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

Jacqueline Godany

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.

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