Probing the edge with cold atoms

The quantum Hall effect is a hallmark of topological physics. It is the first example in which the topology of the system determines a macroscopic phenomenon, the quantization of Hall conductance. In a seminal paper, Halperin related it to the existence of skipping orbits for the electrons at the edge of the sample (1). Although the Hall conductivity is nowadays routinely measured with high precision and used to define the SI unit of electrical resistance, observation of the underlying skipping orbits has been elusive. On pages 1514 and 1510 of this issue, Stuhl et al. (2) and Mancini et al. (3) report a striking visualization of these trajectories using ultracold atoms trapped in a synthetic lattice