In Anton Zeilinger’s dream world, superfast quantum computers will process data using single atoms instead of silicon chips. Such devices will have fantastic powers, including the ability to transpose matter into packets of information and teleport it through space. But to Zeilinger, even that dream is not exotic enough. When science is truly new, he says, the technology that results from it “cannot be imagined” in advance.
He speaks from experience: The Austrian physicist has spent his career on the outer boundaries of understanding, studying some of the greatest mysteries of quantum physics. While classic Newtonian physics does a fine job of describing the world we see around us, it breaks down utterly when confronted with the unpredictable behavior of the quantum world, the realm of atoms and quarks. Quantum physics addresses that breakdown, but it also leads to ideas so bizarre that Albert Einstein said they had to be in error. He particularly objected to “entanglement” —the notion that twin particles could become intertwined across space and time—and predicted it would never be proved.
Yet Zeilinger is doing just that through an elaborate series of experiments, each one cleverer than the last. In his hands, entanglement is not just a scientific oddity but an essential tool. Using photons, the basic unit of light, he demonstrated that multiple particles could be entangled, a key step toward practical quantum computers. He also was the first to accomplish teleportation (pdf), in which the characteristics of one particle are transferred to another, a breakthrough that could lead to the creation of unbreakable codes. A professor of physics at the University of Vienna and scientific director of the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences, Zeilinger was awarded the first Newton Medal, given by the British Institute of Physics, and the 2010 Wolf Prize in Physics. DISCOVER senior editor Eric Powell caught up with him during a visit to New York City.
How did you come to view the world in such an unusual way?
I grew up after World War II in Austria, so we were very poor. We lived in the Soviet zone, which meant housing was scarce. We were put up on the third floor of a castle in a small village. It had these huge rooms, and I liked to look out the window. So my parents got these bars on the window, and they tied me to them with a harness. I would sit there, hanging out of the window for hours just watching and observing cows and people below. The villagers still talk about the strange child hanging from the castle window watching everything.
So you were intensely curious from an early age?
Oh, I used to take apart everything I could. Like my sister’s dolls. I took the arms and the legs off because I wanted to know how they worked, and I never put things back together, which was not always appreciated, as you can imagine. And later in school I had a very good physics and mathematics teacher. He was able to teach us the basic ideas of relativity theory, such that we believed we understood it, which I now know is not true. Then I learned about quantum mechanics on my own at university, from books, and I was immediately struck by its mathematical beauty.
When did you become interested in entanglement?
In the late 1970s, after I came to MIT, I read the famous 1935 Einstein-Podolsky-Rosen paper, which criticized quantum mechanics as incomplete and first raised the idea of quantum entanglement as a thought experiment. If entanglement was correct, Einstein and his coauthors argued, then two particles would be connected over large distances in such a way that by measuring the properties of one you could predict the properties of the other. But, they argued, this scenario violated the Heisenberg uncertainty principle, which said that it’s impossible to know both the position and momentum of a particle at the same time [because the act of measuring one instantly and unavoidably changes the other]. Because the two theories were at odds with each other, they said, quantum mechanics must be incomplete.
Some vital element was missing, you mean?
That’s what they argued. They said that physics has to be about things really existing out there independent of our doing a measurement; that was the basic tenet of Einstein all his life. Today we know that the argument was wrong.
And how do we know that?
Thanks to physicist John Bell, who took quantum entanglement seriously. Bell developed a mathematical proof called Bell’s theorem to test the thought experiment Einstein had suggested, which was based on the assumptions of a theory called local realism. In local realism, it is assumed that particles carry properties independent of observation, and that no information can travel faster than the speed of light between the particles. This leads to experimental predictions conflicting with quantum mechanics, which states that the very act of measuring a particle changes the properties measured and that this change happens faster than light. But back when Bell created his proof, it wasn’t possible to do a real-world experiment that could decide between local realism and quantum mechanics.
We were put up on the third floor of a castle in a small village. The villagers still talk about the strange child hanging from the castle window watching everything.”
Today you can test Bell’s theorem in the laboratory with entangled particles. What do these experiments conclude?
That local realism doesn’t work. For example, say you are experimenting with entangled photons. As soon as you measure one of the entangled photons in a detector and find that its polarization—that is, the orientation of its waves—is horizontal, the other one in the pair is instantly projected into a horizontal state. And this happens not because the photons were both horizontally polarized from the beginning. That is contradicted by the experiments. It doesn’t matter whether you look at the two particles at the same time, separated over large distances, or one after the other; the results are the same. So it seems as if quantum mechanics doesn’t care about space and time.
So does that mean Einstein was wrong?
There are still some technical loopholes in the experiments testing Bell’s theorem that could allow for a local realistic explanation of entanglement. For instance, we don’t detect all the particles in an experiment, and therefore it is conceivable that, were we to detect every single particle, some would not be in agreement with quantum mechanics. There is a very remote chance that nature is really vicious and that it allows us to detect only particles that agree with quantum mechanics. If so, and if we could ever detect the others, then local realism could be saved. But I think we are close to closing all of these loopholes, which would be a significant achievement with practical implications for quantum technologies.
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