Researchers in Germany have made an atomic conveyor belt and catapult from light. Stefan Kuhr and co-workers at the University of Bonn can move single atoms around, put them down where they are needed and launch them into space at a specified speed.

The researchers envisage dropping atoms off the end of the belt, one at a time, into some kind of container. This, they say, would be useful for several technological applications, including quantum computers.

Such super-powerful computers exist only in theory at present; they would use the laws of quantum mechanics to boost processing speed and capacity. Some of the proposed designs involve a handful of atoms held in a cavity. But so far, atoms can be placed in cavities only by chance - rather like hoping a bee will buzz into a milk bottle. The conveyor belt picks atoms up and drops them into place.

It might also allow physicists to arrange atoms in straight chains or wires, to make the smallest of electronic circuits.

One of the big difficulties with this sort of atom-craft is that, like the buzzing bee, atoms are usually moving frantically. To make them sit on a conveyor belt they must be slowed right down.

Kuhr's group uses the forces exerted by laser beams to retard fast-moving caesium atoms, which they then hold in a trap of light and magnetic fields. The team then pulls these cold atoms out of the trap one at a time using two laser beams.

The lasers, shining in opposite directions, again trap each atom in their bright beams. But by detuning the light frequencies slightly, the researchers create a series of light troughs that move steadily along the beam. An atom, trapped in one of these troughs, is borne along with it as if on a conveyor belt.

By retuning the beams to the same frequency, Kuhr's group can stop the atom at any point, and hold it in a stationary trough of a standing wave of light, like the depressions in a vibrating dish of water.

The researchers can move atoms over distances of about a centimetre, and position them to within a thousandth of a millimetre. They can accelerate atoms from zero to ten metres per second within a tenth of a millisecond. And, by suddenly switching off the laser beams while the atom is moving, they can catapult it into space at a chosen speed.