# Photonic quadrupole topological phases

## TL;DR

This paper reports the first experimental realization of a photonic quadrupole topological phase in silicon photonics, demonstrating robust corner states in a 2D system, which could lead to advanced on-chip optical devices.

## Contribution

It provides the first demonstration of a photonic quadrupole topological phase, showcasing robust corner states and their protection in a 2D silicon photonic system.

## Key findings

- Observation of localized corner states in a 2D photonic system.
- Robustness of corner states against certain defects.
- Contrast with trivial systems showing no robustness.

## Abstract

The topological phases of matter are characterized using the Berry phase, a geometrical phase, associated with the energy-momentum band structure. The quantization of the Berry phase, and the associated wavefunction polarization, manifest themselves as remarkably robust physical observables, such as quantized Hall conductivity and disorder-insensitive photonic transport. Recently, a novel class of topological phases, called higher-order topological phases, were proposed by generalizing the fundamental relationship between the Berry phase and the quantized polarization, from dipole to multipole moments. Here, we demonstrate the first photonic realization of the quantized quadrupole topological phase, using silicon photonics. In this 2nd-order topological phase, the quantization of the bulk quadrupole moment in a two-dimensional system manifests as topologically robust corner states. We unambiguously show the presence of localized corner states and establish their robustness against certain defects. Furthermore, we contrast these topological states against topologically-trivial corner states, in a system without bulk quadrupole moment, and observe no robustness. Our photonic platform could enable the development of robust on-chip classical and quantum optical devices with higher-order topological protection.