A Photonic Majorana Bound State

Topological photonics sheds light on some of the surprising phenomena seen in condensed matter physics that arise with the appearance of topological invariants. Optical waveguides provide a well-controlled platform to investigate effects that relate to different topological phases of matter, providing insight into phenomena such as topological insulators and superconductors by direct simulation of the states that are protected by the topology of the system. Previous work has largely focussed on the investigation of states that appear at the edge of domains which have different topological invariants. Here, we demonstrate experimentally for the first time a photonic simulation of a Majorana fermion that binds to a vortex defect which is localised in the bulk of a 2+1D photonic material. We realise the vortex in a photonic graphene analogue by introducing a distortion to the hexagonal lattice. The wavefunction of a single photon in the photonic crystal is analogous to that of a single Majorana fermion. The modes lie mid-gap at zero energy. We show adiabatic transport of the mode as the vortex is moved, as well as topological protection against imperfections of the waveguide lattice. Our research promises to open new avenues in the rapidly developing field of topological photonics and provide new insights in the physics of topological solid state systems.