In 1983 Markus Büttiker, Yoseph Imry, and Rolf Landauer made a bold prediction: Persistent currents, long thought to be the exclusive province of superconductors, should also exist in a normal-metal ring—provided the ring is threaded with a magnetic flux. 1 Controversial at the time, the prediction could hardly have been more counterintuitive. Currents induced in a resistive wire usually decay in less than a picosecond, due to thermal fluctuations and the scattering of electrons with defects, phonons, and each other. The inelastic electron-electron and electron-phonon scattering mechanisms, moreover, alter the quantum states of the electrons and rob them of their phase memory.
If the ring is small enough and cold enough, though—on the order of a micron at temperatures under a kelvin, say—the electrons’ phase coherence length may exceed the ring’s circumference as inelastic scattering becomes increasingly rare. Electrons still scatter elastically from thousands of atomic defects, grain boundaries,...
