Nobel Prize lecture Anton Zeilinger Nobel Prize in Physics 2022

Our final speaker is Anton Zeilinger. He was born in Riet im Inkrais. Upper Austria, Austria in 1945. He studied undergraduate physics and mathematics at the University of Vienna. from which he also received a PhD in physics in 1971. He is currently professor emeritus at the University of Vienna. and senior scientist at the Austrian Academy of Sciences where he also served as president until recently. Please join me in welcoming Anton Zeilinger to the stage. Hello, I'm really happy to be here. I am grateful to the Swedish Academy of Sciences and its various committees for having elected me as the winner of this year's prize. I would like to begin by appreciating some basic facts. One is the fact that I was born into a place at that time. where it became possible to carry out such work which was actually a narrow escape. I was born two weeks after the end of World War II. I am grateful for the family I came from, my father and my mother, who truly nurtured the kind of curiosity I had. They never told me what to do, but it was clear that being curious is important. I would like to thank my family for enduring me, my wife Elisabeth for supporting me very strongly, And she also made her own compromises in her career for me, they say. And I'm grateful for my children and for my grandchildren, that they bear with me. But last, and perhaps foremost, I would also like to thank the taxpayers. The taxpayers in Austria, in Europe, and even in the US, who provide us with the funds to carry out this work. And that is truly a great privilege. I would like to dedicate my lecture to these two people who were very important to me. One is Helmut Rauch, my academic supervisor at the university. who unfortunately passed away three years ago. I worked in his group in Vienna in an environment where the fundamental work was the obvious thing to do. And I know this was very special. There were not many laboratories in the world where this was so easily possible. And from Helmut Rauch I also learned that you can have ideas that are wrong. in the sense that if you argue, the arguments are incorrect. But the idea is correct. Intuition can be far stronger than logical arguments. That was very important to me. The second person I dedicate this lecture to is Mike Horn. He has already been mentioned by both speakers previously. He unfortunately also passed away three years ago. I will mention a little bit more than what you have heard so far from my previous speakers. My entry into the foundations of quantum mechanics was The fact that Rauch, Treimer, and Bonsai developed a neutron interferometer in 1974. Neutrons are massive particles. Massive particles are the opposite of what John demonstrated. For massive particles, the particle nature is obvious, But the wave feature is the one you must demonstrate. This had been known for a long time, but they built a beautiful interferometer. What is this E-shaped object made of perfect crystal? which splits and overlays quantum states. I was not involved in the development, but I had the idea in 1975 to use this to demonstrate what is known as 4π spinor rotation. If you rotate anything by 2π, like me rotating around, I remain the same. If I were a neutron, I would have to spin around twice to return to my original state. It's the phase that is different. And this is what we demonstrated in that experiment. It's actually, to me, one of the first experiments, which later became so obvious that no one mentions again how important it was. It's used everywhere in quantum computers and all that kind of stuff. Then in 1976, and this is important, I think, for all of us, just a year after we had conducted the experiment, I discovered the conference organized by John Bell and Bernard d'Espagnat in Erice in Sicily, which was called Think Shops on Physics, Experimental Quantum Mechanics. I went there and presented this experiment. At the conference I met Alain, I met John Clauser, I met Mike Hoare, I basically met all the players in the field. And they talked about entanglement, And I had no idea what they were talking about. I really didn't understand it. But it took some time. Meanwhile, we carried out other fundamental experiments. It's my journey, as I said, my journey through quantum wonderland. I want to tell you today about one experiment—the famous double-slit experiment. It has already been mentioned by John. It's a picture from the dialogue between Bohr and Einstein. where you have a situation in which some radiation, can be particles or can be light, can pass through the slits here in the middle and they form fringes. And the question is through which slits these two pass. And the modern way to say it is that The fringes back there only appear if you cannot say which path they took. They themselves don't know it. Nobody in the universe knows it. And I say even God doesn't know it. This information is not available. So we conducted the experiment, Gehler, Schall, Treimer and Mampe, with me using very cold neutrons, at the reactor in Grenoble. What you see here is the distribution of neutrons, with the solid line representing the first-principle theory with no free parameters. Now today this is obvious. In the old days, when I gave this talk, even famous professors in the front row stood up and said, It really works that way, one particle at a time. They thought, what do you expect? That's what your theory claims, you know. And this kind of experiment also allowed us to establish limits on possible hypothetical non-linearities of quantum mechanics Another important person in my life was Cliff Schall. He was a professor at MIT. I worked with him on neutron interferometry. He received the Nobel Prize in 1994. And I was actually here for the Nobel celebration. I learned from him one thing, and that is, perform every experiment at least one order of magnitude more precisely, one magnitude better than what you think is necessary. And it always pays off. Then another interesting thing happens in 1985. 1985 was the fiftieth anniversary of the Einstein-Podolsky-Rosen paper. And in the fall of 1984, I came in one morning to the lab, And Mike Horn says, do you want to go to Finland? And I said, sure, anytime. Yes, there is a conference in Finland about Einstein-Podolsky-Rosen. So we said, oh, we have to invent a way that connects interferometry with Einstein-Podolsky-Rosen. And in that way, we invented this Einstein-Podolsky-Rosen interferometry, where instead of spin, the two states are up and down This is two different methods of propagation. And the same mathematics, the same physics, and this was the first suggestion of using external variables for this kind of Einstein-Podolsky-Rosen work. In this situation, this is now quite common. What happened at that time was parametric down-conversion. as a source of entanglement emerged and showed, gave us sources that are much better, much more intense, much cleaner, and so on, than the atomic conditions. This was discovered by Byrne and Weinberg in 1970. The Mandel group began conducting experiments in 1985. In 1985, Eli and Xi performed the first EPR test using down-conversion. And in 1989 we proposed this kind of experiment with momenta, which is exactly what you see here. That was performed by Werner Tebster in 1990. By Werner Tebster and the Chao group in 1990. At that time, something else happened. This is my journey through the world. Something else happened that was interesting. In 1984, Danny Greenberger, who is here in the audience, Arrived in Vienna on a Fulbright Fellowship. And we sat together and discussed what kind of project. And we had both been thinking that maybe it's interesting. to examine three-particle entanglement, not just two. That sounded okay, something acceptable to do, you know, But who says it would be interesting? It turns out that this was mind-boggling. It turns out that if you have a three-particle entanglement, as written down here, three polarizations of photons, then we can have a situation in which, upon measurement of two, You can predict with certainty the polarization of the third one. But a local realist would predict the exact opposite of what quantum mechanics predicts. So it is not a statistical contradiction. Previously, I had naively thought that the difference between quantum physics and classical physics is statistics. No, it's not statistics. It's a definite opposite. And here we are, the three of us discussing this kind of experiment. And since 1988, this has been my goal; this experiment has been my goal. And it took us ten years. And at that time, we had no idea how to create entanglement. of more than two particles. We had no idea of the proper source. We had no idea how to measure it, and so on. All of this was developed over time. And I say this so strongly because today, In many applications, when you apply for money, You have to write down what your goal is. What do you want to achieve in five years? And you must write down which methodology you wish to use. This would have been completely impossible for the GHZ experiment. Completely impossible. And then something else happened in 1990. I moved to Innsbruck, and suddenly I had a lot of money. because of this new position and so on. So I could really buy everything. I could buy the down-conversion crystals, the first one. One of them is now in the Nobel Museum, and so on. Here's an experiment we did. It was the first experiment with down-conversion in our lab. where we pump a crystal, this is the blue pump And the two, the green and the red photons, come out. And what we do is send the pump back, so we create another pair, But actually, it's the same pair. One pair generates the same pair, whether sent this way or sent that way. And then you insert mirrors, and then by moving any one of the mirrors, You can suppress the interference and so on. which is an effect that cannot be identified as an effect on a single photon. In 1905, an important event was that Paul Queer also joined my group. Moreover, the first collaborator in the experiment was Harald Weinfurter. When he arrived, we decided to carry out this experiment. But we had no real idea how a laser works. In 1995, Queer joined us, and we developed this new high-intensity source. for entangled photons, which became the workhorse for an extremely long time. Around the same time, quantum information became a fledgling young bird, you know. It all started in 1995 with Stephen Wiesner, who suggested that If you can print the number on a banknote using quantum states, then it cannot be copied. This is an idea far ahead of its time, and it was only published in 1983. in support by John Bennett in a cryptic journal. This idea already had the feature of no cloning. It's important that you cannot clone. A theorem which was later formulated by Wouters and Zurich, randomness of the individual event and complementarity And the term "qubit" was then coined by Ben Schumacher. the general superposition of 0 and 1 And here is our experiment on dense coding and experimental quantum communication, which, as far as I know, is the first application of entanglement in a quantum communication scheme. So you have a source that produces entangled photons, and then you send them through this diamond here, and you superpose them again on a beam splitter, the two photons. And by manipulating only one of the two photons, You can encode four different pieces of information, instead of only two in a classical polarization. In practice, it was only possible to encode three due to limitations. But that is a separate history. And then finally, this is 1996, with Marek Szukowski, Harald Weinfurt, and Mike Horn, We have been discussing for a very long time how to create entanglement in free particles. And in 1996, we finally found it, and the basic idea was that you pump the left image, You pump the crystal so forcefully that you create two pairs simultaneously. And then you superpose the beams on a beam splitter up there, and one of the photons you allow to propagate away is called T, the trigger. And then if you detect them in very short coincidence, much shorter than the coherence time of the radiation That was one of the key ideas; then, the other three photons become entangled. A similar procedure can be used to generate four photons. So now we have the tools in our hands, and these tools which we had in our hands, we discovered were useful in sorting along the way to conduct other experiments, which had been proposed One experiment was quantum teleportation. proposed by Bennett, Brassard, Crépeau, Joseph, Pérez, Wouter, shown here Along with their cat, I don't know whose cat it was. I don't know what it should be. And when that paper came out, don't misunderstand me, I thought, this is a typical theorist's idea. It's absolutely impossible to do. And I was completely wrong. I didn't realize we had already started moving in that direction. The tools we developed for GSZ could be used here. And that we did. One of the tools is the behavior of two photons at the beam splitter. You take a beam splitter, and you have two photons incident, One from above, one from below. And then what can happen is that both photons can be reflected, Both can be transmitted, or one can be transmitted while the other is reflected, and so on. If these are so-called symmetric states, bosonic states, then the point is that the probability that both are reflected, If both are transmitted, they extinguish each other. And therefore, you get, when you place the detectors here, You get zero coincidences if the flight time difference is zero. This was discovered by Hango Mendel. Unfortunately for photons, three states are symmetric. So you cannot use it to discriminate among states. But luckily, one state is antisymmetric. And for this antisymmetric state, the two photons always emerge in different states. And only in that situation. So that was an important workhorse. In that way, we could identify one Bell state, one entangled state, with certainty. So here is the experimental setup in Vienna for quantum teleportation, quantum state teleportation over a distance of just one meter. And there are the friends, colleagues, participating in that experiment. You see, some of them in the picture look much younger than they do today. And some haven't really changed at all, even when you walk around. Harald Weinfurter is here, for example. Tianwei Pan doesn't look much older either. Then the next idea, also along the road to GHZ what was entanglement swapping. It had already been suggested by Bennett, Brassat, Repo, Choser, Peres, and Brutus. that you could actually teleport one of the entangled states. And if you do that, you get two photons that are entangled. two entangled pairs, two photons which are entangled which have no shared history, which have never met one another. Nothing. Completely independent. We call this teleportation swapping. And we carried it out immediately after the teleportation. This has now become a cornerstone in future quantum networks. that you can connect over longer distances using entanglement swapping and similar techniques. The first experiment was conducted by Tianwei Pan and Thomas Jenewein in my group. It's an in-fiber experiment where you use a beam splitter, which is just a fiber coupler. And we were actually able to demonstrate a violation of the Bell inequality. for the independent photons, for those that have been generated entirely on their own. The most important thing here is an idea by Asha Peres from 2001. He said, okay, you could have these two pairs. And you could actually register the others right away. Write them down, print them out, whatever you want. Permanent record. And the other two you can decide on at a later time. Whether you subject them to an entangling measurement or not. If you perform an entangling measurement on the other two in the middle, Then the outer ones become entangled, even if they have already been registered. If you don't do it, then they are not entangled. The same data is not entangled with each other. So what's happening? The data is certainly invariant. But the meaning of the data, the very physical meaning, depends on a measurement you take at a later time. And Asha Peres says that we only have relational bits. We can only say something about relations. The quantum state does not tell us what is really happening. because that's a completely different way of putting it. There was an experiment we conducted in 2012, Where he did the top violet thing up there. It is the apparatus that determines whether to measure the photons individually or jointly. It's a beam splitter that can be switched, so to speak. And behold, you obtain entanglement. So the fact that two systems are entangled or not, can depend on future experiments. And thus, to me, this serves as confirmation of Bohr's statement. No phenomenon is a phenomenon unless it is an observed phenomenon. Okay? It might be related to what you said at the end. Alright, here is the GHS experiment, exactly as we had proposed it. And here, such multi-particle entangled states are now commonplace. Numerous quantum computing approaches. There is one important message I want to convey here. When you examine the predictions of quantum mechanics regarding multi-particle entanglement, So you could have one measurement here, one there, one earlier, one later, and so on. These predictions are completely independent of the relative arrangements. of these measurements in space and time. That tells you something about its role in space and time. There is no role at all. How much time do I have? Six minutes. Okay, thank you, thank you. Very kind. Here are the pictures of the current theories and the experiments on the GHS experiment. Published in 2000. More than ten years after we decided we wanted to do it. I mean, if we had known the solution in the end, we could have done it much faster. But this is always a different story. Now, I want to tell you, we weren't doing anything else besides that in my group. And this is just an overview up to 2010. We conducted several quantum computing experiments. Some of them have become important in the field, or not. I'm showing you this because it's interesting. It's fun at the time. But I personally did not follow up on that. I'm still interested in the foundations. And I still hope that someday, I will still be alive when we finally find the answer. Also the answer to your question. Now, back to Bell tests. Alain has already mentioned this experiment very kindly. And I would like to thank you, Alain. Because when Gregor Weiss began the experiment, there was a minor motivation issue. And you came and told him, do it, it's very important. You can be extremely, extremely positive. Thank you very much. I didn't hear that. Yeah, do it better than your experiment did. Sure, yeah, right. That was really good, yeah. So we conducted the experiment. We have already heard the essence. Two photons are generated at the down-conversion source. And they are sent to two glass fibers 300 meters apart. And there were rapid electro-optic switches. Which switch the polarization quickly. Driven by a true random number generator. A quantum random number generator. Alright, both sides. In fact, these electro-optic switches have also been donated to the Nobel Prize Museum. Because they were extremely crucial. There were new developments. You couldn't purchase them in that quality in many areas. And in that way, we were able to violate the Bell inequality quite nicely. We were able to apply it to quantum cryptography. Entanglement-based quantum cryptography. Also already mentioned, suggested by Eckhart. There were parallel experiments conducted in the Cuyat group and the Shisai group. And that's the picture. It's a strange language that I use here. We encoded the image of the Venus from Willendorf. It's a famous figure discovered not far from Vienna. It is approximately 30,000 years old. And do you know why we chose Venus? It's because my friends came with a bomba. Or with a tank they wanted to encode. And I told them you have to use something. Which one is Austrian? And without a doubt a peaceful symbol. So we brought Venus here. We took the Venus of Willendorf. And it's the first time and so far the only time that a naked woman was pictured in Physical Review Letters. So we're not far from that. Then you know the story continues. I'm reaching the end of my journey. We began conducting long-distance experiments. For example, between the Canary Islands. Many. I can only show you an image of one of them. This is an ideal location. On both islands, you have clusters of telescopes. Which we can use if we can convince the telescope owners. Then another type of experiment was closing the loopholes. Already mentioned by both speakers. In 2015, a beautiful experiment by the Hanson Group in Delft. By the Weinfurter Group in Munich. Presented almost simultaneously. At least I heard about it for the first time in 2016. And our experiment. And the experiment conducted in Boulder, Colorado, at that time. And that is actually a picture of John Bell. Using images of all participants in the experiments. If you look closely, you see pictures of Bohr and Einstein. Now here's a curiosity about the development of our field. These are the citations of the Einstein-Podolsky-Rosen paper from 1935. And at the beginning, there are only three citations. They're not so bad. Because two are from Schrödinger and one is from Bohr. But afterward it stopped. The issue was regarded as resolved or merely philosophically uninteresting. And it resurfaces again in the 1990s. Philosophical discussion arises once more. And you know, at the time of Bell, after Bell's theorem in the 1960s. It starts strongly and keeps rising. And now the paper is by far the most frequently cited paper by Einstein. It is quoted nearly twice a day. But this does not mean that it is read more frequently than in the past. It's actually very difficult to read. It's not an easy paper. Now two things for preparing for the coffee break. One of the questions is, in principle, in principle, and just for the fun of it. The word can be one minute. It could be a major conspiracy. Perhaps there is an earlier cause that influences the sources, the detectors, and so on. So what are the most independent early causes you can have? Well, we thought this was a suggestion by Kaiser, Friedman, and Galicia. To use something extremely distant in the universe. So we took quasars. Quasars emerged billions of years ago and took the random numbers. Randomness in their light to trigger what's going on. This was a beautiful experiment. Up on the hills there in the night. Looking at this quasar, looking at this quasar, and so on. And to the bellies. The question is how can you proceed from here? Just to provoke a little bit. The next step is to use the cosmic microwave background. Zero minutes, yes. Then we can move on to neutrinos, early neutrinos. But we even, and this is extremely speculative. Perhaps someday we can utilize primordial gravitational waves. But for now, I want to finish. This is my paper with the largest number of quasars. What we did there was conduct 13 experiments across 5 continents. And there was a random number generator created by everyone. There were people participating, typing random numbers into their mobile phones. And these random numbers determined the settings of the experiment. Because, as John Bell said, he is one of our great prophets. Human choices should be regarded as free variables. And here, at the end, I conclude with this image of the METEOR satellite. Which you may have seen. And I end. I want to express my perspective on the world. Perhaps I would suggest that the separation we make between reality and information is artificial. Perhaps we need to unify the two. And here is a picture of all the people I have ever published with over the years. Everyone appears only once here, the first time. But this is only half of it. That's the second half. And I want to thank you all very much. Thank you, thank you all these people. Could I please ask Alena Spey and John Clauset to join? So that we can thank all three speakers for their fascinating presentations.