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Towards Quantum Memory

Storing quantum information is like trying to analyze a dream while you're still having it--simply getting involved can spoil the experience. But now physicists have demonstrated for the first time a way of recording quantum information from a beam of light onto a cluster of atoms without wrecking the message. The new work is a step toward developing computers based on the principles of quantum mechanics and may be a way of linking quantum computers.

The snag with quantum information--such as the position and momentum of subatomic particles--is that measuring it modifies the information. So physicists must figure out how to do the storing without accidentally looking at what it is they are trying to store. Physicist Eugene Polzik and his colleagues at QUANTOP, the Danish Quantum Optics Center in Aarhus, have managed to do just this using so-called squeezed light. Although the rules of quantum mechanics say that one can never simultaneously know both the size and timing of light waves exactly, they do allow a compromise: It's possible to quantum-manipulate, or "squeeze," light to give more precise knowledge of one at the expense of shakier knowledge of the other.

By shining a vanishingly weak beam of squeezed light through a gas of carefully shielded cesium atoms, Polzik and his colleagues have demonstrated that the atoms sense that the light was squeezed and replicate the quantum state of the light among the outermost electrons of each atom. The atoms remember the quantum tickling for about a millisecond, a long time on the scale of atomic physics, the researchers report in 29 July issue of Physical Review Letters. Useful quantum information can be encoded onto the squeezed light either by switching on and off the squeezing or modulating it à la radio transmission.

Light is a good means of communicating between quantum computers, says physicist Samuel Braunstein of the University of Wales in Bangor, U.K. So it's important to find ways of allowing quantum chatter at the interface, he says. The new work is a "substantial step," adds Seth Lloyd of the Massachusetts Institute of Technology, and it has particular significance for communications: "You could imagine this being part of a quantum internet."

Related sites
Eugene Polzik's home page
6th International Conference on Quantum Communications, Measurement and Computing