Quantum Measurement of current one electron at a time redefines the Amp

Graphene Nanopump Zeroes in on the Perfect Ampere - IEEE Spectrum
Thanks to quantum Hall resistance and Josephson voltage measurements, the ohm and volt can be practicably defined within 10 parts per billion. Both, however, depend on empirical measures of current, typically via watt-balance measurements that are accurate to only 100 ppb. (Watt balances, in their turn, depend on the definition of the kilogram, which is still evolving, related initiatives like a Compton-wave definitions of mass.)

That leaves the amp, waiting for a way to produce exquisitely accurate currents.

The Ampere, Redefined | EE Times
Scientists across the globe therefore are working to find a similar constant to determine the unit of 1 ampere. A suitable natural constant could be the charge of a single electron. This charge in principle can be measured by tunneling single electrons in a suited circuit using quantum mechanics. A potential tool towards this end could be a single-electron pump, which has been known since 1990. However, it took the development of PTB researcher Hans Werner Schumacher and his team to transform the theoretical knowledge into the real world and measure the charge difference associated with every single "jump" of an electron, directly and very accurately.





Ampere - Wikipedia, the free encyclopedia
The ampere (SI unit symbol: A; SI dimension symbol: I), often shortened to amp,^[1] is the SI unit of electric current^[2][3] (quantity symbol: I, i)^[4] and is one of the seven^[5] SI base units. It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.

In practical terms, the ampere is a measure of the amount of electric charge passing a point in an electric circuit per unit time, with 6.241×10¹⁸ electrons (or one coulomb) per second constituting one ampere.^[6]

The practical definition may lead to confusion with the definition of the coulomb (i.e., 1 ampere-second) and the ampere-hour (A·h), but in practical terms this means that measures of a constant current (e.g., the nominal flow of charge per second through a simple circuit) will be defined in amperes (e.g., "a 20 mA circuit") and the flow of charge through a circuit over a period of time will be defined in coulombs (e.g., "a variable-current circuit that flows a total of 10 coulombs over 5 seconds"). In this way, amperes can be viewed as a flow rate, i.e. number of (charged) particles transiting per unit time, and coulombs simply as the number of particles.