Observation of quantum metric and non-Hermitian Berry curvature in a plasmonic lattice

We experimentally observe the quantum geometric tensor, namely, the quantum metric and the Berry curvature, for a square lattice of radiatively coupled plasmonic nanoparticles. We observe a nonzero Berry curvature and show that it arises solely from non-Hermitian effects. The quantum metric is found to originate from a pseudospin-orbit coupling. The long-range nature of the radiative interaction renders the behavior distinct from tight-binding systems: Berry curvature and quantum metric are centered around high-symmetry lines of the Brillouin zone instead of high-symmetry points. Our results inspire pathways in the design of topological systems by tailoring losses or gain.