Active basic electrical standards

The realization and reproduction of the volt and ampere by national metrology laboratories (NMIs), and in particular in France by the Bureau national de métrologie (BNM), requires the use of special resources. The farad, henry and ohm are the subject of the article "Basic passive electrical standards". .

The unit of electromotive force is reproduced from the alternating Josephson effect, a quantum phenomenon that directly relates a DC voltage U to a constant K _J and a frequency f in the form U = f / K _J . Current devices based on Josephson junction arrays ensure volt reproducibility of the order of 0.1 nV, or 10 ^-10 in relative terms. They can therefore be used to connect secondary standards, such as saturated Weston batteries and Zener diode references, with very low uncertainties.

According to theory, K _J should correspond to the ratio 2e/h, where h is Planck's constant and e the electron's charge. However, the uncertainty surrounding K _J = 2e/h is very large. The value of K _J , expressed in the SI system, is only known with an uncertainty of 4 · 10 ^-7 in relative terms. It is therefore of great interest to improve the SI realizations of the volt. This is one of the aims of the "watt balance" experiment carried out by several national metrology institutes (NMIs).

Subject to certain assumptions, this experiment also leads to a direct determination of h. It would also make it possible to control the kilogram, the only material artifact of the SI defining both the unit and the standard. Eventually, the SI could evolve towards a system based solely on fundamental constants, with mass redefined on the basis of Planck's constant, for example.

In practice, the ampere is reproduced from the ohm and the volt. Eventually, it could also be represented by a quantum phenomenon, the one-electron tunneling effect, particularly for very low current intensities (< 1 nA). This effect, discussed in the last paragraph, would bring the ampere back to the fore by enabling the realization of a quantum current standard whose intensity is directly linked to the electron's charge. To illustrate this point, two experiments based on one-electron systems are discussed. Conducted at several LNMs, they aim to close the quantum metrological triangle by applying Ohm's law either in the form U = RI, by direct combination of the three quantum effects, or in the form Q = CU, by creating a "natural" standard of capacitance....