Observation of chiral edge states with fermionic atoms in a synthetic Hall ribbon

Edge states are a direct manifestation of the effects of topology in fermionic many-body systems. A prominent example is the quantum Hall effect, where chiral edge states are responsible for basic phenomena such as quantized transport. Neutral ultracold fermionic atoms offer a complementary platform to condensed-matter experiments thanks to the high degree of control and tunability over all microscopic parameters. Here we report on the experimental realization of chiral edge states in an ultracold gas of neutral fermions subjected to an effective gauge field. The atoms are confined in a ribbon geometry, with a lattice structure and a tunable width that is provided by a finite-sized ``synthetic'' dimension encoded in the atomic spin. In particular, by imaging individual sites along this synthetic dimension, we detect the existence of chiral edge states and we investigate the breakdown of chirality as a function of the bulk-edge and edge-edge couplings. Our work paves the way towards the investigation of the resilience of chirality against controlled perturbations and to the observation of new topological states of fermionic matter in otherwise inaccessible regimes.