# Entanglement Hamiltonians and entropy in 1+1D chiral fermion systems

## TL;DR

This paper presents an exact method for computing entanglement Hamiltonians and entropy in 1+1D chiral fermion systems, including excited states and temperature effects, advancing understanding of quantum entanglement in these models.

## Contribution

It introduces a novel exact computational approach for entanglement Hamiltonians and entropy in chiral fermion systems, including excited states and temperature corrections.

## Key findings

- Exact formulas for entanglement Hamiltonians of chiral fermions.
- Precise calculation of entropy changes due to particle excitations.
- First-order temperature correction for multiple interval entanglement entropy.

## Abstract

In past work we introduced a method which allows for exact computations of entanglement Hamiltonians. The method relies on computing the resolvent for the projected (on the entangling region) Green's function using a solution to the Riemann-Hilbert problem combined with finite rank perturbation theory. Here we analyze in detail several examples involving excited states of chiral fermions (Dirac and Majorana) on a spatial circle. We compute the exact entanglement Hamiltonians and an exact formula for the change in entanglement entropy due to the introduction of a particle above the Dirac sea. For Dirac fermions, we give the first-order temperature correction to the entanglement entropy in the case of a multiple interval entangling region.