# Fermionic time-reversal symmetry in a photonic topological insulator

## TL;DR

This paper demonstrates that photonic systems can exhibit fermionic-like time-reversal symmetry protected edge states, challenging the assumption that bosonic systems cannot support such topological features.

## Contribution

It introduces a photonic topological insulator with a $ ext{Z}_2$ invariant supporting counter-propagating edge states under time-reversal symmetry.

## Key findings

- Experimental demonstration of chiral edge states in a photonic system.
- Encoding of effective spin as a waveguide lattice degree of freedom.
- Fermionic properties realized in bosonic photonic structures.

## Abstract

Much of the recent enthusiasm directed towards topological insulators as a new state of matter is motivated by their hallmark feature of protected chiral edge states. In fermionic systems, Kramers degeneracy gives rise to these entities in the presence of time-reversal symmetry (TRS). In contrast, bosonic systems obeying TRS are generally assumed to be fundamentally precluded from supporting edge states. In this work, we dispel this perception and experimentally demonstrate counter-propagating chiral states at the edge of a time-reversal-symmetric photonic waveguide structure. The pivotal step in our approach is encoding the effective spin of the propagating states as a degree of freedom of the underlying waveguide lattice, such that our photonic topological insulator is characterised by a $\mathbb{Z}_2$-type invariant. Our findings allow for fermionic properties to be harnessed in bosonic systems, thereby opening new avenues for topological physics in photonics as well as acoustics, mechanics and even matter waves.