Common sense is useless in the world of the extremely tiny, where the rules of quantum mechanics apply. One of the most amazing differences is that two particles —like two photons of light — can be entangled in such a way that what happens to one determines what happens to the other, even though they are very far apart. It is what Einstein, skeptical, called “spooky action at a distance.” The 78-year-old physicist Anton Zeilinger, born in the small Austrian town of Ried im Innkreis, has spent a quarter of a century proving that the most absurd predictions of quantum physics are correct. A little over a decade ago, his team succeeded in teleporting a quantum state between two entangled photons of light. One photon was in La Palma and the other in Tenerife, both part of Spain’s Canary Islands. There were 89 miles (143 kilometers) between them.
Zeilinger, of the University of Vienna, won the 2022 Nobel Prize in Physics for teleporting information and paving the way for exponentially faster and more secure quantum computers. The Austrian physicist sat down for an interview over coffee on the terrace of a hotel in Valencia overlooking the Mediterranean, during a break in his activity as a member of the jury for the Rey Jaime I Awards, which recognize achievements in research and entrepreneurship.
Question. You first heard about quantum entanglement at a conference in 1976. What did you think?
Answer. I didn’t understand what was going on. I just realized that it must be interesting.
Q. How do you explain entanglement to people with no prior background in this field?
A. No one is completely without prior background. The entanglement of two particles is like you have a pair of dice. Three is rolled on one die and three is also rolled on the other. If one die shows six, the other also shows six. And the same number always comes up on both dice.
Q. Einstein said that God “does not play dice.”
A. I believe that God puts the numbers so that we believe that he plays dice, but he does not play dice. God says: now it is three, now it is two, now it is six. And we believe that God plays dice.
Q. In your Nobel lecture, you stated that “not even God” knows what information is in the particle.
A. Maybe he knows. Or maybe not. We cannot know.
Q. Do you use God as a metaphor or do you believe in God?
A. Yes. Why not believe? The famous Isaac Newton published books on many subjects, but he wrote much more about religion than physics. He was a religious person.
Q. Two entangled particles can be imagined as twin brothers who behave similarly at a distance because they share the same DNA, but that's not how it works.
A. In entanglement, the two quantum siblings behave the same, but without DNA.
Q. It’s more than counterintuitive. It’s crazy.
A. It's crazy, yes.
Q. Einstein defined entanglement as “a spooky action at a distance.” Does it seem spooky to you?
A. Einstein used the German word geisterhaft, which means something like spiritual. It is a phantasmagorical phenomenon if you try to explain it with the usual rules. But in quantum physics, you know how it works.
Q. In your Nobel lecture, you projected a question on the screen: “Is the Moon there when no one is looking at it?” What is your answer?
A. The important thing is that to prove that the Moon is there, you have to look at it. If you don’t look at it, you can only use your experience and your logic to say that it is there. But, with quantum particles, you can’t tell the system is there if no one is looking. Einstein asked: “Do you really believe that the Moon is not there when no one is looking?” And [Danish physicist Niels] Bohr replied: “Can you prove otherwise? Can you prove that the Moon is there when no one is looking?” And no, you can’t.
Q. Niels Bohr stated: “It is a mistake to think that the task of physics is to find out what nature is like. Physics is concerned with what we can say about nature.”
A. I would go one step further and say: What can be said about nature, in principle, also defines what can exist. So nothing can exist without the possibility of saying something about it.
Q. What is reality then?
A. In physics, we have always made great progress without answering the question of what this is. We only answer the question of what can be measured and how can we observe something. We can observe reality, we can make measurements, but I don’t think we can say anything about the essence of reality.
Q. Are you a Christian?
A. Yes, I was raised Catholic, but my mother was a Protestant, so I learned from both. Sometimes I went to the Protestant church with my mother and sometimes to the Catholic mass with my father. It was interesting.
Q. When you see this world of particles doing crazy things, how does that craziness fit in with the idea of an organized God?
A. The Jesuit theologian and philosopher Karl Rahner said: “The Christian of the future will be a mystic or he will not exist at all.” I agree. It cannot be said that God is organized or is like this or like that. God is not subject to our definitions.
Q. Perhaps God does not exist without the gaze of the observer.
A. It is a different type of observation: it is not with the eyes, it is an observation with the soul.
Q. After your experiment in the Canary Islands, you stated that the teleportation of information “plays a vital role in the vision of a global quantum internet, since it provides secure communication without restrictions [...] and an increase in exponential of computing speed.” When will we see those promises delivered?
A. Good question. As to when we will have full quantum computing, we don’t know. In fact, today I would be more cautious with my statements, because the challenge is enormous. In small quantum computing systems there is a lot of work underway, but for big computers there is still a lot to do.
Q. Google is already making big announcements about imminent quantum computers.
A. They have a quantum computer, but it is small and can only be used for very specialized problems, not more general problems. To have a complete quantum computer, you need about 1,000 quantum bits. And now we are talking about systems with 30 or 50 quantum bits.
Q. You predicted in an interview in 2010 that in 15 or 20 years we would have an interesting quantum computer.
A. Yes, I would say the same today [laughs]. It is impossible to speak about 20 years from now.
Q. You also said, perhaps provocatively, that in the future we will have quantum computers on cell phones.
A. That will be in 50 or 100 years. I didn’t say it to provoke, but as a challenge. When the first computers were built, they were huge, taking up an entire room. And then nobody thought that you could have it on a mobile phone.
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