Britannica Sources used in this answer

Why can Josephson junctions provide an accurate standard of voltage?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Figure 1: Specific heat in the normal (Cen) and superconducting (Ces) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (Tc) and above Tc. — superconductivity Figure 1: Specific heat in the normal (C_en) and superconducting (C_es) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (T_c) and above T_c.

Josephson junctions serve as an accurate standard of voltage because the voltage across the junction is directly related to the frequency of oscillating supercurrents through fundamental physical constants.1

When a DC voltage ( $V$ ) is applied across a Josephson junction, the supercurrent oscillates at a frequency ( $ν$ ) defined by the relationship:
$ν = \frac{2 e V}{h}$
In this equation, $e$ represents the charge of an electron and $h$ is Planck’s constant. Because frequency can be measured with extreme precision, this relationship allows for the determination of voltage with high accuracy.23

Prior to the use of the Josephson effect, voltage standards relied on Weston cadmium cells, which were prone to drifting and caused inconsistencies between laboratories. The Josephson method has eliminated these issues, providing a standard that remains consistent across different times and locations to within a few parts in $10^{8}$ . By interacting the junction with microwave radiation, researchers can observe current steps that allow for the precise maintenance of this voltage standard.23