Creation of topological states of a Bose-Einstein condensate in a square plaquette of four optical traps

We study a square plaquette of four optical microtraps containing ultracold 87Rb atoms in the F = 1 hyperfine state. In the presence of an external resonant magnetic field, the dipolar interactions couple the initial mF = 1 component to other Zeeman sublevels. This process is a generalization of the Einstein–de Haas effect to the case when the external potential has only C4 point symmetry. We observe that vortex structures appear in the initially empty mF = 0 state. Topological properties of this state arise due to competition between the local axial symmetry of the individual trap and the discrete symmetry of the plaquette. For deep microtraps vortices are localized at individual sites, whereas for shallow traps only one discrete vortex appears in the plaquette. States created in these two opposite cases have different topological properties related to C4 point symmetry.