One strategy for taking computers into a whole new realm of miniaturization is based on harnessing the unfamiliar quantum laws of the atomic world. Quantum computers, which could be enormously faster than today's machines, are still largely a dream, and they could stay that way unless physicists can find a way to prevent fragile quantum information from being corrupted. But now a team has shown in the 7 September issue of Physical Review Letters that quantum computers can identify errors and fix them.
Current "classical" computers process information as digital ones and zeros. But because a quantum particle can exist in several states at once, quantum computers use a richer unit of information called a qubit, a blend of one and zero. This intermingling allows an array of qubits to carry a whole swathe of numbers simultaneously, so that the computer can act on a vast range of values at once, greatly accelerating the computation. Unfortunately, a passing atom can interact with a qubit, causing some of its information to leak away.
The basic idea for correcting errors is to spread the information of one qubit into a family of linked qubits so that, should any be corrupted, the information can still be recovered from its partners. Raymond Laflamme and his Los Alamos colleagues teamed up with a group of specialists in nuclear magnetic resonance (NMR), led by David Cory of the Massachusetts Institute of Technology in Cambridge, to demonstrate this scheme in a group of molecules where the magnetic orientation of each nucleus encodes zero (up) and one (down).
Cory and the Los Alamos team tested two molecules: alanine, an amino acid, and trichloroethylene. Both provide a single information qubit plus two neighboring control qubits for error correction. First the researchers used NMR pulses to align all three molecules, then left them at the mercy of their surroundings. Itinerant atoms then jostled the molecules out of position. When the researchers realigned all three molecules with another pulse, the response of the two control molecules revealed what error had crept into the information qubit. That error could then be corrected with an additional pulse. "We had enough control to do the right operation and preserve the quantum information," says Laflamme.
Although enthusiastic about the demonstration of principle, researchers emphasize that this is just the first step towards full quantum error correction. "It's a long way from three qubits to a quantum computer powerful enough to solve significant problems," says Peter Shor of AT&T Bell Labs.