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See Inside August 2007

The Gedanken Experimenter

In putting teleportation, entanglement and other quantum oddities to the test, physicist Anton Zeilinger hopes to find out just how unreal quantum reality can get

Part of his fun is confirming the strangeness of quantum mechanics. Quantum indeterminacy notoriously bothered Albert Einstein, who called the theory incomplete. A particle should know where and what it is, he believed, even if we do not, and it should certainly not receive signals more quickly than at light speed.

Einstein’s view remained a matter of interpretation and in the realm of gedanken, or thought, experiments until 1964, when Irish physicist John Bell proved that measurements of entangled particles could distinguish quantum mechanics from Einstein’s position, a mix of locality (signals flow at light speed) and realism (particles possess definite, albeit hidden, properties).

Light-based tests of Bell’s theorem require two detectors to rapidly switch the directions along which they measure the polarizations of entangled pairs. Statistically, local realism dictates that the polarizations can be linked, or correlated, only for a certain percentage of measurements. In a classic 1982 Bell test that set the standard for future attempts, French physicists upheld quantum mechanics—and upended local realism—by observing a greater percentage.

Zeilinger’s first foray into entanglement was as a theorist, when, in 1989, he co-invented a nonstatistical version of Bell’s theorem for three entangled particles—called GHZ states, after the last names of the discoverers (Daniel M. Greenberger of the City College of New York, Michael A. Horne of Stonehill College in Easton, Mass., and Zeilinger). The trio imagined three entangled photons each striking a detector set to measure polarization in one of two directions, either horizontal-vertical or twisted left or right. In principle, four combinations of detector settings would set up a single measurement capable of distinguishing quantum mechanics from local realism.

“It was the biggest advance in the whole business of the comparison of quantum mechanics to local realistic theories since Bell’s original work,” says physicist Anthony J. Leggett of the University of Illinois. Realizing the GHZ experiment took Zeilinger until 2000.

The year before, he also closed a loop­hole in the 1982 French experiment (other loopholes remain) by using two briskly ticking atomic clocks to preclude any chance that the detectors were somehow comparing notes sent at light speed.

A few months ago Zeilinger reported implementing a new kind of statistical Bell test, devised by Leggett, that pits quantum mechanics against a category of theories in which entangled photons have real polarizations but exchange hidden particles that travel faster than light. In principle, such faster-than-light theories might have perfectly mimicked quantum strangeness and let realism go unmolested. Not so, according to the experiment: the results could be explained only by quantum unreality.

So what idea replaces realism? The situation calls to mind one of Zeilinger’s favorite books, the humorous novel The Hitchhiker’s Guide to the Galaxy, by Douglas Adams, in which a mighty computer crunches the meaning of life, the universe and everything and spits out the number 42. So its creators build a bigger computer to discover the question. (An avid sailor, Zeilinger named his boat 42.)

If quantum indeterminacy is like the number 42, then what idea makes it intelligible? Zeilinger’s guess is information. Just like a bit can be 0 or 1, a measured particle ends up either here or there. But if a particle carries only that one bit of information, it will have none left over to specify its location before the measurement.

Unlike Einstein, Zeilinger accepts that randomness is reality’s bedrock. Still, “I can’t believe that quantum mechanics is the final word,” he says. “I have a feeling that if we get really deep insight into why the world has quantum mechanics”—where the 42 comes from—“we might go beyond. That’s what I hope.” Then, finally, would come understanding.

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