where ℏ ℏ is the Planck constant divided by 2 π π , m is the mass of the superfluid particle, and Δ Δ tot ϕ ϕ v is the total phase difference around the vortex. Because the wave-function must return to its same value after an integer number of turns around the vortex (similar to what is described in the Bohr model), then Δ Δ tot ϕ ϕ v = 2 π π n , where n is an integer. Thus, the circulation is quantized: