The 1996 Nobel Prize in Physics, awarded to David M. Lee and Robert C. Richardson of Cornell University, and Douglas D. Osheroff of Stanford University, recognizes the discovery of superfluidity in helium-3. This discovery, made in 1972, required them to cool liquid helium-3 to a very low temperature: 2 millikelvin, or two thousandths of a degree above absolute zero.
A superfluid is a liquid that flows without viscosity or inner friction. For a liquid to become superfluid, the atoms or molecules making up the liquid must be cooled or "condensed" to the point at which they all occupy the same quantum state. A liquid of helium-3, an atom whose nucleus is made up of an odd number of particles, is a type of particle known as a fermion. Groups of fermions are not allowed to occupy the same quantum state.
By cooling the liquid to a low enough temperature, helium-3 atoms can pair up (left panel). The number of particles in each nucleus adds up to an even number, making it a type of particle known as a boson. Groups of bosons can fall into the same quantum state, and therefore superfluidity can be achieved. Helium-4 (middle panel), a boson, does not need to pair up to form a superfluid; groups of helium-4 atoms condense into the superfluid state at about 2 degrees above absolute zero. Superfluidity, especially the kind that exists in helium-3, is analogous to conventional low-temperature superconductivity, in which electrons flow through certain metals and alloys without resistance. In a superconductor (right panel), electrons, which are fermions, pair up in the metal crystal to form "Cooper pairs," bosons which can then condense into a superconducting state. (Illustration by Malcolm Tarlton, AIP)