Two intervals R\'enyi entanglement entropy of compact free boson on torus

TL;DR

This paper calculates the second R'enyi entanglement entropy for two intervals in a finite-temperature 2D compact free boson on a torus, revealing T-duality invariance and matching known zero-temperature results in certain limits.

Contribution

It provides an exact computation of the two-interval R'enyi entanglement entropy on a torus, including classical and quantum parts, and demonstrates T-duality invariance.

Findings

Exact expression involving theta functions for the entropy.

Agreement with zero-temperature results in small interval limits.

Universal thermal correction form for small separation between intervals.

Abstract

We compute the $N=2$ R\'enyi entanglement entropy of two intervals at equal time in a circle, for the theory of a 2d compact complex free scalar at finite temperature. This is carried out by performing functional integral on a genus 3 ramified cover of the torus, wherein the quantum part of the integral is captured by the four point function of twist fields on the worldsheet torus, and the classical piece is given by summing over winding modes of the genus 3 surface onto the target space torus. The final result is given in terms of a product of theta function and certain multi-dimensional theta function. We demonstrate the T-duality invariance of the result. We also study its low temperature limit. In the case in which the size of the intervals and of their separation are much smaller than the whole system, our result is in exact agreement with the known result for two intervals on an infinite system at zero temperature \cite{eeoftwo}. In the case in which the separation between the two intervals is much smaller than the interval length, the leading thermal corrections take the same universal form as proposed in \cite{Cardy:2014jwa,Chen:2015cna} for R\'enyi entanglement entropy of a single interval.