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Boffins go dotty over quantum teleportation

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Researchers at the University of Singapore have demonstrated that it is possible to "teleport" information from one so-called quantum dot to another.

The team used a computer model to show that decoherence, the tendency of quantum information to leak into the environment, is not a problem when working on such a small scale, and that quantum information about one dot can be transferred to another without loss.

Quantum dots are large clusters of atoms that can be described by a single quantum state, in the same way that an atom is. This makes them ideal candidates for use as quantum bits, or qbits, in quantum computing.

They used a computer model to study the transfer or swapping of states between two dots, and determined that it can be done perfectly, under certain very specific conditions. The model revealed that the fidelity of the teleportation is proportional to the entanglement of the two dots, and that a two-dot system would still work under a moderate amount of decoherence.

(Entanglement means that the mathematical description of one dot must refer to the other dot, regardless of how far apart from one another they are. Decoherence is the fact of information being lost by a system through interaction with the environment.)

In theory the work paves the way for functioning large scale quantum computers, which depend on being able to move information around, just like normal computers do. But the reality may be a bit more complex.

Professor Ben Murdin, head of the Photonics group at the University of Surrey says he remains cautious.The real test of all these systems comes when you try to scale up.

"It is relatively easy to build one or two identical quantum dots. The problem comes when you want more than that."

Making identical dots is hard enough, he explained. Making more is even harder because you increase your chances of making an error. Even if you could replicate your original dot multiple times without errors, you run into serious problems with information loss caused by the decoherence which increases with the complexity of the system.

"The more dots you have, the more they tend to leak information into the environment, and the less equal they are. Which has a similar sort of effect," he told us. "Essentially, the bigger the system is, the noisier it becomes."

Despite his misgivings, Professor Murdin notes: "This research is a critical step to get quantum computers into real world applications. Right now, information is whizzing around all over the place on your computer. Quantum information will only really be useful once we can move it around, too.

"But as to when we will see a quantum computer for sale...that is a tough question."

The research is by K.W. Choo and L. C. Kwek, entitled Quantum dot as a resource for teleportation and state swapping and published in Phys. Rev. B 75 205321 (2007). ®

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