Superfluidity

Superfluidity is a state of matter in which a fluid flows without viscosity and displays macroscopic quantum behavior, such as quantized circulation.

Bose–Einstein condensate: A state in which bosons occupy a shared lowest-energy quantum state, producing macroscopic quantum behavior. A shared quantum state underlies superfluidity in dilute atomic gases.
Liquid helium: Helium cooled below its boiling point while remaining liquid because quantum effects suppress solidification. Helium provides the classic setting in which superfluid behavior was discovered and studied.
Viscosity: A fluid's resistance to deformation and flow under applied stress. Superfluid flow lacks the ordinary viscous dissipation that defines everyday fluid motion.
Superfluid helium cryogenics: The use of liquid helium below its lambda transition to cool equipment and remove heat. Helium II's high thermal transport makes it useful for cooling superconducting magnets and instruments.
Superconductivity: A state in which a material conducts electrical current without resistance and expels magnetic fields. Both involve macroscopic quantum coherence, but superconductivity concerns charged matter and electromagnetic response.
Landau criterion: A condition for a moving object to create excitations in a superfluid without dissipating energy. It predicts when flow can remain dissipation-free and when excitations begin.
Helium-4: The common helium isotope, whose bosonic atoms form a superfluid below the lambda transition. Its superfluid phase established the classic connection between bosons and superfluidity.
Quantum coherence: A stable phase relationship among quantum states that enables wave-like interference. Long-range coherence lets many particles contribute to one collective superfluid state.
Superfluid gyroscope: A rotation sensor that detects changes in the phase or circulation of a superfluid. Quantized circulation makes superfluid flow a precise probe of rotation.
Supersolid: A phase of matter with crystalline order and superfluid behavior. It combines superfluid transport with spatial order absent from an ordinary superfluid.

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