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As first suggested by Fritz London, superconductivity and superfluid flow in liquid helium are macroscopic quantum phenomena. They depend on the fact that the energy states of even macroscopic objects, although closely spaced, are discrete, and on the statistical mechanics of systems made up of identical particles. The electrons in a superconducting metal, with a spin of one‐half, obey Fermi‐Dirac statistics and the exclusion principle. Helium atoms of isotopic mass 4 obey Einstein‐Bose statistics, in which there can be many particles in the same quantum state, as is the case with photons, the quanta of radiation, if they are regarded as particles.
A diverse class of physical systems—including superconductors, superfluid helium, lasers and quasi‐one‐dimensional conductors—derive their unusual properties from the macroscopic occupation of a single quantum state.