Josephson voltage standard

A Josephson voltage standard is a complex system that uses a superconductive integrated circuit chip operating at 4 K to generate stable voltages that depend only on an applied frequency and fundamental constants. It is an intrinsic standard in the sense that it does not depend on any physical artifact. It is the most accurate method to generate or measure voltage and, by international agreement, is the basis for voltage standards around the World.

In 1962, Brian Josephson, a graduate student at Cambridge University, derived equations for the current and voltage across a junction consisting of a thin insulating barrier separating two superconductors – now generally known as a Josephson junction. His equations predicted that if a junction is driven at frequency ${\displaystyle f}$, then its current-voltage (I-V) curve will develop regions of constant voltage at the values ${\displaystyle nhf/2e}$ , where ${\displaystyle n}$ is an integer and ${\displaystyle h/e}$ is the ratio of the Planck constant ${\displaystyle h}$ to the elementary charge ${\displaystyle e}$. This prediction was verified experimentally by Shapiro in 1963 and has become known as the ac Josephson effect. This effect found immediate application in metrology because it relates the volt to the second through a proportionality involving only fundamental constants. Initially, this led to an improved value of the ratio ${\displaystyle h/e}$. Today it is the basis for all primary voltage standards. Josephson's equation for the supercurrent through a superconductive tunnel junction is given by

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