6. Quantum matter

When vast numbers of particles cluster together, they can, under special conditions, display coherent quantum behaviour at the macroscopic level. ‘Quantum matter’ explains how this produces some of the most exotic matter of which we know: superfluids, in which a quantum fluid suddenly loses all its viscosity, and superconductors, where all electrical resistance is lost. The results are dramatic and spectacular. These phenomena have led to the development of measurement devices that allow the basic unit of mass, the kilogram, to be defined in terms of the fundamental constants of nature to an unprecedented degree of precision.