Contact person
Gunnar Eklund
Forskare
Contact GunnarElectric dc voltage
Electric dc voltage is an area of expertise at the National Measurement Laboratory for Electrical Quantities. We realise the unit related to electrical potential difference (voltage), volt, by utilizing the Josephson effect. We also perform research within this area. In addition, we offer calibration of voltage and can assist with measurements and evaluations related to electric voltage.
The voltage unit
The dc volt (V) is realised by utilizing the Josephson effect, which is an example of a macroscopic quantum physical effect. It is the so-called ac-Josephson effect that is employed, which in short means that a known microwave frequency is sent via a waveguide to an array of Josephson junctions. Depending on the type of array a simultaneous current can be driven through the device, or the output can be measured completely without any additional external input. In both cases, the voltage developed across the array, takes a quantized value which is a multiple of an expression depending solely on the microwave frequency and the numerical value for the ratio of Planck’s constant to twice the elementary charge, the so-called Josephson constant, KJ. Consequently, the traceability originates from Planck’s constant, the elementary charge and the frequency in hertz. When the SI system was revised in May 20, 2019, Planck’s constant and the elementary charge were given exact values to make the redefinition possible. However, in practise, the volt has been realized this way for several decades before the official definition was changed. Before the redefinition an international consensus value for the Josephson constant was used. To ensure traceability, realisation of the voltage unit is performed at least twice a year and between realisations it is maintained by a group of Zener references. The mean value of this group has a yearly stability of ±0.03 μV.
Research and development
Our voltage references are SIS (superconductor-insulator-superconductor) and SNS (superconductor-normal metal-superconductor) arrays which operates around 70 GHz and can generate voltages between 145 μV and 10 V depending on the number of junctions. A 10 V array comprise of 20 000 or more Josephson junctions. The measurement uncertainty for the realisation is of the order of 1 nV/V at 1 V. We continuously work on further development and automatization of the techniques used for voltage realisation.
Contact us
Contact us if you have any questions about our work on electric voltage. We offer calibration of voltage generating and voltage measuring devices and can also assist in the development of new calibration methods. More information about our calibration can be found on our service pages.