Quantized Photocurrents in Gapless Topological Matter

The quantum Hall effect in gapped systems represents a defining signature of nontrivial topology. Realizing this principle in gapless matter has remained a central challenge in quantum materials. Chiral topological semimetals provide a unique platform to achieve this aim via symmetry-protected multifold crossings that act as Berry-curvature monopoles. When optical transitions are confined to a single multifold node, the resulting circular photogalvanic effect is predicted to be quantized in terms of the topological charge of the node. In real materials, however, this phenomenon remains experimentally elusive, obscured by trivial band transitions, the energy separation between the node pairs, and their relative positions with respect to the Fermi level. Here we observe a quantized circular photogalvanic effect in the chiral topological semimetal Rh0.95Ni0.05Si. Ni substitution opens a photon-energy window dominated by interband optical transitions at the {\Gamma}-point multifold node. This allows circularly polarized near- to mid-infrared pulses to drive a helicity-odd terahertz response that manifests three hallmarks of quantization: a sharp onset, a wavelength-independent plateau governed by the magnitude of monopole charge, and an abrupt cutoff imposed by Pauli blocking. Our work establishes an all-optical analogue for the quantum Hall effect and a new paradigm for topological quantization in gapless matter.