Roland Piquepaille's Technology Trends
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dimanche 29 août 2004

An international team of physicists has entangled five photons for the first time in the world, reports Technology Research News in "Five photons linked." Why is this important? Because it's the minimum number of qubits needed for universal error correction in quantum computing. In other words, they found a way to check computational errors in future quantum computers. The physicists also demonstrated what they call 'open-destination teleportation,' a way to teleport quantum information within and between computers." "They teleported the unknown quantum state of a single photon onto a superposition of three photons. They were then able to read out this teleported state at any one of the three photons by performing a measurement on the other two photons," adds PhysicsWeb in "Entanglement breaks new record". This will be used in about ten to twenty years to move information among quantum networks.

Researchers from the University of Science and Technology of China, the University of Innsbruck in Austria, and the University of Heidelberg in Germany have entangled five photons. "Our experiment demonstrated for the first time the ability to manipulate five-particle entanglement," said Jianwei Pan, a physics professor at the University of Science and Technology of China and a fellow at the University of Heidelberg in Germany.

This makes possible to check quantum computations for errors and teleport quantum information within and between computers. But how?

Error correction uses mathematical codes to detect when a bit has been accidentally flipped, and is widely used in classical computing because electronic and magnetic bits occasionally switch accidentally from a 1 to a 0 or vice versa. Quantum bits are more delicate and require an error correction method to be feasible.
Researchers have devised many quantum error correction codes that can correct these errors and restore a qubit to its proper superposition state. But these codes require entangling multiple particles. Properties of particles that are entangled remain in lockstep regardless of the distance between the particles.
The simplest code calls for entangling three qubits, but requires repeating the process twice using two additional sets of three qubits. The one-step code requiring the fewest entangled qubits uses five. "Five-particle entanglement is the threshold number of qubits required for universal error correction," said Pan.

As it's said above, the researchers also demonstrated open-destination teleportation, which could be used for distributed quantum computing.

Quantum teleportation is akin to faxing a document and in the process destroying the original. Teleportation is a potential method of transferring information within quantum computers, and can also eventually be used to transport information among quantum networks.
Open-destination teleportation The illustration shown on the left describes the open-destination teleportation (Credit for the image: unknown, but found on PhysicsWeb). "To demonstrate open-destination teleportation, Pan and co-workers first teleported the unknown quantum state of a single photon onto a superposition of three photons. They were then able to read out this teleported state at any one of the three photons by performing a measurement on the other two photons," writes PhysicsWeb.

For some more details, you can read "Manipulation and preparation of five-particle entanglement," a news release from the Chinese Academy of Sciences.

The research work about this five-particle entanglement was published by Nature under the name "Experimental Demonstration of five-Photon Entanglement and Open-Destination Teleportation" in its July 1, 2004 issue. Here are the links to the abstract and to the full paper -- via arxiv (PDF format, 19 pages, 379 KB).

And remember that it will probably take twenty years before full-scale quantum computers replace our current silicon-based computers.

Sources: Eric Smalley, Technology Research News, August 25/September 1, 2004; Belle Dumé, PhysicsWeb, June 30, 2004; The Chinese Academy of Sciences, News release, July 5, 2004; Nature, Vol. 430, No 6995, Pages 54 - 58, July 1, 2004

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