==Vortex in a superfluid==In a superfluid, a quantum vortex is a hole with the superfluid circulating around the vortex axis; the inside of the vortex may contain excited particles, air, vacuum, etc.
* Hagen Kleinert (1985), «Towards a Quantum Field Theory of Defects and Stresses--Quantum Vortex Dynamics in a Film of Superfluid Helium», Int.