Quantum Interference of Topologically Protected Photonic States in a Laser-Written Waveguide Array

The topological properties of matter strongly influence quantum transport in solid-state systems. The same ideas can be applied to photonics systems, e.g. waveguide arrays or photonic crystal structures, for which a proper band engineering can lead to the creation of topologically protected surface states of light [1, 2]. Such photonic states, thanks to their remarkable robustness against scattering with lattice imperfections and defects, potentially find important applications in the field of integrated quantum photonics, where the high fidelity of devices operations is among the primary requirements. In fact, several topological photonics systems have been studied experimentally [1, 2], but the quantum interference between topologically protected single photon states, which is the key ingredient for topological photonics-based quantum devices, is yet to be explored.