Since 2019, the units of the International System (SI) have been defined by fixing the value of seven constants of nature. The ampere, the basic electrical unit, is now linked to the elementary charge, e. Previously, and since 1948, the definition of the ampere was based on Ampère's theorem, which linked the unit of electrical current to the newton. Complex electromechanical experiments were therefore needed to implement this definition, which were incapable of achieving the accuracy required to trace electrical measurements in the SI. As a result, electric current was measured in the national metrology institutes from the farad using capacitance charge experiments or from the volt and the ohm by applying Ohm's law, but with measurement uncertainties of more than one millionth.

The new definition of the ampere has opened the way to practical realizations that exploit quantum phenomena. The challenge now was to develop a quantum current standard capable of controlling a flow of elementary charges with a target measurement uncertainty of less than 10 billionths. A great deal of effort has gone into developing nano-devices capable of manipulating electrons one by one in a controlled manner. However, these systems, which deliver very low currents well below 1 nA, have never been able to demonstrate such an accuracy. In 2016, an alternative route based on applying Ohm’s law to the Josephson voltage and quantum Hall standards, which provides realization of the volt and the ohm respectively, was explored [1]: a programmable quantum current generator (PQCG) reached the target uncertainty in the milliampere range. However, this first result was obtained at the expense of the application of error corrections.

In this work, we report on a new quantum standard that generates currents at perfectly quantized values, without error. It is based on the combination of Josephson voltage, Hall resistance and superconducting amplifier quantum standards in an original quantum circuit (Fig. 1). The accuracy of the currents generated, in the microampere range, at quantized values, ±(n/p)ef_J (where n and p are integer control parameters and f_J is the Josephson frequency), has been demonstrated with relative uncertainties of less than 10 billionths [2].  The ampere can now be realised over a wide range of current values, with uncertainties improved by two orders of magnitude (Fig.2).

At the heart of a complete quantum instrumentation comprising several quantum devices, the new quantum current standard lays the foundations for a universal quantum realisation of the electrical units of voltage, current and even resistance, with a single experiment. With this in mind, cryogenic fluid-free refrigerators and quantum devices operating under less extreme experimental conditions, such as those based on graphene or magnetic topological insulators, will simplify the set-up and operation of the experiment, making the realisation of electrical units more practical and efficient.

[1] J. Brun-Picard, S. Djordjevic, D. Leprat, F. Schopfer and W. Poirier, “Practical quantum realization of the ampere from the elementary charge”, Phys. Rev. X, 6, 041051 (2016). https://doi.org/10.1103/PhysRevX.6.041051.

[2] S. Djordjevic, R. Behr and W. Poirier, “A primary quantum current standard based on the Josephson and quantum Hall effects”, Nat. Commun., 16, 1447 (2025). https://doi.org/10.1038/s41467-025-56413-9.