Macroscopic Quantum Interference Effects through Superconducting Point Contacts

A small-area contact between two pieces of superconducting metal or alloy is a remarkably sensitive and convenient device for demonstrating long-range quantum interference effects. It has been shown in many experiments that the critical supercurrent through a pair of such contacts in a parallel superconducting circuit has a component periodic in the flux enclosed by the topological ring formed by the parallel circuit, the periodicity being $\frac{h}{2e}$, showing that the wave function is coherent around the ring. With this configuration we have demonstrated coherence lengths of the order of millimeters or centimeters in Nb, V, Ta, Sn, Pb, and ${\mathrm{Nb}}_{0.85}$ ${\mathrm{Zr}}_{0.15}$, in rings where the enclosed area was as large as 1 ${\mathrm{cm}}^{2}$, so that the periodicity in applied field was of the order of ${10}^{\ensuremath{-}7}$ G. Both bulk and thin-film materials have been used. These experiments provide general proof of London's concept of long-range phase coherence in both type-I and type-II superconductors, and in addition lead to useful laboratory instruments. We have used such devices to show quantized flux pinning in bulk type-II superconductors and to measure the magnetic field of a rotating superconductor.