Logo of the Physikalisch-Technische Bundesanstalt
symbolic picture: "magazines"

The ampere - on its way to a new definition

Especially interesting for
  • fundamentals of metrology

A new basis consisting of fundamental constants is to be provided to the International System of Units (SI). In this system, the electric units are represented by the base unit ampere. Contrary to the ohm and the volt, which are already traceable to fundamental constants, such a realization has, to date, not been achieved for the unit of current. PTB has now succeeded in developing a current standard which generates a current based on the clocked transfer of single electrons and simultaneously measures the precision of the generated current independently.

This semiconductor structure can measure single electrons and their respective charge. Three singleelectron pumps are operated on the chip; these are connected in series by a semiconductor wire (marked with A). The transferred electrons are detected by means of the two single-electron detectors (C). (Idle elements that do not influence the array are coloured in grey. B: Semiconductor channel)

The ampere is a base unit in the International System of Units (SI), and yet, to realize it with metrological accuracy in practice, a detour via Ohm's law (i.e. via the electric units the volt and the ohm) is always necessary. Those two units can be accurately realized on the basis of fundamental constants – the Josephson constant (for the volt) and the von Klitzing constant (for the ohm). The corresponding fundamental constant for the ampere is the charge of a single electron.

It is, in principle, possible to realize the ampere in a new way, by temporally clocking the flow of single electrons by means of so-called “single-electron pumps”. In the pumping mode, electrons coming from the current lead on the left are first trapped one by one and then released into the other current lead. If this procedure is repeated periodically, this generates a current which is determined only by the clock frequency, the number of electrons transferred per cycle and by the electric charge of the electron.

The self-referenced quantum current source developed at PTB is based on such single-electron pumps: several such pumps are arranged consecutively in a semiconductor array and are connected via islands. Highly sensitive detectors are coupled to these islands to count the number of electrons present on the island. This has allowed a clocked current to be generated and checked in situ. Since the new pump only transports a few dozen electrons per second, it is slow enough to allow the corresponding precision measurements. Additionally, it allows validated small currents down to the attoampere range (10−18 ampere) to be generated with a clearly lower measurement uncertainty than would be achievable using conventional current measurement methods. Hence, it enables measuring instruments to be calibrated for small currents, as are used, for example, in radiation protection.

The scientists involved in the development of this current standard were awarded the Hermann von Helmholtz Prize in 2014.


Lukas Fricke
Department 2.5 Semiconductor Physics and Magnetism
Phone: +49 (0)531 592-2422
Opens window for sending emaillukas.fricke(at)ptb.de

Scientific publication

L. Fricke, M.Wulf, B. Kästner, F. Hohls, P. Mirovsky, B. Mackrodt, R. Dolata, Th. Weimann, K. Pierz, U. Siegner, H. W. Schumacher: A self-referenced single-electron quantized-current source. Phys. Rev. Lett. 112, 226803 (2014)