A TRICK of the light is all that's needed to create a new kind of quantum logic gate. It is the latest step in the quest for a practical quantum computer, which should in theory vastly outperform any classical computer.

These logic gates are made in an "optical lattice", an interference pattern created when three laser beams intersect, forming a 3D grid of bright and dark spots. Ultra-cold atoms of rubidium are held by this cage of light, sitting at the dark spots.

Changing the polarisation of the lasers forces pairs of rubidium atoms together, which makes them start to exchange quantum information encoded in the direction of their spins (Nature, DOI: 10.1038/nature06011). "The spin states are forced to interchange as time progresses," says team member Benjamin Brown of the Joint Quantum Institute in Gaithersburg, Maryland.

This leaves the atoms "entangled" in a shared state in which measuring ...