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The ampere enters the quantum era: A breakthrough with global consequences in the field of metrology

The LNE Laboratory has created a quantum standard for the ampere, accurate to within 10-8.

By developing a quantum ampere standard to within 10-8, the National Metrology and Testing Laboratory has cemented the redefinition of this unit of electrical intensity to quantum physics and a fundamental constant, thereby advancing one of the seven basic International Standard units from its electromechanical definition dating back to 1948.

A new definition for the ampere

The International System of units (or SI), which is built on the seven basic measurement units (i.e. kilogram, ampere, meter, Kelvin, second, mole and candela), should be undergoing a major change in 2018: during the General Conference on Weights and Measures held in Paris, scientists will be highly motivated to update this SI system by aligning it with modern physics. Some of the units can no longer be defined based on physical artifacts, but instead must rely on fundamental constants from the field of physics (e.g. elementary charge for the ampere, Planck's constant for the kilogram) and moreover on quantum physics. This transformation will result in lower measurement uncertainties, provided that the standards serving to undergird these definitions can be properly derived.

Such has already come to pass for the ampere at LNE, France's National Metrology and Testing Laboratory.

Let's recall that the ampere is the unit used to measure the intensity of an electrical current. For nearly 70 years, its definition (founded in 1948) has been correlated with an electromechanical force expressed in Newtons (a force measurement unit), while in reality electrical currents are described at the microscopic scale by a per-second flux of elementary charges. This former definition limited the precision of electrical current measurements as well as all related quantities.

The world's first quantum standard for electrical current

A breakthrough of global significance was attributed to LNE's quantum electrical metrology team, working in the Trappes facility and spearheaded by members Jérémy Brun-Picard, Sophie Djordjevic, Dominique Leprat, Félicien Schopfer and Wilfrid Poirier, who produced an electrical current quantum standard that's both universally applicable and practical.

Their work has led to improving the uncertainties on current measurement traceability by two orders of magnitude compared to uncertainties announced by national metrology institutes. Published in the scientific journal American Physical Society, this team has successfully generated currents whose intensities, over the range of a microampere to a milliampere, relate to the elementary charge, with a relative uncertainty of ten parts per billion (i.e. 10-8).

This strong performance could be achieved thanks to the tweaking of an original electrical circuit that allows for error-free application of Ohm's Law (which is the relationship between current, voltage and a resistance value) using quantum resistance and voltage standards that rely on both the quantum Hall effect and the Josephson effect. These two macroscopic quantum effects are on display, one in two-dimensional conductors the other in supraconductors. Moreover, they are solely correlated with two fundamental constants from the field of quantum physics, namely Planck's constant and the elementary charge.

This new quantum current generator paves the way to an electrical metrology that in the future will be completely based on the engineering of quantum standards.

Jérémy Brun-Picard, Sophie Djordjevic, Dominique Leprat, Félicien Schopfer and Wilfrid Poirier

With, as possible practical consequences in applied research and industry, electrical measurements, and much beyond that, in taking advantage of quantum accuracy and reproducibility.

LNE contact

Press contact : Valérie MULOT - +33 (0)1 40 43 40 93 - valerie.mulot@lne.fr

 

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