Quantum matter

Abstract ‘Quantum matter’ describes how superconductors and superfluids can have properties like those found in the strange quantum world of atoms, electrons, and photons. In quantum theory, properties such as energy cannot have any possible value, but are quantized. Energy comes in discrete lumps. Particles can act like waves, be in more than one place at the same time, and interfere with themselves. The weirdness of quantum theory is captured in the paradox of Schrödinger’s cat, where quantum effects occur on the macroscopic scale. This is realized in a superconductor, where magnetic flux is quantized. Brian Josephson proposed an electrical circuit that is the basis for a Superconducting Quantum Interference Device (SQUID). These devices are of technological significance as they are used in quantum computing, and to make precise measurements of fundamental physical constants and of magnetic field strengths. SQUIDs are now used in metrology, the science of precise measurements, being the basis for the international standard for the volt, the unit of electrical voltage, and are used in quantum computing.