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The superfluid transition is displayed by quantum liquids below a characteristic transition temperature. Helium-4, the most abundant isotope of helium, becomes superfluid at temperatures below 2.17 K (−270.98 °C). The less abundant isotope helium-3 becomes superfluid at a much lower temperature of 2.6 mK, only a few thousandths of a kelvin above absolute zero.

Although the phenomenology of superfluidity in these two systems is very similar, the nature of the two superfluid transitions is very different. Helium-4 atoms are bosons, and their superfluidity can be understood in terms of the Bose statistics that they obey. Specifically, the superfluidity of helium-4 can be regarded as a consequence of Bose-Einstein condensation in an interacting system. On the other hand, helium-3 atoms are fermions, and the superfluid transition in this system is described by a generalisation of the BCS theory of superconductivity. In it, Cooper pairing takes place between atoms rather than electrons, and the attractive interaction between them is mediated by spin fluctuations rather than phononA phonon is a quantized mode of vibration occurring in a rigid crystal lattice, such as the atomic lattice of a solid. The study of phonons is an important part of solid state physics, because they contribute to many of the physical properties of materials. A unified description of superconductivity and superfluidity is possible in terms of gauge symmetry breakingSpontaneous symmetry breaking in physics takes place when a system that is symmetric with respect to some Lie group goes into a vacuum state that is not symmetric. At this point the system no longer appears to behave in a symmetric manner. It is a phenome.

One important application of superfluidity is in dilution refrigeratorA dilution refrigerator is a cryogenic device first proposed by Heinz London. Its refrigeration process uses a mixture of two isotopes of helium: helium-3 and helium-4. When cooled below 700 mK, the mixture undergoes spontaneous phase separation to form as.

Recently in the field of chemistry, superfluid helium-4 has been successfully used in spectroscopic techniques, as a quantum solventAny superfluid can theoretically act as a quantum solvent . In practice, the only viable superfluid medium that can be used is helium-4, and it has been successfully accomplished in controlled conditions. Any inhomogeneity in the bulk superfluid medium wi. Referred to as Superfluid Helium Droplet Spectroscopy (SHeDS), it is of great interest in studies of gas molecules, as a single molecule solvated in a superfluid medium allows a molecule to have effective rotational freedom - allowing it to behave exactly as it would in the gasFor other meanings see gas (disambiguation). A gas is one of the phases of matter. Gases are, like liquids, fluids: they have the ability to flow and do not resist deformation. Unlike liquids, however, unconstrained gases do not occupy a fixed volume, but phase.