March 11, 2004

Quantum computing gets a step closer

Researchers have created the flying qubit.

by Mark Peplow
Nature News

RESOURCES Ion-trap quantum computer Silicon quantum computer Many-ion quantum computer RELATED SITES Monroe's Lab Centre for Quantum Computation Print this article Register for updates when new articles are posted Discuss this article

Scientists have witnessed an atom and a photon - a small packet of light - share the same information. This is an important milestone in the quest to create a 'quantum computer', which could operate much faster than conventional computers.

A quantum computer would process information using atoms and other tiny particles, rather than the transistors and circuit boards of standard computers. The research, published in Nature1, shows that an atom can act as a bit of 'computer memory', and that light can carry the atom’s information from one place to another.

Chris Monroe and colleagues from the University of Michigan used a cadmium atom trapped in an electric field to ‘store’ information about the atom's magnetic state. By pumping energy into the atom with a laser, they forced it to spit out a packet of light. That photon carried an imprint of the atom's information with it, which could be read by a detector.

These mobile bits of quantum information, known as 'flying qubits', have the potential to travel over many kilometres, according to Eugene Polzik, a physicist at the University of Aarhus, Denmark. “Ultimately a quantum link over a very long distance could be created,” he says.

Information is transported using a process called entanglement, says Monroe. When two objects are entangled, they can be in separate physical locations but share the same information at the same time, he explains.

Researchers have already entangled pairs of atoms, and pairs of photons. But this is the first time that scientists have seen a single atom entangled with a single photon. "This has probably been going on in other experiments, it is just that no one has looked for it before," says Monroe.

Here comes the spooky bit

Computers store information as a series of bits: switches that can be 'on' or 'off'. In the cadmium atom, the tiny magnetic fields of the nucleus and an outer electron can either point in the same direction (on) or opposite directions (off). Once the atom is in one of these states it will stay that way for thousands of years, says Monroe.

But in the quantum world, things are different. The key is that the cadmium atom can be both on and off at the same time. This ambiguity is what gives quantum computers their edge over the humble desktop, because it allows a group of atoms to store an enormous amount of information, which they can share through entanglement.

Entanglement is like breaking a penny into two pieces, explains Monroe. By looking at one half, you can tell exactly what the other half looks like because they both share information about the original penny, he says. The tricky, quantum part is that the exact nature of one half is only decided when someone actually looks at the other half, even if they are miles apart.

"Einstein called this 'spooky action at a distance'," says Monroe. "It is as if there are hidden wires connecting the two. We do not know how they got there, but they are essential for quantum computing."

Monroe's next step is to run two of these experiments side by side and mix up the two photons that fly away from the atoms. He hopes that this will remotely entangle the two cadmium atoms, providing another important component for a quantum computer.

Article Copyright © 2004 MacMillan Publishers Ltd. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.