Superfluids are substances such as
Superfluids placed in a closed loop, can flow endlessly without friction.
Vortices were observed
A trapped ultracold gas of Rb atoms. This is a Bose-Einstein condensate (BEC). [From Dalibard et al, ENS.]
A vortex in a dilute ultracold gas of Rb atoms. [From Dalibard et al, ENS.]
It is believed that superfluid properties arise when we have a phase-coherent quantum mechanical macroscopic system.
Maybe, this observation was behind the laconic remark of Onsager (1949):
Thus the well-known invariant called the hydrodynamic circulation is quantized; the quantum of circulation is $h/m$.
whose ramifications keep scientists busy up to this day.