spin subsystem entanglement for eigenstates of the Hamiltonian \[ H= \sum_{1\leq j< k \leq N} (\frac{1}{r_{j,k}})^{\alpha} {\mathbf \sigma}_j\cdot {\mathbf \sigma}_k \] for a ring of $N$ spins 1/2 with asssociated spin vector operator $(\hbar /2){\bf \sigma}_j$ for the $j$-th spin. Here $r_{j,k}$ is the chord-distance betwen sites $j$ and $k$. The case $\alpha =2$ corresponds to the solvable Haldane-Shastry model whose spectrum has very high degeneracies not present for $\alpha \neq 2$. Two spin subsystem entanglement shows high sensistivity and distinguishes $\alpha =2$ from $\alpha \neq 2$. There is no entanglement beyond nearest neighbors for all eigenstates when $\alpha =2$. Whereas for $\alpha \neq 2$ one has selective entanglement at any distance for eigenstates of sufficiently high energy in a certain interval of $\alpha$ which depends on the energy. The ground state (which is a singlet only for even $N$) does not have entanglement beyond nearest neighbors, and the nearest neighbor entanglement is virtually independent of the range of the interaction controlled by $\alpha$.