Credit: © 2007 Nature Physics

Developing new measurement standards that are based on fundamental physical constants, such as Planck's constant and the mass of the electron, is a top priority in metrology laboratories around the world. However, the unit of electrical current, the ampere, is still defined in terms of the forces acting on wires carrying currents, which is why a number of groups are working on alternative measurement standards.

A central element in some of these approaches is a 'turnstile' that opens and closes at a well-defined frequency, allowing a specific number of electrons to pass through the turnstile every time it opens. Jukka Pekola and co-workers1 at the Helsinki University of Technology and Stony Brook University have now made a new type of turnstile, based on a hybrid single-electron transistor that consists of an island of superconducting aluminium that is separated from non-superconducting electrodes by aluminium oxide tunnel barriers. The electrons must tunnel through these barriers to enter or leave the island. The device is controlled by a combination of a.c. and d.c. voltages.

Pekola and co-workers measure the current through the turnstile as these voltages are varied. The robust plateaux they observe in their data (see figure) demonstrate that they are able to control the number of electrons that pass through the turnstile when it is open. A quantum measurement standard for current would bring it into line with the standards for resistance and voltage, which are based on the quantum Hall effect and the Josephson effect respectively, thus completing a metrological triangle for electrical units based on Ohm's law.