Fig. 1.

Experimental scheme and general strategy. (A) Trapped cold atomic fermions move on a square optical lattice in the presence of a synthetic uniform magnetic flux . Two repulsive potentials, initially forming holes in the atomic cloud, are suddenly removed at time . At all times, atoms are confined by an additional circular potential. We generally assume that the confining barriers are perfectly sharp but eventually discuss the case of smoother potentials. (B) The system is initially prepared in a quantum Hall phase: Chiral edge states propagate along the edges determined by the repulsive walls and the external confinement. After releasing the walls, the edge states tend to propagate along the Fermi radius determined by the circular confinement: They encircle the initially vacant regions.