Observation of unidirectional backscattering-immune topological electromagnetic states

A one-way street for electromagnetic waves A hallmark of the quantum Hall effect, and a consequence of nontrivial topology of the electronic properties of systems manifesting this phenomenon, is the existence of so-called chiral edge states. These are a class of unique electronic states through which two-dimensional electron systems subject to a large magnetic field propagate in a unidirectional fashion, essentially unimpeded by scattering. Recent theoretical studies predicted that analogues to such electron 'one-way' edge modes could be realized for electromagnetic waves in photonic crystals, materials with periodic variations of the refractive index. Zheng Wang and colleagues now experimentally realize such photonic chiral edge states in a designed magneto-optical photonic crystal and demonstrate their unidirectional and scattering-protected nature. The finding of such photonic chiral edge states may enable new classes of photonic devices as well as new experimental realizations of the quantum Hall phenomenon in classical and bosonic systems.