Superfluids are substances such as

• Liquid Helium in temperature $T < 2.17$ Kelvin.
• Vapours of alkali metal gases (Li, Rb, etc) in temperature $T\sim 1\, nK$.
• and many other systems.

Superfluids placed in a closed loop, can flow endlessly without friction.

Vortices were observed

• In liquid Helium. First, by J. Vinen in 1958.
• In ultra-cold atomic gases, since 1995.

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.

Schrödinger equation