Observation of Copropagating Chiral Zero Modes in Magnetic Photonic Crystals

Topological singularities, such as Weyl points (WPs) and Dirac points, can give rise to unidirectional propagation channels known as chiral zero modes (CZMs) when subject to a magnetic field. CZMs, as distinct zeroth Landau levels (bulk modes) with high degeneracy, are responsible for intriguing phenomena like the chiral anomaly in quantum systems. The propagation direction of each CZM is determined by both the applied magnetic field and the topological charge of the singularity point. While counterpropagating CZMs have been observed in 2D and 3D systems, the realization of copropagating CZMs has remained elusive. Here, we present the first experimental observation of copropagating CZMs in magnetic photonic crystals hosting a single pair of ideal Weyl points. By manipulating the crystal's structural configuration and applying a uniform bias magnetic field, we spatially alter the locations of the WPs, creating pseudo-magnetic fields of opposite directions for different WPs. This arrangement results in a pair of CZMs that possess the same group velocity and copropagate. Our work opens up new possibilities for the topological manipulation of wave propagation and may lead to advancements in optical waveguides, switches, and various other applications.