# Observation of non-Hermitian topology and its bulk-edge correspondence in an active mechanical metamaterial

## TL;DR

This paper experimentally demonstrates a novel bulk-edge correspondence in a non-Hermitian topological phase within an active mechanical metamaterial, revealing unique edge mode behaviors linked to bulk invariants.

## Contribution

The study provides the first experimental observation of non-Hermitian topological phases and their bulk-edge correspondence in a mechanical metamaterial with non-reciprocal interactions.

## Key findings

- Observation of a change in edge mode localization linked to bulk topological invariant shifts
- Experimental validation of non-Hermitian bulk-edge correspondence in a mechanical system
- Demonstration of robustness of the non-Hermitian topological phase

## Abstract

Topological edge modes are excitations that are localized at the materials' edges and yet are characterized by a topological invariant defined in the bulk. Such bulk-edge correspondence has enabled the creation of robust electronic, electromagnetic and mechanical transport properties across a wide range of systems, from cold atoms to metamaterials, active matter and geophysical flows. Recently, the advent of non-Hermitian topological systems---wherein energy is not conserved---has sparked considerable theoretical advances. In particular, novel topological phases that can only exist in non-Hermitian systems have been introduced. However, whether such phases can be experimentally observed, and what their properties are, have remained open questions. Here, we identify and observe a novel form of bulk-edge correspondence for a particular non-Hermitian topological phase. We find that a change in the bulk non-Hermitian topological invariant leads to a change of topological edge mode localisation together with peculiar purely non-Hermitian properties. Using a quantum-to-classical analogy, we create a mechanical metamaterial with non-reciprocal interactions, in which we observe experimentally the predicted bulk-edge correspondence, demonstrating its robustness. Our results open new avenues for the field of non-Hermitian topology and for manipulating waves in unprecedented fashions.