Superfluidity was discovered in liquid helium below about 2 kelvins (-271°C) at the end of 1937, in Cambridge (J.F. Allen and A.D. Meisener) and independently in Moscow (P. Kapitza). The following year, in Paris, F. London understood that this strange behaviour was a consequence of a phenomenon predicted by Einstein in 1925: in some fluids, atoms can adopt a coherent collective behaviour; together they merge in a wave of matter that moves with no friction. This wave is called a “Bose-Einstein condensate”.
All particles cannot form such a state: Einstein’s prediction only works for particles belonging to the “boson” family. Indeed, because of different symmetries, quantum mechanics classifies particles in two families with very distinct properties: bosons and fermions. Fermions cannot form a condensate. A helium-4 atom is a boson; it was in that isotope of helium that superfluidity was discovered in 1937. Electrons are fermions; however, a pair of electrons is a boson: today, we know that in a superconductor, electrons merge in a collective wave similar to superfluidity but, in order to do that, they have to make pairs. Because the nucleus of a helium-3 atom has one less neutron than helium-4, the helium-3 atom is a fermion. In 1973, it was discovered that liquid helium-3 also becomes superfluid, but at a temperature ten thousand times lower than its heavier isotope (helium-4) and only if the helium-3 atoms form pairs, just like superconducting electrons.
The absence of viscosity in a superfluid was explained thanks to the works of Lev Landau in 1941 and Bogoliubov in 1947. Landau noticed that in order for a superfluid to lose energy, quantum states of energy superior to that of the fundamental state had to be excited, and that this could only happen below a minimum speed. This phenomenon has been confirmed in details in liquid helium since 1995, but also in many superfluid gases such as cold atomic vapours (rubidium, sodium, hydrogen, cesium, etc.).
