THE search for gravitational waves has a new recruit. These ripples in space-time, which are thought to emanate from violent events such as the collision of black holes and the first moments of the universe’s existence, could be picked up using the spooky phenomenon of quantum entanglement. Machines based on the principle might even outperform the enormous gravitational wave detectors now operating in the US.

When two quantum particles interact they sometimes retain a link between their quantum states. Anything that later changes the state of one of the entangled pair will instantly affect the state of the other – even if they are

separated by a huge distance. This phenomenon is put to use in quantum cryptography, for example, where messages are encoded using a key generated with entangled pairs, usually photons. Trying to listen in on an entanglement-encrypted communication will alter the entanglement between the particles and betray the eavesdropper.

A group of Italian and South African researchers say gravitational waves could also disturb entangled pairs, affecting the relative orientations and spins of an entangled pair of photons, for example. The effect is slight, but by measuring a sufficiently large number of photon pairs the team’s

proposed detector should be sensitive enough to spot a passing gravitational wave, says team member Fabrizio Tamburini of the University of Padua in Italy.

He says the team’s detector should be at least as powerful as the US-based Laser Interferometer Gravitational-Wave Observatory (LIGO), which aims to detect waves by their influence on beams of laser light. Unlike LIGO, Tamburini’s machine could also pick up high-frequency waves. Such waves might have emerged from inflation, the period of rapid expansion thought to have happened just after the big bang. The researchers’ paper has been accepted for publication in Physical Review A.

Spooks enlisted in the hunt for gravity waves

James Hough of the University of Glasgow in the UK, who is part of the LIGO team, welcomes the proposal. “It is always exciting and stimulating to see new ideas for making gravitational-wave detectors,” he says. However, he is far from convinced that it will outdo LIGO. “The entangled-photon system looks to be at least 1000 times less sensitive in the same frequency range,” he says. It might also struggle to see the echoes of inflation, he adds. “Predicted signal levels tend to be very small, below the claimed sensitivity.”

Tamburini is undeterred. He says that the modest sensitivities calculated by the team are based on the faint sources of entangled photons that are currently available. Brighter sources will send the entangled photon system past the sensitivity of LIGO. “It’s just a question of technology,” he says. Michael Brooks ●

“By measuring a large number of photon pairs, the detector may be sensitive enough to spot a passing gravitational wave”

www.newscientist.com 9 August 2008 | NewScientist | 11

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