Quantum computers still work with half their bits missing

6 August 2009
by Katie Silver

In the wacky, unfamiliar world that is quantum computing, a study has found that these curious devices can continue to work perfectly even if half their components, or qubits, are missing.

Quantum computers harness the power of quantum physics where, "strange things can happen," said lead author of the study, Tom Stace at the University of Queensland (UQ) in Brisbane, Australia.

Working on the principle that electrons can be in multiple places at the same time, quantum computers can operate beyond the binary capabilities of ordinary computers. That is they can encode both a '0' and a '1' at the same time, making a third 'superposition' – a concept that is not at all intuitive.

Risk of information loss

These parts or qubits, are tiny particles kept at very cold temperatures and can be a single atom or a photon. Quantum computers can operate in parallel when completing calculations, giving them far greater power than their sequentially processing counterparts.

"If we can take advantage of this phenomena, we can do certain computational tasks that on a regular computer would either be impossible or take so long so as not to be practical," said Stace.

Whilst providing more scope than an ordinary computer, a quantum computer is also at far greater risk of errors and information loss. This is because the world at the atomic level is so sensitive, that qubits can easily be lost or suffer a 'bitslip', whereby they are accidentally zapped to the opposite state.

To get a handle on how serious this problem could be for the future development of quantum computers, Stace along with Andrew Doherty, also at UQ, and Sean Barret at Macquarie University in Sydney, created a complex mathematical model to test what happens when losses or errors happen within the quantum information.

Powerful encryption systems

The study, published in the Physical Review Letters suggests that quantum computers are still able to function normally even when 10% of the information has errors, or up to 50% of the qubits are missing.

One of the major implications of the finding, said the authors, is that both errors and losses can coexist at the same time. Previously, theory suggested that either errors or losses would need to be exclusive, so as not to disrupt the system.

This is the first study to "rigorously prove" that "quantum error correction for loss is possible, just like in classical machines," said David Reilly, a physicist from the University of Sydney, who was not involved in the research. "Now we only have to get to a certain level of [accuracy]… because we know quantum correction will take care of the errors," he said.

Researchers hope that quantum computers could one day make very powerful encryption systems, but they will also have applications ranging from long distance communications networks to machines capable of modelling very complex systems.

Source: http://www.cosmosmagazine.com