Non-Hermitian non-Abelian topological transition in the S=1 electron spin system of a nitrogen vacancy centre in diamond

Topological phases and transitions are of fundamental importance in physics, which provide a deep insight into the understanding of materials. Recently, non-Abelian topological transitions have been investigated in Hermitian systems, revealing important topological features. With non-Hermiticity introduced, non-Hermitian non-Abelian topological transitions bring about more intriguing topological features, yet has not been experimentally explored. In this work, we report the observation of the non-Hermitian non-Abelian topological transition at the atomic scale utilizing a nitrogenvacancy center in diamond. While the well-established topological numbers, failed to recognize this transition, we successfully characterized such a transition with the measurement of the complex eigenvalue braids. We obtained the braid invariants from the measured relative phases between eigenvalues. The observed change in braid invariants provides a clear signature of the non-Abelian topological transition. Furthermore, we experimentally revealed an intriguing consequence of this transition, which is the creation of a third-order exceptional point through the collision of two second-order exceptional points with opposite charges. Our experimental findings shed light on the abundant non-Abelian topological phenomena involving non-Hermiticity, and provide insights into manipulating the spectral topology in atomic scale systems to achieve exotic functionalities arising from non-Abelian band braiding.