One-way transport in quantum dots

Physicists have built a novel device that relies on electron spin to rectify current. Rectifiers are devices that only allow movement in one direction. Examples include self-winding wristwatches and rectifiers in electrical circuits. Even though the exact mechanism is different for each, all rectifiers share a common principle: the rectification is based on an asymmetry in the system that makes it much easier for motion to occur in one direction than another. In recent years there has been much interest in making rectifiers that work in the quantum regime, prompted by the availability of smaller and smaller semiconductor nanostructures and the prospect of technological applications. The first quantum rectifier was built by physicists at Lund University in Sweden and the Niels Bohr Institute in Copenhagen in 1999. This device was able to steer electric current but was not able to restrict flow to just one direction. Now Keiji Ono of Tokyo University in Japan and co-workers have developed an entirely different rectification mechanism in which the spin of the electrons plays a crucial role (K Ono et al. 2002 Science 297 1313-1317). Their quantum rectifier, which consists of two weakly coupled quantum dots, has two important advantages: it is fully controllable and is capable of blocking current entirely in one direction. In the October issue of Physics World, Miriam Blaauboer from the Technical University of Delft in the Netherlands explains how the device works. (U.K.)