Junctra

Meet Junctra, a Josephson junction: two superconductors with a whisper-thin barrier between them. Give her a voltage and she oscillates.

In a superconductor, electrons pair up into Cooper pairs that share a single quantum phase. Pairs can tunnel through a thin insulating barrier between two superconductors. Brian Josephson predicted in 1962 that if a steady DC voltage V is held across the junction, the phase difference grows steadily in time, and the supercurrent alternates at a frequency f = 2eV/h. The 2e is there because the carriers are pairs of electrons.

The ratio 2e/h is about 483.6 megahertz per microvolt. So a junction biased at just ten microvolts oscillates at about 4.8 gigahertz.

Run the relation backwards and a frequency, which can be measured extremely precisely, defines a voltage. Arrays of many junctions are used as voltage standards in metrology labs, and since 2019 the SI fixes e and h exactly, which gives the Josephson constant an exact value.

Junctra only behaves this way when she is cold enough to be superconducting, typically a few kelvin for the niobium junctions used in standards.