Josephson constant

In 1962 Brian Josephson, a 22-year-old Cambridge graduate student, predicted that Cooper pairs would tunnel through a thin insulating barrier between two superconductors, and that a DC voltage across the junction would drive an AC current at frequency 2eV/h. John Bardeen — already a double Nobel laureate and the B in BCS — publicly doubted it. The effect was confirmed within a year, and Josephson shared the 1973 Nobel Prize.

The consequence for metrology is enormous: irradiate a junction array with a microwave signal of known frequency and it locks onto exact voltage steps of nf/K_J. Since frequency is the best-measured quantity in existence, this turns voltage into a frequency measurement. From 1990 to 2019 a conventional value K_J-90 = 483597.9 GHz/V defined the practical volt; the 2019 SI made h and e exact, so K_J = 2e/h became exact and the volt is now realised, not merely represented, by a Josephson array.