Quantum fluid

A quantum fluid refers to any system that exhibits quantum mechanical effects at the macroscopic level: such as superfluids, superconductors, and ultracold atoms. Typically, quantum fluids occur in situations where both quantum mechanical effects and quantum statistical effects are significant.

Most matter is either solid or gaseous (at low densities) near absolute zero. However, for both isotopes of helium, helium-4 and helium-3, there is a pressure range within which they can remain liquid down to absolute zero, because the wavelengths of the quantum fluctuations within the helium atoms are larger than the inter-atomic distances.

In the case of solid quantum fluids, it is only a fraction of its electrons or protons that behave like a "fluid". One prominent example is superconductivity, where quasi-particles made up of pairs of electrons and a phonon act as bosons. As these are not subject to the Pauli exclusion principle, they can collapse into the ground state, and thus establish a supercurrent with a resistivity near zero.