Developing new measurement standards that are based on the 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 standards.

A central element in some of these approaches is a 'turnstile' that opens and closes at a well-defined frequency, f, allowing a well-defined number of electrons, N, to pass through the turnstile every time it opens, leading to a current I = Nef, where e is the charge of the electron. Writing in Nature Physics, Jukka Pekola and co-workers at the Helsinki University of Technology and Stony Brook University report a new type of turnstile based on a hybrid single-electron transistor (Nature Phys. doi:10.1038/nphys808; 2007).

The hybrid device consists of an island of superconducting aluminium that is separated from copper source and drain 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 d.c. voltage across the source and drain, and a combination of a.c. and d.c. voltages that are applied to a metal gate electrode.

Pekola and co-workers measure the current through the turnstile as these voltages are varied, with the plateaux in the data (left) demonstrating that they are able to control N. In both figures the vertical axis is the current and the horizontal axes are different components of the voltages. The figure on the far left, which was recorded at a frequency of 12.5 MHz, shows N increasing from zero up to ten. The plateaux in the other image, which was recorded at 20 MHz, correspond to N = 1, 0 and −1 (that is, a current flowing in the opposite direction).

A quantum 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.