Josephson junction

A Josephson junction is a sandwich of two superconductors separated by an ultra-thin insulating barrier. In an ordinary insulator no current flows, but here something remarkable happens: pairs of electrons that carry superconducting current can tunnel coherently across the barrier, so that a supercurrent flows with zero voltage across the junction. Brian Josephson predicted this in 1962, as a graduate student, and it earned him a Nobel Prize.

The junction behaves quantum-mechanically in a way you can almost watch. The supercurrent depends on the difference in quantum phase between the two superconductors — a property of the collective wavefunction that the electron pairs share. If you instead hold a steady voltage across the junction, that phase winds round and round at a rate fixed precisely by the voltage and two fundamental constants, making the junction emit a current that oscillates at a frequency proportional to the voltage. Quantum mechanics here turns a voltage into a clean, calculable frequency.

These exact relations make Josephson junctions extraordinarily useful as precision instruments. Linked in loops, they form SQUIDs — superconducting quantum interference devices — the most sensitive magnetometers known, able to detect the faint magnetic fields of a beating heart or a thinking brain. The voltage-to-frequency link defines the international standard volt, and Josephson junctions also serve as the basic building block of many superconducting qubits in quantum computers.