# Multipartite entanglement after a quantum quench

## TL;DR

This paper investigates how multipartite entanglement evolves in a quantum many-body system after a quench, using quantum Fisher information, and finds that the entanglement generally remains high, especially near critical points.

## Contribution

It introduces a formalism to quantify multipartite entanglement after a quantum quench using quantum Fisher information and applies it to the quantum Ising chain.

## Key findings

- Asymptotic states are predominantly more than bipartite entangled.
- Multipartite entanglement diverges for small quenches near the critical point.
- The formalism relates QFI to observable fluctuations in the diagonal ensemble.

## Abstract

We study the multipartite entanglement of a quantum many-body system undergoing a quantum quench. We quantify multipartite entanglement through the quantum Fisher information (QFI) density and we are able to express it after a quench in terms of a generalized response function. For pure state initial conditions and in the thermodynamic limit, we can express the QFI as the fluctuations of an observable computed in the so-called diagonal ensemble. We apply the formalism to the dynamics of a quantum Ising chain after a quench in the transverse field. In this model the asymptotic state is, in almost all cases, more than two-partite entangled. Moreover, starting from the ferromagnetic phase, we find a divergence of multipartite entanglement for small quenches closely connected to a corresponding divergence of the correlation length.