Superconductivity and Quantum Mechanics at the Macro-Scale

Superconductivity is perhaps the most spectacular macroscopic quantum phenomenon. A “persistent current” in a ring of superconducting wire will continue to flow forever – a laboratory realization of perpetual motion. A voltage across a junction between two superconductors produces an oscillating current with a frequency that is determined exactly by the voltage and the fundamental constant of quantum mechanics, Planck’s constant. Superconductivity is the quintessential example of an “emergent phenomenon” in physics, in which the collective behavior cannot be understood in terms of the properties of any finite collection of microscopic constituents (i.e. electrons). Notable physicists including Einstein, Heisenberg, and Feynman tried and failed for half a century to achieve the basic understanding of superconductivity that was only achieved in the mid 1950’s and early 1960’s. However, many fundamental issues remain to be resolved, including those related to the more recent discovery of unconventional “high temperature superconductivity” in a variety of synthetic metals and the construction of coherent superconducting “Q-bits” which act as laboratory realizations of Schrodinger’s cat.