Alice Ring in a Bose-Einstein Condensate*

Topological excitations analogous to 't Hooft-Polyakov magnetic monopoles have recently been observed in spinor Bose-Einstein condensates. While the singular point defect is topologically stable, it can undergo a continuous transition to a more energetically favorable but topologically equivalent structure consisting of a half-quantum vortex ring with a ferromagnetic core. This nonsingular excitation is known as an Alice ring. We observe the transition from monopole to Alice ring experimentally in a spin-1 Bose-Einstein condensate in its polar phase, with numerical simulations matching our experimental results. We further characterize the Alice ring and observe its oscillations in the harmonically trapped condensate.