THE smallest motor ever built has been chugging away merrily in a German lab. Its mechanism consists of a single molecule, and it's powered by nothing more than a beam of light. The team who built it think that such photon-powered motors could be the next little thing in nanotechnology.

Hermann Gaub, a biophysicist at the University of Munich, and his colleagues built their nanomotor from a synthetic azobenzene polymer. This molecule contains pairs of nitrogen atoms with a benzene ring attached on either side. The nitrogen "bridge" between the rings is kinked, but when exposed to a particular wavelength of light, it straightens out and makes the molecule longer. A different wavelength returns it to the kinked form again. The researchers wondered if they could harness this motion to do useful work.

First they chemically tethered molecules of the azobenzene polymer to a coating on the surface of a ...