Josephson Junction

A Josephson junction is two superconductors separated by a thin insulating barrier, thin enough that Cooper pairs tunnel through it. The junction behaves as a nonlinear inductor, an element whose response to current is not proportional the way an ordinary inductor's is. That nonlinearity is what makes the junction useful: it bends the energy spectrum of an oscillator so the first two energy levels are unevenly spaced, which lets engineers address the 0 to 1 transition of a qubit without accidentally exciting higher levels. Nearly every superconducting qubit, transmon, flux qubit, phase qubit, builds on a Josephson junction, and the junction's fabrication quality, the oxide barrier in particular, is one of the main limits on qubit coherence and reproducibility. The junction is named for Brian Josephson, who predicted the tunneling effect in 1962.