  The frictionless, directional propagation of particles at the boundary of
topological materials is one of the most striking phenomena in transport. These
chiral edge modes lie at the heart of the integer and fractional quantum Hall
effects, and their extraordinary robustness against noise and disorder reflects
the quantization of Hall conductivity in these systems. Despite their central
importance, controllable injection of edge modes, and direct imaging of their
propagation, structure, and dynamics, is challenging. Here, we demonstrate the
distillation of individual chiral edge states in a rapidly-rotating bosonic
superfluid confined by an optical boundary. Tuning the wall sharpness, we
reveal the smooth crossover between soft wall behaviour in which the
propagation speed is proportional to wall steepness, and the hard wall regime
exhibiting chiral free particles. From the skipping motion of atoms along the
boundary, we spectroscopically infer the energy gap between the ground and
first excited edge bands, and reveal its evolution from the bulk Landau level
splitting for a soft boundary, to the hard wall limit.
