Physicist Anton Zeilinger may not understand quantum mechanics, but he has not let that stand in his path. Besides paving the way for ultrapowerful computers and unbreakable codes that run on quantum effects, the 62-year-old Austrian has a gift for pushing the limits of quantum strangeness in striking ways. Recently he observed the delicate quantum link of entanglement in light flickered between two of the Canary Islands, 144 kilometers apart. He dreams of bouncing entangled light off of satellites in orbit.
Though better known to the world at large for such headline-grabbing experiments, Zeilinger, who is based at the University of Vienna, has gone to comparable lengths to test the underlying assumptions of quantum mechanics itself. His results have left little hiding space from the conclusion that quantum reality is utterly, inescapably odd—so much so that 40 years after first encountering it as a student, Zeilinger still gropes for what makes it tick. “I made what I think was the right conclusion right away,” he says, “that nobody really understands it.”
For almost 17 years Zeilinger’s work has centered on tricks of entangled light. Two particles are called entangled if they share the same fuzzy quantum state, meaning neither of them begins with definite properties such as location or polarization (which can be thought of as a particle’s spatial orientation). Measure the polarization of one photon, and it randomly adopts a certain value, say, horizontal or vertical. Oddly, the polarization of the other photon will always match that of its partner. Zeilinger, whose group invented a common tool for entangling polarization, likes to illustrate the idea by imagining a pair of dice that always land on matching numbers.
Equally mysterious, the act of measuring one photon’s polarization immediately forces the second photon to adopt a complementary value. This change happens instantaneously, even if the photons are across the galaxy. The light-speed limit obeyed by the rest of the world can take a leap, for all that quantum physics cares.
Scientists have come to view entanglement as a tool for manipulating information. A web of entangled photons might enable investigators to run powerful quantum algorithms capable of breaking today’s most secure coded messages or simulating molecules for drug and materials design. For six years Zeilinger pushed the record for most number of photons entangled—three, then four (bumped to five in 2004, then six, by a former researcher in his group). In 1997 Zeilinger first demonstrated quantum teleportation: he entangled a photon with a member of a second entangled pair, causing the first photon to imprint its quantum state onto the other member. Teleportation could keep signals fresh in quantum computers [see “Quantum Teleportation,” by Anton Zeilinger; Scientific American, April 2000].
A few years later his group was one of three to encode secret messages in strings of entangled photons, which eavesdroppers could not intercept without garbling the message. He is not always the first to achieve such a feat, but “he has a very good eye for an elegant experiment and one that will convey the thing that he’s trying to convey,” says quantum optics researcher Paul G. Kwiat of the University of Illinois, a former member of Zeilinger’s lab who is now a collaborator.
“The only reason I do physics is because I like fundamental questions,” Zeilinger says between bites of bagel with cream cheese and honey. He had come to Denver for a physics meeting, where he would tell assembled colleagues of his work beaming entangled photons between La Palma and Tenerife in the Canary Islands—extending the range of secret entangled messages by 10-fold.
Broad-faced and smiling, with oval glasses scrunched between his beard and a puff of frizzy gray hair, he looks a little wolflike—ready to catch quantum prey. “All I do is for the fun,” he says.
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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