Teleporter sends ions on long-distance journey

03rd February 2009 by Stephen Battersby

WE MAY never zap Jeff Goldblum across a room, but this feat of quantum teleportation is impressive nevertheless.

A team in the US has built a teleporter capable of sending the state of ytterbium ions from one side of the lab to the other; something that until now had only been possible with photons. The team say the technique could dramatically increase the distance over which quantum information can be sent.

Before this, teleportation came in two forms, both with severe limitations. Physicists could teleport quantum states between photons. But photons cannot be stored and so cannot form the basis for any quantum information device that needs a memory. In other experiments, the state of an ion has been teleported to another ion held within the same trapping device, a few micrometres away.

Steven Olmschenk at the University of Maryland in College Park and his colleagues have now devised a third method, using photons to set up a long-range teleport between more substantial bits of matter.

They start with two ions of the element ytterbium, each held in its own "electromagnetic bottle", called a Paul trap. Each ion is in a blend of two states with different energy levels. That blend constitutes a quantum bit of information, or qubit.

A laser beam makes each ion emit a photon. Each ion-photon pair is "entangled" in a shared quantum state in which what happens to one particle can affect the other, no matter how far apart they are. The photons then pass through an optical device which entangles them with each other, a process that also entangles the ytterbium ions with each other.

When the researchers make the right kind of measurement on the first ion, the entanglement acts as a link to teleport its state, or qubit, to the second ion. So the second ion becomes an exact copy of the first.

The ions in this experiment were just a metre apart, but increasing this distance would be straightforward.

Photons cannot carry quantum states over long distances along an optical fibre without the states being destroyed. The promise of this new process is that it could form the basis of a "quantum repeater" that will enable quantum communication over almost any distance: ion teleporters placed at regular intervals along a fibre could briefly store quantum states before sending them on their way, so no single photon has to travel too far.

"I think it's a significant step," says quantum physicist Paul Kwiat of the University of Illinois at Urbana-Champaign. But he says the scheme has a very low data rate of just one successful teleport every 12 minutes, on average. Olmschenk thinks he could up the rate to about a qubit per second, which he says could be useful for quantum cryptography.


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