Twisted optical fibres as photonic topological insulators

The breaking and enforcing of symmetries is a crucial ingredient in designing topologically robust materials. In electronic and microwave systems, magnetic fields can break time-reversal symmetry to create Chern insulators. By contrast, at optical frequencies, natural materials cannot respond to magnetic fields, which presents a challenge for the scalable exploitation of topologically enhanced devices. Here we leverage the natural geometry of fibre to build a scalable photonic Chern insulator by twisting the fibre during fabrication. The twist inside optical fibre breaks an effective time-reversal symmetry and induces a pseudo-magnetic field, which we observe via photonic Landau levels. Unavoidably, this twist introduces a competing topology-destroying effect through a parabolic profile in the effective refractive index. Using simulations to guide experimental materials design, we discover the 'Goldilocks' regime where the real-space Chern invariant survives, guaranteeing topological protection against fabrication-induced disorder of any symmetry class.