Entanglement Entropy of the N=4 SYM spin chain

TL;DR

This paper calculates the entanglement entropy of excited states in various sectors of N=4 SYM spin chains, revealing how EE increases with coupling strength and providing exact results in certain limits.

Contribution

It provides the first detailed analysis of EE in all closed rank one subsectors of N=4 SYM, including exact expressions in the BMN limit and the behavior across coupling regimes.

Findings

EE increases monotonically with coupling strength.

Exact analytic expressions for EE in the BMN limit.

Violation of weak coupling EE bounds at strong coupling.

Abstract

We present a detailed study of the Entanglement Entropy (EE) of excited states in all closed rank one subsectors of N=4 SYM, namely SU(2), SU(1|1) and SL(2). Exploiting the techniques of the Coordinate and the Algebraic Bethe Ansatz we obtain the EE for spin chains with up to seven magnons, at leading order in the coupling expansion but exact in the length of the spin chain and of the part of it that we cut. Focusing on the superconformal primary operator with two magnons in the BMN limit, we derive analytic and exact, in the coupling $\lambda'$, expressions for the Renyi and the EE. The interpolating functions for the Renyi and the EE monotonically increase as the coupling increases from the weak coupling $\lambda' \rightarrow 0$ regime to the strong coupling $\lambda' \rightarrow \infty$ regime. This results to a violation of a certain bound for the EE that is present at weak coupling and confirms the physical intuition that entanglement increases when the coupling increases.