By means of the state-of-art quantum many-body simulations, performed on the world’s fastest supercomputers, researchers have achieved accurate model calculations for a rare-earth magnet TmMgGaO4 (TMGO). They found that the material, under the correct temperature regime, could realize the the long-sought-after two-dimensional topological Kosterlitz-Thouless (KT) phase, which completed the pursuit of identifying the KT physics in quantum magnetic materials for half a century.