The “new” ohm and the “new” volt already exist. However, they have not yet truly established themselves within the international system of units (SI). This is about to change with the “new” ampere. Within the planned “new” SI, the base unit of current will then be traced to the corresponding fundamental constant – the electric charge of a single electron. In time for the (perhaps critical) decisions on the change to the “new” SI at the Conférence Internationale des Poids et Mesures this November, a group of scientists from the Physikalisch-Technische Bundesanstalt (PTB) has succeeded in developing a current standard that not only generates a single-electron current, but also simultaneously measures this current independently. For this considerable advance, the scientists under Hans Werner Schumacher's lead have now been awarded the Hermann von Helmholtz Prize.This prize is endowed with €20,000 and is deemed one of the most significant international awards in the field of metrology – the science of accurate measurement.

The ampere has long been a problematic unit: in spite of its being a base unit – and thus a basis for all electric measurements – it can only indirectly be realized in a metrologically accurate way via other electrical units (the volt and the ohm). It has long been possible to accurately realize these two units on the basis of fundamental constants – the Josephson constant (for the volt) and the von Klitzing constant (for the ohm). Scientists worldwide are now putting great effort into doing the same with the ampere. The appropriate fundamental constant is the charge of a single electron. In principle, this constant can be measured by having single electrons “tunnel” quantum-mechanically in corresponding circuits. This takes place via a so-called "single-electron pump", a device first designed in 1990. However, it is only recently that scientists from PTB have succeeded in measuring the change in the charge of each individual “electron jump” directly and with extremely high accuracy.

To this end, Prof. Schumacher and his team developed a so-called "self-referenced" quantum current source. This is a semiconductor circuit which is equipped with several pumps and detectors and is operated near absolute zero temperature. The single-electron pump is a minuscule semiconducting island with two current leads. In pumping operation, first an electron coming from the current lead on the left is loaded onto the island and then released into the other current lead on the right. If this procedure is repeated periodically at a clock frequency, then a current is generated which is only determined by the clock frequency and the single-electron charge. Such semiconductor circuits have already been considered as promising candidates for the realization of the ampere for some time.

Prof. Schumacher's group has now succeeded in measuring, for the first time, the current that is involved in each single-electron jump. Their pump transports only a few dozen electrons per second. This is slow enough to allow precision measurements. As the scientific journal "Nature" announced some time ago, PTB's new development represents a decisive step towards a new definition of the ampere. It furnishes proof that the realization of the new definition on the basis of the charge of a single electron can indeed work.

Additionally, the new current source allows the generation of validated small currents down to the attoampere range (10^-18 amperes) with a clearly lower measurement uncertainty than would be achievable using conventional current measurement methods. Hence, it enables precision calibrations of measuring instruments for small currents, as are used, for example, in radiation protection.