At a microscopic level, electrical current is most often described as a flow of charged particles. While the size of an electrical current can therefore be quantified using a ‘particle flow rate’, this is not the way that the base unit of electrical current in the International System of units (SI) the ampere is presently defined. Instead, the present definition of the SI ampere is based on an electromechanical force expressed in newtons.

One technique to create a ‘quantum current standard’ combines quantum resistance and voltage standards.

This definition, set in 1948, limits the accuracy of electrical measurements and of derived quantities. A major overhaul, planned in 2018, aims at put SI units in line with modern physics to reduce measurement uncertainties, by fixing the value of certain fundamental constants, among them the elementary charge that will be used to define the ampere.

Researchers from the Laboratoire national de métrologie et d’essais (LNE) have developed a quantum current standard, universal and practical, able to generate for the first time, currents of values in a range from microampere to milliampere that are accurately linked to the elementary charge with the targeted relative uncertainty of 10 parts in a billion.

This breakthrough relies on an error-free application of the Ohm’s law to the highly-accurate quantum standards of resistance and voltage, based on two macroscopic quantum effects only linked to fundamental constants, the quantum Hall effect and the Josephson effect, occurring in two-dimensional semiconductors and superconductors respectively.

This novel quantum current generator improves the accuracy of current standards by two orders of magnitude and paves the way to fully quantum-based electrical measurements in the SI.

This research was published in the Physical Review X (PRX) journal of the American Physical Society (APS):
J. Brun-Picard, S. Djordjevic, D. Leprat, F. Schopfer and W. Poirier,
“Practical Quantum Realization of the Ampere from the Elementary Chargeˮ, Phys. Rev. X, 6, 041051 (2016).

It was also covered by a Viewpoint commentary “A New Era for the Ampere”, in the on-line Physics journal of the APS. 

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