In 1962, the British physicist Brian Josephson predicted a surprising effect between weakly coupled superconductors that would allow reference voltages to be reproduced with high accuracy.

This Josephson effect can be observed in a current-carrying junction comprising two helium-cooled superconductors separated by a thin, non-conducting or normal-conducting layer. If a microwave of frequency f is emitted onto this junction, its current-voltage characteristic curve demonstrates steps of constant voltage U_n = n ∙ K_J^–1 ∙ f. The Josephson constant K_J = 2e/h (with the electron charge e and Planck’s constant h) is around 5 ∙ 10¹⁴ Hz/V. With frequencies of 10 to 70 GHz, voltages of several 100 μV can be generated in this way.

The creation of a Josephson voltage standard was pioneered by Dr. Bruno Fuhrmann of TU Braunschweig and Dr. Volkmar Kose of PTB Braunschweig, who were awarded the 1975 Helmholtz Prize for their work titled “Accurate electrical DC and DC-voltage source with long-term stability”. The award also served as an acknowledgement of the successful collaboration between two scientists, one working in a university and the other in a non-university setting.

The voltage standard developed by Fuhrmann and Kose consisted of two Josephson junctions, an ohmic voltage divider and a SQUID (Superconducting Quantum Interference Device), all of which were cooled with liquid helium to 2 K. By irradiating the junctions with 70 GHz microwaves, a reference voltage of 3 mV was first generated from which the researchers obtained a voltage in the range of 1 V by means of the voltage divider. To this end, they allowed a constant 30 mA direct current to flow through the voltage divider and compared the voltage that dropped across a partial resistance of 0.1 Ω with the reference voltage. They measured the difference between the two voltages by means of a SQUID picovoltmeter. After the voltage divider had been set in accordance with the reference voltage, a voltage of 960 mV dropped across the total resistance (32 Ω) of the voltage divider; it was then possible to compare the voltage of 960 mV with conventional voltage standards based on Weston standard cells. By means of the Josephson voltage standard, it was possible to stabilize and reproduce DC voltages and direct currents with an accuracy of 3 ∙10^–9.

Since then, Josephson series arrays have been manufactured at PTB that can be used to reproduce voltages of up to 15 volts with a relative uncertainty of around 10−9. By triggering the individual Josephson junctions, DC voltages between -10 V and +10 V can be set precisely and almost at will.