Anton Zeilinger - Quantenverschränkung und Quantenkommunikation

Above the roofs of Vienna. Here the ground station stands for one of the most exciting experiments of the present. Physicists are testing the quantum communication in space from here. The Vienna Quantum Space Test Link, the technical term for this station, is to transmit secret messages between space and earth in connection with a satellite. Quantum restrictions over a distance of thousands of kilometers. Of course, it all depends on the restriction of these photons and thus on this spooky long-range interaction, which is then triggered locally here at the measurement in a defined state, which we then write down with a date stamp, with a time stamp, and then be able to compare it with the data from the transmitters and generate the key. The station is now the same for transmitters and receivers. Photons, the smallest particles of light, are sent from here to the church on Leopoldsberg on the outskirts of Vienna and reflected from there. Experiments that form the basis for a completely new data encryption, the so-called quantum cryptography. Quantum cryptography will allow things to be encrypted in such a way that even an NSE or another large, well-equipped institution cannot stop it. The quantum physicists of the Austrian Academy of Sciences are already very close to this goal. Together with researchers from the Academy of Sciences in China, they will carry out the quantum physical experiments in space. And be able to transmit data with absolute certainty. The scientists test how the connection between the ground station and the reflectors here at Leopoldsberg works. A task at least as challenging as communication in space. The light quantities sent to Leopoldsberg are reflected there with the help of mirrors and re-registered in a small telescope here in the ground station. If it works here, it will also work on the satellite, the researchers are convinced. An optical link has now been built up here from the roof of the Institute for Quantum Optics and Quantum Information. What we see here is about six kilometers. Six kilometers is about the effective thickness of the earth's atmosphere. So the earth's atmosphere will be much thinner at the top, but if it would not be, but would effectively remain the same, it would only be six kilometers. The experiments that we do here in Vienna over the roofs are definitely representative of a quantum optical link between a satellite in an earth orbit to a ground station. The atmospheric disturbances, the absorption, all these problems that occur here are almost equivalent. And quite expressive. So we have structures here with which we can characterize and measure the atmospheric disturbances. We then evaluate this and can conclude whether a link from a satellite to a ground station is realistic. Albert Einstein discovers something seemingly crazy together with his colleagues Boris Podolsky and Nathan Rosen in 1935. At first glance, completely absurd. The phenomenon of distortion. Austrian physicist Erwin Schrödinger now calls it the characteristic property of quantum mechanics. Two particles, e.g. light particles, are closely connected at any distance by magic hand. Einstein speaks of the spooky effect of particles. There is the so-called distortion, where two particles are so closely connected, even at great distances, that the measurement of one of the two changes the state of the other immediately, without there being a signal from one to the other. And then of course you say, how is that possible? And then you have to say, quantum physics can describe this mathematically wonderfully. There is no question at all that this is correct. But to understand it visually, that is the challenge. And we haven't actually solved that yet. Nevertheless, quantum physics has been proven a thousand times in experiments. Now the scientists also want to use the universe as a laboratory. On board a Chinese satellite, a source is set up for limited photons. This enables quantum communication with individual photons between satellites and the Earth. An intercontinental, auditory data exchange would then be out of the question. The goal of this satellite project, which we are doing together with the Chinese Academy of Sciences, is to send a source of limited photons to the satellites. And they can then send the two particles down to two different points on the Earth. And then you can show this spooky effect at even greater distances. First, that it exists, which would be a new result, because at distances of a thousand or a few thousand kilometers it has not yet been proven. And secondly, you can use it. You can use it, for example, for transmitting secret messages. With the help of the phenomenon of the spooky long-distance effect, messages from space can be encrypted and sent. An unbreakable cult, which is protected by a natural law against hearing. This encryption technology, the so-called quantum cryptography, is considered to be the most widely developed application of encryption in information technology. Restricted photons can form their own secret key. A practical application of a phenomenon that has not yet been fully understood. The phenomenon of restriction. It is more comparable to the situation of a cube. If a cube is thrown, on one side it randomly shows a number between 1 and 6. If a cube is thrown elsewhere, from a bob, as we say, for example, the first cube is thrown by Alice, the second by Bob. So Bob is now also throwing his cube. He also randomly has a result. Randomly a number of eyes between 1 and 6. Both have random results and it is no wonder, nothing is exciting about it. But if you put the measurement results together, if you always throw them at the same time, we can, and this is such a source, produce such cubes, which show single random events at each throw, but always show the same number of eyes together. That is, everything I know about this pair-cube system is that they have common properties, they always show the same number of eyes, but no individual ones. This is still random. In classical physics, this is impossible. Every coin has two sides. A quantum-mechanical coin would have all positions equal. And only the measurement determines a state. A coin randomly falls on a head or a number. That is, in half of the cases, they will simply have the same side on top, and in the other half of the cases they will have different sides on top. This is not even a classic correlation. It is as good as we know it from everyday life. Or in the lottery it is something more complicated. But in fact, it is apparently possible, with such an experiment, two cubes, two coins, but the cube is a better example, because it is even higher in dimension, we can produce two cubes, which always, without exception, always show the same number of eyes. This cannot be compared with our everyday understanding of physics. How do you produce polarized photons? An ultraviolet laser beam is focused on a non-linear crystal. A photon breaks down into two new photons. They can be polarized in their polarization, i.e. vibration. They are transmitted via fiberglass to so-called detection units and analyzed on the computer. These two photons have the same frequency and have the same polarization. So no matter if I measure a polarization on one side, I will measure the same polarization on the other side. And this is the quantum-mechanical miracle. This is now possible with any polarization. I can adjust any linear polarization and still the other photon, the second photon, will have the same polarization. And that's the crazy thing. These two photons are brought to overlap at the polarizing beam divider. The photons that leave this beam divider I have no information about in which direction they were generated in my periodically polarized crystal, in my non-linear crystal. So I have no information about which polarization the individual photon carries. The only thing I can determine by a measurement, and you can see that back here, the only thing I can determine by the measurement is in which polarization I want to measure the one photon. And I see instantly, and that's what made a stunning long-range effect, that the other photon carries the same polarization. Experiments have shown that this remains upright up to a distance of 150 kilometers. So these photons retain their indistinguishability, their restriction over incredibly long distances. Distances for which quantum mechanics was not developed. Quantum mechanics was developed for long scales of atoms and photons. In fact, at the time, one wanted to understand how atoms and photons interact with each other. We use these similar structures, at the time they were called thought experiments because they were so crazy, but today we can actually build these things in the laboratory, measure in the laboratory, which is a great technical effort. Individual photons are very small energy units. We can measure that today and demonstrate it over very, very long distances. It reminds of a scene from the famous film The Third Man, a British thriller from 1949. Because deep down in the sewer system of the city of Vienna, the quantum physicists have set up their laboratory. As early as 2004, they managed to teleport photons over a distance of 600 meters across the Danube. From Station A, the physicists call them Alice, they transfer the state of vibration of one of the two photons to a photon 600 meters away at Station B, called Bob for short, without the light particles coming into contact with each other. If you were to listen to the channel in between, or look at what information is being transmitted, it is neither in the quantum channel, the channel that connects the source with Bob, nor in the classical channel, the full information is available. In neither of the two channels, there is the full information about the bit, the Q-bit that I want to transmit. So I have to have both information. That is the first thing that is interesting. The second thing that is interesting about quantum teleportation is that in conventional communication processes, I always have particles whose property I change so that I code my bits in polarization, in charge, in whatever, in the phase of the light, whatever coding mechanisms I know on the Internet. In quantum teleportation, this is not the case. It is only characterized in retrospect. It is actually a spooky action on a distance, as Einstein called it. So it is actually true that information is transmitted without having a direct line. First, the restriction is transmitted, and here we know that a restriction is not an individual property of a photon, but a sum of two properties of two photons or more. It can be many more particles. The restriction of light particles is possible even over a distance of more than 140 kilometers, show the Viennese physicists. An important point is that the restriction is experimentally proven many times, and also over great distances. And that means not only that it exists, but that it can also be used technically. Only the obvious explanation is missing. The Viennese quantum physicists for the first time manage to transmit quantum information over great distances, in this case between the Canary Islands of La Palma and Tenerife. With 144 kilometers of free space, a record distance. Two photons that remain connected over this great distance and can be used for the transmission of quantum information. As a next step, the researchers try to establish The question is now whether these technologies, which were actually developed to allow classical high-speed data rates at every point on Earth, can also be used for these sources, these optical structures. We have done many theoretical studies and also practical studies in our institute. Among other things, on a link between La Palma and Tenerife, where you have about the same weaknesses. Photons are transmitted from one island to another and you can see the atmospheric disturbances, the losses that actually occur on a satellite link. Already in the near future, a satellite with a source for limited photons is to be launched. Safest encryption methods, such as intercontinental quantum cryptography, will become applicable. The essential thing about quantum cryptography is that I can generate a key through these limited photons, with which the transmitter can encrypt the message, the receiver can decrypt it. And the essential thing now is that if someone tries to listen to this connection via which the key is created, it is discovered immediately. And then the key is not used. Because it is discovered immediately because these are individual light particles and I cannot subject individual light particles to a measurement without changing their state. And I see this change immediately. Depending on the need, the non-hackable quantum keys can then be sent to the users. Another big advantage is that these keys can always be re-created for the respective purpose. The sequence is absolutely random Secret listeners disturb the system and are therefore discovered immediately. We have a receiver in Graz, we have a receiver in Vienna and the satellite illuminates the area between Vienna and Graz at the same time. This means that anyone who builds a telescope in the meantime can also pick up these photons, cut them up and evaluate them. The thing is that it does not allow us to decrypt our data or to create our own key. A data transmission on a global level seems to have returned to tangible proximity with the help of satellites. Here at our institute, we want to carry out experiments on which we realize space-to-ground links where we actually transfer limited photons from space to the ground and thus create a quantum cryptographic key. This would make it possible to create global quantum communication. In cooperation with the Chinese Academy of Sciences, to establish a cooperation in which a Chinese satellite transfers these single photons that are generated in a very similar way on the satellite to the ground and we here in Austria and others in Graz, in Steiermark, receive these photons, measure them and create a quantum cryptographic key which at the end of this procedure is symmetrical to a key that was created in China. This is the secret of quantum physics that it will also work over such long distances. And this is the very first time in the history of mankind that an information-theoretical, hearing-safe communication method can be established in a distance, in an intercontinental distance between Graz and, in this case, the ground station Quantum cryptography naturally arouses great interest In banks, governments and not least in the military. The US Department of Defense supports quantum physicists in the USA in this work. The quantum-physical encryption technology is only rarely used by governments so far. The technology was used in various ways for demonstration purposes. We made a bank transfer in Vienna, but there is only one single transfer with a huge effort. Various other applications have already taken place, but there is no really routine technical application yet, because there are two challenges that still have to be overcome. One is the distance. So far, it's only about distances of 10-20 km, the size of a city, for example. And the second is the data rate. The data rate is, compared to what the Internet needs, far too low. I claim, however, that this is a technical development problem. That is, if you take a lot of money into your hands and develop better methods, then you can probably solve it. Our understanding of the world has changed radically over the years. Categories such as space and time seem to fail here. Nevertheless, quantum theory is a fixed part of our everyday life. Quantum physics is actually the most economically successful theory, because semiconductor chips, for example, laser, magnetism, all of this would not exist without quantum physics. That is, the whole modern high-tech, including the television, through which the rulers now watch this film, would not exist. The modern world of today cannot be imagined without quantum physics. The core spin tomography is a model example of a blessed technological application. Quantum physics enables great inventions, but ultimately it is not fully understood in its concepts and methods. Where is the problem? The problem is that quantum physics makes some predictions and has some phenomena that contradict the intuition or, let's say, the healthy human mind. Restricted quantums contradict the classical worldview, because their properties are not clearly defined and are much more strongly connected after the measurement, i.e. much more closely correlated than this could be explained by a classical interaction. Does quantum mechanics now provide a complete description of physical reality? This is what the debate between Albert Einstein and Niels Bohr is about. In contrast to Bohr, Einstein rejects the statement that particles, as long as you do not observe them, can be in several places at the same time from the time of their life. Paradoxes that are solved today? Certain things are already understood. For example, that you can only speak about the properties of a system if you have actually observed it, measured it, otherwise it has no properties. This is, for example, one of the things that you take away from quantum mechanics. However, this is already an interpretation, this is already the Copenhagen interpretation. Albert Einstein actually has almost all important problems and thought experiments for quantum mechanics. He tries to understand the basics of quantum theory and begins to develop completely new concepts for their understanding. Nevertheless, in connection with the restriction of particles, he speaks of a spooky far-reaching effect and as a follower of the classical, so-called local realistic world view, poses the question, is the moon also there when no one sees it? Albert Einstein was a local realist, but for very, very good reasons, because he really understood what it was about. And it turns out that at that point he was on the wrong track. And I would give a lot to know Einstein's opinion on the current situation. It is certainly not so easy to just say that this is nonsense, but he would certainly have said something cleverer. How do quantums behave? It is not possible to understand the behavior of restricted systems if one assumes certain so-called reasonable assumptions about how the world should be created. This was the great discovery of the Irish physicist John Bell. With this, he throws the classical world view over the heap. What did John Bell show? He said, if I now have these restricted particles and I make a measurement here and I influence the others, can I perhaps explain this by the fact that they somehow have properties that I can use to produce the measurement result? And Bell says, no, that's not possible. But it can only be a spooky far-reaching effect in some form. And this is not only important philosophically, but the so-called Bellian inequality says nothing more than that if these were properties, then the two particles could not be so strongly connected as they can be connected by quantum mechanics, but only weaker. That is, the practical meaning today is that this is a method to check how well my device works. If it hurts the Bellian inequality very strongly, then I have a wonderful distortion. And that is also important in cryptography. If my particles hurt the Bellian inequality nicely, then I can use them for cryptography and I can be sure that a listener has no chance of hearing this. Until the late 1980s, scientists saw the distortion of two particles. Anton Zeilinger discovers with his colleagues Greenberger and Horn the distortion of three particles. The so-called Greenberger-Horn-Zeilinger state. This means that with measurements of two particles I can determine the state of the third particle. For me personally, the most important experiment that really cost me 10 years is this realization of these three-particle distortions. That was so difficult and challenging that we sometimes said, this doesn't exist. Why doesn't it work again? Maybe it doesn't exist at all. And it took so long until we found a way to see it. Today this is a matter of course, but back then it was quite a story. It took us about 10 years. What no one suspected at the time, the distortions of three and more particles, were an important prerequisite for the development of quantum computers. Experiments that clearly confirm the quantum-mechanical prediction. Why are we convinced of a prediction? Because in this case it was an incredibly coherent prediction. It all fit together. It was all mathematically beautiful. It was very simple. You can show that in a few lines. And we physicists believe that if something is simple and mathematically beautiful, then it has a truth content. And that's usually the case in physics. The Viennese quantum physicists can show almost 100 years after the discovery of the new physical theory, that in the world of quantum, the concept of information has a fundamental meaning. Future applications such as the intercontinental quantum cryptography prove this. But how big is the quantum world? Are the laws of quantum physics only valid for small particles? Quantum physics has no limits. In principle, it should also apply for macroscopic objects, for something like that. However, this is a very big challenge to show quantum phenomena for macroscopic effects for the simple reason that if a system interacts with the environment then it loses its quantum state because information is passed on to the environment. That means I have to isolate the system very well from the environment to see these phenomena. And that is much more difficult with a large object than with a single atom, for example. But that is a technical challenge for me and not a fundamental question. There is no transition between micro- and macro-worlds. Ultimately, this is also the driving force for this experiment, in which quantum effects such as distortion are examined in terms of the macroscopic order of magnitude. The spatial structure of light and quantum systems I would say macroscopic is everything you see in our order of magnitude. Quantum effects only occur on single photons, single particles or very few particles. You don't see quantum effects in our world at all. In our world we are in orders of magnitude of 10 to the power of 23 particles. You don't see the effects there. They fade out. What we are trying to do here is to bring the effects precisely in the direction of macroscopy. Maybe to understand them better, maybe to get an intuitive picture of them. Maybe to see new effects. Nobody knows what happens when quantum effects occur in the macroscopic world. With the help of a computer program the structure of liquid crystals can be changed in the order of magnitude. This has an immediate effect on the light. It receives a phase information and changes, for example, by a rotation. What we do here is we change the structure of the light. We do this with a laser. We receive the light from a laser, and it goes to a device that can make holographic transformations. This works with liquid crystals. You can see a liquid crystal display and you can control the liquid crystals electrically. This works with a computer. You can program it like a HD screen and give the liquid crystals information on how to change the light. If you give the liquid crystals information you can change the structure of the light that comes out here. This is the foundation of our experiments that we continue here. The physicists test the order of magnitude of the limitations of fast-rotating light particles. A world record is achieved. They produce the largest quantum numbers so far. There is a possibility to create particularly strong rotational impulses and high rotational impulse quantities. Theoretically, you can become as strong as you want. You could give such strong rotational impulses to one light particle. If you send it to your hand, you would notice in which direction your hand would start to get a rotational kick. Even though it has such a strong rotational impulse, the light particle could be limited to this rotational impulse. Not like polarization here, but a rotational impulse. This is one possibility to produce macroscopic values, i.e. such strong rotational impulses that you can feel them without a super-sensitive camera or detection possibility, and still have these strange properties of quantum physics, such as restriction. This is one possibility. This is a fundamental approach. In the experiment we did with these high rotational impulses, we were able to show that you can use them to get improved angle resolution when measuring angles. With a few photons you can measure very precise angles. This is one application. Or you can transfer the impulse to other objects where you can only send a few photons and still have a strong rotational impulse. We wanted to show that, although we only work with individual photons, the rotational impulse can be very strong. We were able to extend this to two dimensions. Here you can see Einstein's spook. In real time. For the first time in the history of science, it is possible to ban the quantum phenomenon on film. The film shows how a measurement on a particle of light affects a particle with it. A large number of photons create these complex spatial patterns. To make the whole thing a little clearer, we used a camera. For the first time we were able to use a camera that can be switched on and off very quickly. We used the camera to measure these rotational impulses. When light has such a rotational impulse, it has a very special spatial structure. It is in a ring mode or in different points. We were able to measure that with this camera. We only converted one photon and measured the polarization of the other. This measurement gave us the same signal to switch the camera on or off. The pictures we took with the camera showed exactly this one photon. This is a white dot on the black camera image. But if we do this often, we see this spatial structure. You can't see much of the distortion yet. The exciting thing was that it was so fast that we were able to change the polarization of the measurement of another photon that we didn't even bring over here. At the same time, the structure has also changed. You could see the effect of distortion live. The pattern that is created in the camera depends on how the partner particles are measured. This distortion of rotational impulse was only shown in a very small area. That is, one or two rotational impulse quantities per photon. We were able to do this with this trick that we first had polarization distortion and then converted it to rotational impulse. We were able to extend that to 300 rotational impulse quantities per photon. 300 is a large number. It is by far the largest we have measured so far. Distortion with such large quantum numbers is not easy to do. On the other hand, the value is still much too small if you really feel something in your hand. We would have to go a lot further. Many people hope for a computer revolution and new software applications from quantum physics. Quantum computers could theoretically be millions of times faster than ordinary computers. Quantum teleportation is the ideal means for transmitting information between possible quantum computers. The information is not represented with bits, but with quantum bits, so-called qubits. They can not only be 0 or 1, but 1 and 0 at the same time. Complex information could therefore be stored in superimposition at the same time. There have been interesting developments in recent times. Originally, a universal quantum computer could solve every problem. Today, it is probably more clever to build a special quantum computer that simulates certain problems. For example, problems in solid-body physics. How a solid body is built, how it works, which is again important for the whole semiconductor technology and all that, for example. And nanotechnology as well. And it looks relatively interesting that in the space of, I would say, 10 years, systems have been able to do something that a previous computer could not do. For a universal quantum computer, we will probably have to wait longer. But again, it may be that 2-3 young students or postdocs suddenly have a great idea and the whole thing works. Many research groups around the world are working on the realization of the supercomputer. New quantum processors are to be used. And perhaps a global quantum internet can soon be built on the basis of teleportation. The Viennese quantum communication researchers can transmit quantum information with the help of limited photons over long distances. They have shown this again and again. The resources to produce quantum mechanical limitations and to generate them over long distances, that is our domain, we have shown that. That is technically feasible. A global internet is technologically within reach. Relativity and quantum. This is one of the unsolved problems of physics so far. The laws of general relativity determine the motion of the planet in the solar system as well as the development of the entire universe. The second pillar of modern physics is quantum theory. It is based on the elementary particles of atomic and solid-body physics. But both theories together, relativity theory and quantum theory, are not compatible. We do not know today how to combine these two theories. Experiments are necessary. Experiments are necessary, as was the case at the time when quantum physics was discovered and invented by their spiritual fathers. There were experiments that could not be explained. The same goes for the theory of relativity. There were experiments, where one wanted to measure the speed of the earth in the ether. This effect was not found. In fact, experimental data emerged that could not be explained in the standard theory, the classical theory at the time. Interestingly, in a very similar time, these two incredibly unthinkable theories emerged for today's technologized society. The theory of relativity, without which satellite navigation would be a strange word for us, and quantum physics, where computers would be a strange word for us today. These two theories are very important, very successful, very efficient, but not compatible. I think this is the most interesting area of research in the future, to see where there are problems, where effects occur in which theory. This seems to me to be one of the most dominant and burning questions in the physics of the present. The reality of the quantum can, although mathematically beautifully describable, almost 100 years after the establishment of quantum mechanics, still not quite understood. It should probably be that an apparent understanding is not possible, but what you might understand, and that is the challenge, why is the world so strange that there are these phenomena? And that is an open question. Do you have a guess? A guess that is also discussed, that our usual idea of space and time is not quite right. That two places from which we believe that they are separated, in a deep philosophical sense, are perhaps not separated, but that space is a construction that we make to be able to describe the world. Our world is a quantum world, and quantum physics is a wonderful description of nature. But it will also develop further. The experiments are in progress.