# Quantum interference of topological states of light

## TL;DR

This paper demonstrates high-visibility quantum interference of topological states of light in an integrated photonic circuit, showing their potential for robust quantum information processing.

## Contribution

It reports the first high-visibility Hong-Ou-Mandel interference of single photon topological states, enabling scalable topological quantum photonics.

## Key findings

- Achieved 93.1% HOM interference visibility with topological states
- Demonstrated control of indistinguishable single photon topological states
- Showed feasibility of integrating topological states in quantum photonic circuits

## Abstract

Topological insulators are materials that have a gapped bulk energy spectrum, but contain protected in-gap states appearing at their surface. These states exhibit remarkable properties such as unidirectional propagation and robustness to noise that offer an opportunity to improve the performance and scalability of quantum technologies. For quantum applications, it is essential that the topological states are indistinguishable. Here we report high-visibility quantum interference of single photon topological states in an integrated photonic circuit. Two topological boundary-states, initially at opposite edges of a coupled waveguide array, are brought into proximity, where they interfere and undergo a beamsplitter operation. We observe $93.1\pm2.8\%$ visibility Hong-Ou-Mandel (HOM) interference, a hallmark non-classical effect that is at the heart of linear optics-based quantum computation. Our work shows that it is feasible to generate and control highly indistinguishable single photon topological states, opening pathways to enhanced photonic quantum technology with topological properties, and to study quantum effects in topological materials.