The ampere is a special case in the physics of electricity: Though it is a very basic dimension, and, like volt and ohm, it is one of the central units of electricity, it hitherto was not possible to measure it directly. Instead, it was necessary to take a detour over voltage and resistance to measure the current. Volt and ohm can be implemented based on natural constants -- the Josephson constant and the von-Klitzing constant.
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.
Schumacher and his team developed a so-called self-referencing quantum current source -- a semiconductor circuit with multiple electron pumps and detectors. The device is operated very closely to absolute zero (0°K). In terms of topology, the single-electron pump is a tiny island with two electric connections. In pumping mode, an electron is placed on the island across one of the connections. In a second step, it is "fired" from the island across the other connection. If this process is repeated periodically, a current is generated which is determined only through the clock cycle and the charge of a single electron.
Such circuits have been said to be a promising candidate for the implementation of a physical ampere for quite a while. The merit of Schumacher and his team is that they, for the first time, succeeded in measuring the current generated at each electron jump. The electron pump devised by the PTB team transports only a few dozens electrons per second -- few enough to enable the high-precision measurements necessary to determine the real value of 1 A. The development is believed to be a decisive step towards a new definition of the ampere.
In addition, the current source from the Braunschweig, Germany, institute enables generating extremely low currents down to the attoampere range (10-18 amperes) at significantly higher accuracy than possible with conventional current measurements. This facilitates calibrating instruments for the measurement of very small currents, as they are used, for instance, in radiation protection applications.
For its achievement the Schumacher team received the €20,000 Helmholtz prize, which will be awarded on June 24 at the Helmholtz Symposium.
This article originally appeared in EE Times Europe with the headline "What is an ampere?"