IT鈥橲 getting even spookier out there. Particles can be strangely
connected over at least ten kilometres, according to results from physicists in
Geneva. Using pairs of 鈥渆ntangled鈥� photons, Nicolas Gisin and his colleagues
from the University of Geneva have shown that the measurement of one particle
will instantaneously determine the state of the other.
More than sixty years ago, Albert Einstein argued that such 鈥渟pooky action at
a distance鈥� was a flaw in quantum theory. But the new results back the theory鈥檚
prediction that pairs of particles can be 鈥渆ntangled鈥� so that a property of
one鈥攊ts spin or position, for example鈥攊s linked intimately with that
of another.
Both particles in an entangled pair are in a 鈥渟uperposition鈥� of more than one
state. If one of the particles is disturbed, by a third particle, say, or a
measuring device, then its superposition collapses into a definite state.
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But the theory holds that this collapse will immediately cause a similar
collapse in the other particle, no matter how far away it is
(鈥淚t takes two to tangle鈥�, 快猫短视频, 28 September 1996, p 27).
Gisin鈥檚 team generated photon pairs in entangled states near Geneva鈥檚 train
station, and sent them along separate optical fibres toward detectors 10
kilometres apart in Bellevue, near Lake Geneva, and Bernex, southwest of Geneva.
Using a statistical analysis of a huge number of pairs, the researchers showed
that the measurement of one photon instantaneously influenced the result
obtained for the other.
Crucially, the measurements on each photon were made almost simultaneously,
so that no signal鈥攅ven one going at the speed of light鈥攃ould have
travelled the 10 kilometres between the photons. As a result, the entanglement
of properties could not have arisen from a signal passing from one photon to the
other, but instead implies a mysterious link between the two.
Such links were originally demonstrated 15 years ago by a team led by Alain
Aspect of the University of Paris, but only for particle pairs separated by no
more than a few metres. Gisin and his colleagues鈥� results, which are yet to be
published, represent a thousandfold increase in the distance over which the
connections hold. 鈥淕isin鈥檚 experiment is really significant,鈥� says Aspect,
鈥渂ecause he shows that he can maintain these amazing quantum correlations over a
very long distance.鈥�