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Watchful eye keeps quantum computing on the boil
11 November 2007
From New Scientist Print Edition.
Zeeya Merali

Enlarge Qubit control

They say a watched pot never boils. Thanks to a quirk of quantum mechanics, a similar thing can be said of atoms, which refuse to decay while you peek at them. A new plan to exploit this effect may overcome one of the biggest obstacles to building a quantum computer.

In theory, quantum computers should be far superior to their conventional counterparts because they will rapidly manipulate huge amounts of information stored in quantum bits, or qubits. To build a working quantum computer, the atoms that make up these qubits must be "entangled" - inextricably linked so that making a change to one qubit instantaneously alters the quantum state of its partner. Unfortunately, entanglement is an extremely delicate property, so qubits quickly disentangle, or decay, when they interact with their physical surroundings, spoiling attempts to carry out complicated calculations.

Now Sabrina Maniscalco at the University of Turku in Finland and her colleagues believe they have found a way to slow down the quantum disentanglement long enough to perform calculations by "watching" the qubits' surroundings. The strategy is based on a weird quantum phenomenon called the quantum Zeno effect, which was predicted in the 1970s and demonstrated in the 1980s.

Essentially, says Maniscalco, the effect allows you to slow an unstable atom's decay by measuring various aspects, such as its energy level. "It's counter-intuitive, but in these experiments you seem to freeze time."

However, using the effect to protect entangled qubits is far from straightforward because, according to quantum mechanics, making observations of quantum-scale particles alters their properties. That means you risk breaking the very entanglement that you are trying to shield, she says.

So rather than meddling with the qubits directly, the researchers realised that they could delay disentanglement simply by observing the region around them. They analysed how this could be done in the case of two entangled qubits trapped in an optical cavity (see Diagram). If these qubits disentangle, they emit a photon. Curiously, the researchers found that setting up a detector to watch for this photon at regular intervals would, in theory, delay the emission of the photon - and by extension disentanglement - for a few microseconds. Such indirect observation would temporarily block the environment's ability to disrupt the qubits.

To induce the delay, the builder of a quantum computer would need to perform a high-speed series of individual photon measurements, says Maniscalco. "Freezing time" completely would be impossible, because you would need to perform measurements with an infinitely small delay between each one, which cannot be done. However, measurements roughly every 100 nanoseconds - the best today's technology could achieve - would be enough to win the extra few microseconds needed for computation, she says.

Barry Garraway, a quantum physicist at the University of Sussex, UK, is impressed, but notes that today's optical cavities usually contain defects which may leak energy and disrupt the Zeno effect. "The team will require the very best cavities, stretching our current ability to make them to the limit," he says.

The team have submitted their work to Physical Review Letters (www.arxiv.org/abs/0710.3914).

From issue 2629 of New Scientist magazine, 11 November 2007, page 16

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