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Spooky Particles Survive Einstein's Trap

15 March 2000 6:00 pm
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The most unnerving idea in quantum mechanics may be "spooky action at a distance"--the notion that certain particles can affect one another almost instantly across vast reaches of space. Recently in Geneva, this aspect of quantum surreality survived the most cunning trap so far, a series of experiments that pitted it against basic principles of Einstein's relativity.

Spooky action requires "entanglement," an intimate linkage between quantum particles. In a classic example, one of a pair of entangled photons is measured, which forces it to choose a polarization. Instantly, its twin must assume the opposite polarization--even if it's billions of light-years away. Somehow the first photon has sent a signal to its distant twin--a counterintuitive phenomenon that's been shown many times.

But this long-distance relationship might break down under special circumstances. Einstein showed that the flow of time, and even the order of events, are relative: They depend on how fast an observer is moving. That's no problem if two entangled particles are measured by two stationary detectors; it would be easy to tell which particle arrives first. But if one detector is moving close enough to the speed of light, relativistic distortions might wreck the entanglement. Each particle will think that it is measured first, choose its polarization, and then signal the other particle to do the opposite. Hence the headache: If two particles disagree about who is the sender and who is the receiver, how can they be communicating? Will the laws of quantum mechanics break down?

Scientists led by Nicolas Gisin, a physicist at the University of Geneva, put this question to an experimental test. In their lab, they created pairs of entangled photons and sent them down different fiber-optic cables to detectors in the nearby villages of Bernex and Bellevue, 10.6 kilometers apart. To create a relativistic anomaly, the scientists needed to crank a detector up to superfast speeds by spinning it at 10,000 revolutions per minute. An ordinary measuring device would have flown apart in seconds, so the Geneva team had to make a clever--and controversial--substitution.

First they split the Bellevue beam, sending the entangled photon down one of two paths. One path led to a dummy detector made of a sheet of black paper wrapped around a whirling drum, and the other to a stationary detector. Silence at the stationary detector meant that the photon had struck the black paper. Thus, the researchers had created their relativistic-speed detector, says Gisin. By comparing thousands of pairs, the team found that entangled photons stayed entangled, even if each thought it had struck a detector first. Einstein's laws had no effect on spooky action.

Other scientists say the results, which have been posted on the Los Alamos National Laboratory preprint archives, are impressive but not quite airtight. "They are very beautiful from an experimental point of view, but there are too many assumptions," says Anton Zeilinger, a physicist at the University of Innsbruck in Austria. For example, no one is sure that a moving piece of paper is, in fact, equivalent to a moving detector. However, scientists agree that the Geneva experiments are a technological feat. "This is, in a certain sense, a new line in experimental work," says Swiss physicist Antoine Suarez. "You are putting quantum mechanics in a relativistic frame."

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