Observation of nonlinear edge states in an interacting atomic trimer array.

Exploring the interplay between topology and nonlinearity leads to an emerging field of nonlinear topological physics, which extends the study of fascinating properties of topological states to a regime where interactions between the particles cannot be neglected. For ultracold atomic systems, although many exotic topological states have been recently observed, the nonlinear effect remains elusive. Here, based on the laser-driven couplings of discrete atomic momentum states, we synthesize a topological trimer array, where the atomic interactions give rise to tunable nonlinearities. We observe the formation of nonlinear edge states in the density population evolution and participation ratio with increasing interaction, in contrast to the diffusive transport in a broad interaction range in nontopological arrays. Furthermore, we show the impact of interactions on the population distribution evolved from the initialized single-site population. Our work opens the avenue for exploring emergent nonlinear topological behaviors in ultracold atomic gases.