Abstract
Antiferromagnetism, characterized by alternating magnetic moments, has garnered renewed interest for its potential applications in spintronics and axion electrodynamics. Its synergy with topology may yield an exotic topological phase unique to a certain magnetic order, termed antiferromagnetic topological insulators (AF TIs). A hallmark signature of AF TIs is the presence of a single surface Dirac cone-a feature typically associated with strong three-dimensional (3D) topological insulators but lacking direct observation. Here, we theoretically and experimentally discover a 3D photonic AF TI protected by the combined symmetry of time reversal and half-lattice translation. By measuring both bulk and surface states, we directly observe the symmetry-protected single-Dirac-cone surface states and their remarkable robustness against random magnetic disorders. To our knowledge, our work constitutes the first realization of photonic AF TIs and photonic analogs of strong 3D topological insulators, opening a chapter for exploring topological photonic devices and phenomena incorporating additional magnetic degrees of freedom.