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Quantum computers hit 'fundamental limit'

Theoretical physicists cast doubt over future storage technology

Robert Jaques, vnunet.com 12 Jul 2005

Next-generation quantum computers that store information in so-called quantum bits, or 'qubits', will be confronted with a fundamental scientific limitation on the life of data stored on the devices, scientists have warned.

According to Dutch theoretical physicists from the Foundation for Fundamental Research on Matter and Leiden University, a quantum computer can only function if the information retains coherence for long enough to be processed.

It is the so-called coherence of the qubit which ensures that quantum information remains intact.

The researchers claim that this coherence spontaneously disappears over the course of time and with it the stored information. This could pose a considerable problem for the development of a quantum computer.

A quantum computer makes use of the fact that a quantum mechanical system - an electron, an atom or even a larger system such as a superconducting quantum bit - can simultaneously exist in two states.

Normally one of the two states disappears as soon as the system comes into contact with the outside world. The coherence then disappears as a result of the 'decoherence' process and the information in a quantum bit is lost, the researchers have warned.

A quantum bit typically consists of a large number of particles, with an unavoidably large number of possibilities to be influenced by the environment and thus be subjected to decoherence.

Jasper van Wezel, Jeroen van den Brink and Jan Zaanen, all attached to the Lorentz Institute at Leiden University, have now investigated whether it is possible to maintain the coherence in an isolated qubit.

Much to their surprise they discovered that the coherence tends to " spontaneously disappear" even without external influences.

"The degradation process is thought to be linked to the occurrence of quantum mechanical spontaneous symmetry breaking," the Dutch scientists explained.

"In classical physics an equivalent example of this process is spontaneous crystallisation in a solution. At a certain position a crystal is spontaneously formed, as a result of which the fluid structure is broken."

According to the researchers' predictions, the coherence in some highly promising concepts for qubits will disappear after about a second.

"Moreover, the smaller the qubits the faster that process occurs. All of this would seem to pose a fundamental limitation on the development of qubits," said the researchers.

"Experimental research will now have to demonstrate whether this phenomenon actually occurs."

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