‘We were considered complete strangers’

What sparked your interest in quantum physics? When you studied physics at the University of Vienna, you had never heard a lecture about it.

Zeilinger

At that time, quantum physics existed only in special applications of solid, particle and nuclear physics. No one has directly dealt with the individual quantities. Here’s what the other two Nobel Prize winners, Alan Aspect and John Clauser, say: We were considered total strangers at the time. For this reason, John Closer did not get a good job at the university.

So how did you get into quantum physics?

Zeilinger

At the end of the course, my colleague and I wanted to know more about it. We obtained the manuscript of Otto Hetmer’s Quantum Lecture from Vienna University of Technology. We went through it from cover to cover and we were totally fascinated.

What surprised you so much?

Zeilinger

Beautiful math that was used. This is totally unique. Nobel laureate Paul Dirac wrote Formalities of Quantum Mechanics and they are simply remarkable. The book by Nobel laureate Claude Cohen-Tnodege was also fascinating to us. But at the beginning of the books it was always said, we’ll get to what it all means. Then you read it, but what you promised never came. That’s when I realized it was a topic that hadn’t been considered.

Is it also a passion to use the language of mathematics to penetrate a world that cannot be experienced in everyday life?

Zeilinger

As a physicist, direct experience doesn’t really bother you. You find it very practical. There are mathematical predictions and empirical methods for testing them.

Do you need certain qualities or talents to do this kind of science?

Zeilinger

Richard Feynman wrote at the end of his famous physics lectures that he would like to apologize. Specifically for those students to whom he explained only what they already knew in principle. This is the talent. I felt the same way in mathematics lectures. When the professor developed an argument on the board, I always knew what to do next in advance. In physics, there is the experience of observing the world and the experiences that cannot be derived purely by logic.

Quantum physics is about things that are obviously true, but often do not correspond to the visible world. Doesn’t this inevitably lead to the question of what reality really is?

Zeilinger

This is the central question in the basics of quantum mechanics. It motivated the three of us, Aspect, Clauser and myself. We realized that the world is not what we think of it from everyday experience. A key point of the everyday experience is this: I see a blue car and I am correct in assuming it is blue even when I am not looking. In quantum physics, this does not always work. When entanglement occurs, for which the Nobel Prize was awarded, it certainly does not work.

How do I deal with this? Then what is correct?

Zeilinger

On a deeper level, all we can say now is what isn’t true. It is not correct to assume that the characteristics observed by one existed prior to the observation. If measurements are made on several entangled particles, first the individual results are random, but secondly strongly correlated, so the particles must know each other somehow. Third, observations are completely independent of space and time. It doesn’t matter if something happened before, after, here, or thousands of miles away. So we also have to question the idea of ​​space and time. There have been heated debates about this for a long time, which is why these questions have been referred to as a philosophical wobble. However, experiments have shown that this is true.

Is there any solution?

Zeilinger

We don’t know what it looks like, but I’m sure there is. In fact, I think it is very clear and simple once we see it. We’ll scratch our heads and ask ourselves, why didn’t we witness this sooner?

Is the principle still applied that nature can be described in simple and beautiful formulas?

Zeilinger

This is still one of the great miracles. Even in complexity research, the basic laws are simple. Why is mathematics so effective in describing nature and the world? In fact, this is illogical. Many have scratched their heads about this. When I talk to teachers, I always say they should pass this on to all children: Math is fantastically accurate. When I get on a plane, I trust the mathematical descriptions to the nearest percentage. Not everyone who gets on a plane or uses a smartphone can be anti-science.

So is nature written in the language of mathematics, or is mathematics our way of approaching the laws of nature?

Zeilinger

My personal opinion is that the separation between mathematics and nature is an artificial one. We need a new perspective, an understanding of unity.

Is it also about the unification of previously incompatible things such as the quantum world and gravity?

Zeilinger

The combination of quantum and gravity has fascinated physicists for nearly 100 years. Some of the smartest people have been brainstormed about it – but they just haven’t made it. This means we may be looking in the wrong place.

Is our mind even capable of understanding everything?

Zeilinger

I believe in the natural sciences: Yes. Our brains have been shaped by evolution, and more complex processes involve social strategies. Why on earth can we do quantum physics with this? In fact, we don’t need that. But the brain is great at processing information, and its limits are still a long way off.

This was a key word: What is information anyway?

Zeilinger

In teleportation, information is something completely unnoticed, unperceived. The question of what gets transferred from A to B is itself illogical. All that matters is where you come from and where you go. I think separating reality from information is a mistake – it does not mean that everything is just information. We need a new concept, like that devised by Isaac Newton with gravity.

So is the information going to be some kind of picture of reality?

Zeilinger

No, this is very realistic. Information is more a way to gain knowledge.

What is your most important experience so far?

Zeilinger

The 1997 teleportation was just one step on the way to experiencing the 1998 GHZ, which is my most important experience to date. The GHZ state is named after Daniel Greenberger, Michael Horn and I, who discovered it in theory. In the experiment, we were able to show three-particle entanglement and refute the existence of hidden variables for final properties, that is, non-statistical properties. This came as a big surprise to many. Because for a long time many believed that particle entanglement was due to unknown statistical effects. The result from that time is also significant because GHZ states are a central aspect of quantum computers.

How do you design a quantum physics experiment?

Zeilinger

In the late ’80s, I made the decision to stop working with neutrons and work with photons because you can produce them at much higher densities. So I looked for the leading research groups in this field. I found them in California and Rochester, New York. Colleagues were quite open, showed me their laboratories, came to Innsbruck and conducted experiments with our students.

How much creativity does such experiments require?

Zeilinger

I have to think: how should I direct the beam, what mirrors do I use, and what are the crystals to generate the entangled photons? I made a lot of phone calls and asked my colleagues what was possible. Creativity comes from the set – and over time, if you deal with a problem for a long time.

Their experiments have grown in scale, most recently using a Chinese satellite to send quantum-coded messages between Vienna and Beijing. What’s Next?

Zeilinger

We want to try this all over the world, as well as on other continents. If you connect proof of a quantum telephone tap between Vienna and Antarctica, it will be in all the papers. We do this mainly to get people excited about quantum physics – and that’s very important to me. But we are also working on other experiments that do not occupy more than one square meter.

What are you trying to find out with this?

Zeilinger

You can imagine if I told you now, it would be a race. I don’t want that, I want to work on it in peace.

Google, IBM, and many other companies are already building tiny quantum computers. When is commercial application possible?

Zeilinger

This is completely open. Look at classic computers. A few decades ago, no one expected that we would do what we do today: make video calls across many networked computers.

What could be the first tasks of quantum computers?

Zeilinger

I think they will be given very specific tasks, for example to simulate a problem in solid state physics. The quantum computers that everyone uses at home are still in the realm of fantasy.

They did the first quantum-coded bank transfer in 2004. Why is this still not possible in everyday life after nearly 20 years? After all, technology is completely error-resistant.

Zeilinger

I discussed this once with a high-ranking banker. He said the existing technology should be discarded and the system completely rebuilt. There are currently small security holes, but filling them will not justify the massive effort. “Of course I will immediately implement quantum cryptography if the competition does,” he said.