# Topology with broken parity-time symmetry

## TL;DR

This paper experimentally demonstrates the existence of topological edge states in one-dimensional photonic quantum walks with broken parity-time symmetry, showing topological phenomena persist beyond the unbroken regime and are not solely driven by $ ext{PT}$ symmetry.

## Contribution

It provides the first experimental evidence of topological edge states in the $ ext{PT}$-symmetry-broken regime and establishes a global Berry phase as a topological invariant in non-unitary quantum walks.

## Key findings

- Topological edge states are observed in $ ext{PT}$-broken regimes.
- Global Berry phase defines topological invariants in non-unitary dynamics.
- Topological phenomena are independent of $ ext{PT}$ symmetry status.

## Abstract

Topological edge states arise in parity-time ($\mathcal{PT}$)-symmetric non-unitary quantum dynamics but have so far only been discussed in the $\mathcal{PT}$-symmetry-unbroken regime. Here we report the experimental detection of robust topological edge states in one-dimensional photonic quantum walks with spontaneously broken $\mathcal{PT}$ symmetry, thus establishing the existence of topological phenomena therein. We theoretically prove and experimentally confirm that the global Berry phase in non-unitary quantum-walk dynamics unambiguously defines topological invariants of the system in both the $\mathcal{PT}$-symmetry-unbroken and broken regimes. As topological edge states exist in both $\mathcal{PT}$ unbroken and broken regimes, we reveal that topological phenomena are not driven by $\mathcal{PT}$ symmetry.