Figure 2 - Unwinding the phase twist.
From the following article
Quantum coherence: Just what is superconductivity?
Steven M. Girvin
Nature Physics 1, 83 - 84 (2005)
doi:10.1038/nphys158
a, Going from a state with twisted phases through intermediate states b and c to reach the phase-aligned state of d requires passing through a discontinuity. This singularity, which corresponds to a vortex passing through the ring, comes with an energy cost that prevents current decay.