Evidence for an atomic chiral superfluid with topological excitations

Topological superfluidity is an important concept in electronic materials as well as ultracold atomic gases¹. However, although progress has been made by hybridizing superconductors with topological substrates, the search for a material-natural or artificial-that intrinsically exhibits topological superfluidity has been ongoing since the discovery of the superfluid ³He-A phase². Here we report evidence for a globally chiral atomic superfluid, induced by interaction-driven time-reversal symmetry breaking in the second Bloch band of an optical lattice with hexagonal boron nitride geometry. This realizes a long-lived Bose-Einstein condensate of ⁸⁷Rb atoms beyond present limits to orbitally featureless scenarios in the lowest Bloch band. Time-of-flight and band mapping measurements reveal that the local phases and orbital rotations of atoms are spontaneously ordered into a vortex array, showing evidence of the emergence of global angular momentum across the entire lattice. A phenomenological effective model is used to capture the dynamics of Bogoliubov quasi-particle excitations above the ground state, which are shown to exhibit a topological band structure. The observed bosonic phase is expected to exhibit phenomena that are conceptually distinct from, but related to, the quantum anomalous Hall effect^3-7 in electronic condensed matter.