Quantisation and its breakdown in a Hubbard-Thouless pump
Geometric properties of waves and wave functions can explain the appearance of integer-valued observables throughout physics. For example, these 'topological' invariants describe the plateaux observed in the quantised Hall effect and the pumped charge in its dynamic analogon, the Thouless pump. However, the presence of interparticle interactions can profoundly affect the topology of a material, invalidating the idealised formulation in terms of Bloch waves. Despite pioneering experiments in solid state systems, photonic waveguides, and optical lattices, the study of topological insulators under variation of inter-particle interactions has proven challenging. Here, we experimentally realise a topological Thouless pump with tuneable Hubbard interactions in an optical lattice and observe regimes with robust pumping, as well as an interaction-induced breakdown. We confirm the pump's robustness against interactions that are smaller than the protecting gap, which holds true for both repulsive and attractive Hubbard $U$. Furthermore, we identify that bound pairs of fermions are responsible for quantised transport at strongly attractive $U$, supported by measurements of pair fraction and adiabaticity. For strong repulsive interactions, on the contrary, topological pumping breaks down. Yet, we can reinstate quantised pumping by modifying the pump trajectory while starting from the same initial state. Our experiments pave the way for investigating interacting topological insulators, including edge effects and interaction-induced topological phases.