The article "Observation of Unidirectional Backscattering-Immune Topological Electromagnetic States" by Zheng Wang, Yidong Chong, J. D. Joannopoulos, and Marin Soljačić reports the first experimental observation of electromagnetic chiral edge states (CESs) in a magneto-optic photonic crystal (PhC) fabricated in the microwave regime. These CESs, predicted by Haldane and Raghu, exhibit unidirectional propagation, with forward and backward transmissions differing by nearly 50 dB. Unlike their electronic counterparts, these photonic CESs are immune to backscattering from large metallic scatterers, as the absence of backward-propagating modes prevents any reflection. This unique form of unidirectional transport is a consequence of the nontrivial topological properties of the bulk band structure, which can be characterized by the Chern number. The authors demonstrate the realization of a photonic CES using a square lattice of magnetized ferrite rods, which localizes a unidirectional waveguide mode at the edge of the structure adjacent to a metallic wall. The experimental setup and results show strong unidirectional transmission and robustness against disorder, opening up new opportunities for photonic devices and applications, such as optical isolators and slow light.The article "Observation of Unidirectional Backscattering-Immune Topological Electromagnetic States" by Zheng Wang, Yidong Chong, J. D. Joannopoulos, and Marin Soljačić reports the first experimental observation of electromagnetic chiral edge states (CESs) in a magneto-optic photonic crystal (PhC) fabricated in the microwave regime. These CESs, predicted by Haldane and Raghu, exhibit unidirectional propagation, with forward and backward transmissions differing by nearly 50 dB. Unlike their electronic counterparts, these photonic CESs are immune to backscattering from large metallic scatterers, as the absence of backward-propagating modes prevents any reflection. This unique form of unidirectional transport is a consequence of the nontrivial topological properties of the bulk band structure, which can be characterized by the Chern number. The authors demonstrate the realization of a photonic CES using a square lattice of magnetized ferrite rods, which localizes a unidirectional waveguide mode at the edge of the structure adjacent to a metallic wall. The experimental setup and results show strong unidirectional transmission and robustness against disorder, opening up new opportunities for photonic devices and applications, such as optical isolators and slow light.