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.
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.
Your own experiments with entangled particles took off in the 1980s, through your collaboration with theoretical physicist Daniel Greenberger. How did that come about?
Daniel came to Vienna on a Fulbright Fellowship that I had organized for him. I remember the first morning we were sitting there together and we were thinking, “What shall we work on?” It turned out that we both had been asking ourselves how we could extend Bell’s theorem to new domains where it hadn’t been applied yet. And we both thought it might be interesting to investigate whether or not more than two particles could be entangled.
No one else had considered that type of experiment? Nobody had worked on entanglement with more than two particles, though the approach is kind of obvious in hindsight. People have worked with two photons, so why not three, you know? We discovered that multiparticle entangled states could exist in theory [some of them are now called Greenberger-Horne-Zeilinger, or GHZ, states], and I decided that my goal was to realize them in the laboratory. And it turns out that most of the tools you need to create them simply did not exist. The light sources and detectors were not good enough to observe multiparticle entanglement. A lot had to be done.
How were you eventually able to create these states? We found a way. To do this you need four photons—two photons entangled in one state and the other two photons entangled in another state. Then we send one photon from each pair into a detector, and you measure only one in such a way that you don’t know where this one photon you measure came from, from the first pair or from the second pair. If you do that right, the remaining three photons end up being entangled.
On the way to creating multiparticle entangled states, you also found a way to transfer the properties of one particle to another—the closest thing to teleportation we have. How did you become interested in that? Well, that’s actually a funny story. In 1993 a proposal by theoretical physicists came out about quantum teleportation, basically the transfer of one particle’s properties to another that could be arbitrarily far away. When I read this I said, “What are they are talking about? This is a typical theoreticians’ proposal; they don’t have any idea how impossible this experiment is.”
What were the theorists proposing? They had a simple thought experiment. You have two individuals, by convention we call them Bob and Alice, who want to communicate via entangled pairs of particles. Alice has a quantum state, which can represent a particular piece of information, and she wants to transfer this quantum state to Bob. And let’s suppose that the communication channels between the two are very bad, such that Alice cannot send the quantum state itself because it will be disturbed by the environment or whatever. These theoreticians proposed that the way to transfer that quantum state was to entangle a third particle with an already entangled pair. So if Alice has one particle from an entangled pair and Bob has the other, then what Alice does is entangle her particle with a third particle that carries the quantum state she wants to send to Bob. Just by doing this entangling procedure, the information carried by the third particle is teleported over to Bob’s. This is a very elegant idea, but there was no way to do the experiment in the old days.
But on the road to realizing multiparticle entangled states, we actually ended up developing the tools that enabled us to do the teleportation experiment. This was amazing. For teleportation you have to start with two entangled photons, and then you have to be able to entangle one of those with a third photon. That’s the idea. And the big surprise is that this has implications for the new field of quantum information science.
What is the connection between quantum teleportation and quantum computation? A quantum computer needs what is called a universal quantum gate, a device where the quantum state of one system changes depending on the state of another system. It’s similar to logic gates, the circuits that regulate output in classical computers. And teleportation can actually serve as that universal quantum gate.
Before this, what would you tell people if they asked you about the practical implications of your research into the fundamentals of quantum mechanics? I would openly tell them that it had no use whatsoever.
You would say that you were just satisfying your natural curiosity? This is part of being human. It must go back to prehistoric times when people stood there and looked up at the sky and wondered what’s going on up there. We would not have our civilization if people weren’t curious about things. To me this is the most important driving force in science. Even for developing new technology, if you really want to do something new, then by its very nature future technology cannot be imagined.
I mean, who had thought of the mobile phone? When the computer chip was developed, nobody had any idea that this would be in a mobile phone, that you would be able to see photographs of other people and download whole books. Our fantasies just aren’t strong enough. We can imagine a lot about the future but they’re usually the wrong things. If you look at things written in the 1950s about the future, most of it was simply wrong. And the same is true when people talk about the possible future today.
Some of your most recent work involved sending entangled photons across long distances in the Canary Islands. What did you get out of those experiments? Well, two things. One is making progress in developing methodology for worldwide quantum communication with satellites, because the distance [from the ground to low-earth orbit] is about the same. The second is closing another loophole in Bell’s theorem—the freedom of choice loophole. This loophole assumes that the source that creates the two particles in the experiment also somehow creates information that influences the choice of what measurement to make on the particles. This is a completely logically consistent position, and the way to rule it out is to make the decision of what to measure before any information can have reached you from the source.
To test this we designed an experiment on the two islands of La Palma and Tenerife, which are separated by 90 miles. On La Palma we created a pair of photons and sent one of the photons over to Tenerife using a free space telescope link, and on Tenerife we decided a long time before the photon arrived which polarization would be measured.
And when you say a long time, you mean. . . A long time means that—for modern electronics, it’s a long time—it takes half a millisecond for the photon to go from A to B, so if you decide something like a few tenths of a microsecond before, then that’s a long time. Then the other photon is kept locally in a glass-fiber spool and measured at a later time in La Palma. And which kind of measurement to perform on this photon is decided by a random-number generator at the same moment when the photon pairs are created at a distant location, about a mile away, which means that no signal could travel fast enough to influence this.
So were you able to close that loophole? That loophole was closed, and now the big carrot is to do a definitive experiment that closes all loopholes at once. The most important point in closing all the loopholes is that it would lead to completely secure quantum cryptography. The loopholes can in principle be used by an eavesdropper to hide, to cover up his presence and get some of the information. But if you have a loophole-free test of Bell’s theorem, then this is not possible anymore. So there is a practical consequence to these experiments.
What are the philosophical implications of your work? The quantum state represents measurement results; it represents information about a concrete situation, and it allows me to make predictions about future measurement results. So it is information both about a situation that I know and information about the future. I often say that quantum theory is information theory, and that the separation between reality and information is an artificial one. You cannot think about reality without admitting that it’s information you are handling. So we need a new concept that encompasses the two. We are not there yet.
Have any philosophers picked up on the conceptual implications of your research? I have a program where I invite philosophers to see what goes on in the lab, because it changes your intuition. A great majority of philosophers are realists, though sometimes naive realists. I often ask them, “Why are you so realistic? If you analyze your fundamental notions you might conclude that these things are more counterintuitive than you think.” Often the answer is, “Yes, but I want to describe reality.” And then I say, “I also want to describe reality, but why are you not satisfied with describing the reality of the observations? Why do you want a hidden reality that exists independent of the observation?” And I don’t get satisfactory answers.
What is the most fascinating new scientific question you see looming on the horizon? To me the most interesting question is, how do we get to the next theory? I find it extremely unlikely that quantum mechanics will not be superseded some day by a deeper theory, because why not? So far in the history of physics we have always found something deeper. That deeper thing is usually more counterintuitive than what we had before and takes awhile to get used to. Just compare relativity theory [which postulates that time is relative and depends on the observer] to Newtonian space-time. This is part of the motivation for our experiments. We want to peel out in as much detail as possible what the conceptual issues are.
As a person who has spent a life with quantum mechanics, do you think you have a deeper sense of the absurd? It could very well be. I’ve noticed that I’m less surprised by unexpected developments than many other people. I seem to take the unpredictability of things as more natural than many other people. When young people join my group, you can see them tapping around in the dark and not finding their way intuitively. But then after some time, two or three months, they get in step and they get this intuitive understanding of quantum mechanics, and it’s actually quite interesting to observe. It’s like learning to ride a bike. Sure, there’s a lot of interesting physics behind it. But practical experience works too. It’s the same in doing quantum experiments. People learn how to play with the stuff.
You have spent time with the Dalai Lama and have taught him the fundamentals of quantum mechanics. What was that like? Is he a good student? He has a very clear scientific mind. He’s very analytic, very precise. I explained the superposition principle and entanglement and the randomness of measurement events, and he always asked the right questions. I invited him to visit a laboratory in Innsbruck, which has ion traps for individual atoms, and you can usually look at an atom there. I wanted to show this to the Dalai Lama because he didn’t believe in atoms. And interestingly, when he came it didn’t work.
The Buddhism practiced by the Dalai Lama embraces an unbroken chain of cause and effect. How did he respond when you explained the random nature of quantum events? This was something he didn’t like. He said, “You have to look closely, you have to find the cause.” And then he said something interesting: “If this is really true and you can convince us, then we have to change our teaching.” That is a flexibility which not every religion has.
Doesn’t that bother you, too? Don’t you find the random nature of the quantum world a little disturbing? Not at all. I find a reality where not everything is predefined much more comforting because it’s an open world. It’s much richer. To me, the most convincing indication of the existence of a world independent of us is the randomness of the individual quantum event. It is something that we cannot influence. We have no power over it. There is no way to fully understand it. It just is.
You have said that children should be introduced to quantum phenomena at an early age. Why? Our brains develop according to the mental activities that we engage in intensely. If you present children with the basics of quantum mechanics, there is opportunity for the development of a different perception of reality. The question is whether we want to take the responsibility of putting somebody, an individual, on a different track than everybody else. Will that person be happy or unhappy in later life?
Do you think you are happier because of your understanding of quantum mechanics? I am happier, sure. I consider myself very, very privileged to be working on these questions.