Prof. Arno Rauschenbeutel, HU Berlin © Arno Rauschenbeutel

16 February 2024

"We are entering a realm where we are manipulating quantum states in a complexity that has never before been possible in humanity or in nature."

Prof. Rauschenbeutel, thank you very much for the interview and your time. Can you briefly introduce yourself and explain your background in experimental quantum physics? How did you become interested in this field of research? And what prompted you to work on optical quantum effects?

My name is Arno Rauschenbeutel and I am an experimental physicist at Humboldt University, where I head the "Fundamentals of Optics and Photonics" working group, and how did I become interested in this field of research? It was by chance, during my studies at Imperial College in London I came across quantum optics. This is the interaction of light and matter on a microscopic scale, i.e. individual particles of light interacting with individual atoms and molecules play a role. So I fell in love with this field. It's also always down to people. I had a tutor who introduced me to it and a great professor, Peter Knight, who gave motivating lectures. After that, I went back to Germany and got advice on where I could go if I wanted to do this kind of physics and ended up in Bonn, with Dieter Meschede, at the Institute for Applied Physics at the University of Bonn.

There I did further experiments in this direction as part of my diploma thesis, and that was the time when quantum communication and quantum computing began to be in vogue and deliver the first exciting results. Then I said I wanted to do that and was lucky that Dieter Meschede "arranged" a doctoral position for me in the group of Serge Haroche at the École normale supérieure in Paris, who later received the Nobel Prize for the manipulation of individual quantum particles, and I did my doctoral thesis there and then just stuck with it. I have followed the development of quantum technology 2.0 almost from the beginning and know what is behind it and what incredible progress has been made.

I'm currently a visiting researcher at the Institut de Physique de Nice in France. There is a working group here that is looking at the interaction of light with ultracold atoms, and we want to launch a research project with them.

What are the main aims of your research and what potential applications do you see for your findings?

First of all, the main goal is to study this interaction of light and matter on the microscopic scale, i.e. individual particles of light interacting with individual quanta of atoms, molecules or artificial atoms, on the one hand, and to control and harness them. The problem with this is that if you hit a single atom with a laser beam and then track a single particle of light, the chance that this particle of light will interact with precisely this atom, i.e. be absorbed and scattered, is very, very, very small. In other words, nothing usually happens. What you want, however, is that this interaction, with which you can perhaps generate or process light in the form of individual photons, for example, takes place deterministically with a high probability. What we have to do to achieve this is not allow the light to get past the atom. 

And how do you achieve that? By focusing the light very strongly, so strongly that the shadow of the atom, if you imagine the atom now as a small disk, becomes as large as the focus of the light and what you then pay as a price for this is that it will only work at a single point, namely exactly at the focus. Because if you don't focus strongly, it will quickly diverge again. It is then only constricted so tightly at one point in space that you can achieve this efficient interaction. What we have now demonstrated as a pioneering achievement is that we can constrict the light so strongly with the help of glass fibers, which we pull incredibly thin, like chewing gum, thinner and thinner until their diameter is smaller than the wavelength of the light, that when we then place atoms on the surface of these glass fibers, the light cannot pass this atom either. But now the advantage is that you have a long string on which you can now line up many atoms. The light does not pass any of these atoms. In other words, you increase this interaction not only by constricting, but also by the number of atoms and can then exploit collective radiation effects.

How can we imagine this in more detail in the application?

First of all, this interface between quantum emitters, such as atoms and molecules, which are capable of emitting individual particles of light and which you then want to use for quantum technology, such interfaces are simply technically necessary. If you want to store information on a USB stick, you also need a USB interface and we have created an optical interface with which we can couple light to atoms very efficiently. On the one hand, this is interesting for generating quantum light, which is already being propagated in optical fibers. The charming thing about this approach is that you don't have to couple the light into an optical fiber after you have generated it, because it is always lossy, but that the whole thing happens in a normal optical fiber, which is only made thin at one point. That is a technical advantage. The other is that you can then also connect atoms as storage for the light that runs through the glass fibers. That doesn't sound so exciting now. Intermediate storage of information is something quite normal in normal information processing. You can store information on your hard disk and retrieve and process it later, but it turns out that it's different in the quantum world and that you can't copy information so easily.

What you can do with normal information carriers, copying information from the hard disk to a USB stick and then having it in two places, does not work. That's why we need so-called quantum memories that circumvent these problems or still allow information to be transported over long distances. Because when you send light via optical fibers as an information carrier, there are always intermediate amplifier stages in the communication path during normal information processing, because the optical fibers are incredibly transparent, so they can send light over 100 kilometers and only 2/3 of the original power is missing. That's great. But if you want to cross the Atlantic, there wouldn't be enough light on the other side of the Atlantic to work with. That's why you have intermediate amplifiers on such a long communication link, and these intermediate amplifiers don't exist in the form they do in quantum mechanics. They cannot simply amplify signals. That sounds strange, but that's the way it is and you just have to accept it. But there are tricks and these tricks can be realized with ensembles of atoms, for example, which then temporarily store the light and retrieve it again.

You then need so-called quantum repeaters. However, I would like to emphasize that what we are doing is basic research. Not only that, we have actually started a project on October 1, 2023, which is the first applied project, so we have promised to produce a prototype and it's not the first time in my career either. The exciting thing is that when you realize a system like the one I described and then start to investigate it and really understand it in detail, you come across new physics - and that's what happened to us - so that it doesn't hit on effects that nobody had on the radar. That is perhaps another main goal of our work. In other words, to gain knowledge in order to expand our understanding of the interaction of light with atoms. Finally, I would like to say that my working group is based at a university, so our mission is clearly also teaching. This means that we are not a research institute at Humboldt-Universität, but a teaching and research institute. As far as the goal of our work is concerned, the safest bet is actually to train excellently qualified physicists who can then apply their knowledge and skills in a wide variety of areas for the benefit of society.

Quantum technologies have been a much-discussed topic in recent years. In your opinion, how important is quantum physics for shaping future technologies and scientific breakthroughs?

It can be said that quantum physics is one of the main pillars of today's technologies and many of the scientific breakthroughs that have been made in the last 100 years. Our economy and our prosperity are largely based on quantum physics. We are talking about quantum technology 1.0, traditional quantum technology, so to speak, and semiconductor physics, i.e. integrated electronic circuits, lasers for long-distance communication and magnetic resonance imaging in medicine. These are just three examples of technologies that are enormously important for our modern communications society and which would not have been possible without an understanding of the microscopic world. Your question is now more about the extent to which these more recent developments are now important for shaping future technologies and scientific breakthroughs. We are talking about quantum technology 2.0, which is now emerging or has already emerged to some extent. 

This quantum technology 2.0 goes even further in the utilization of quantum effects. Quantum technology 1.0 has a lot to do with understanding the discrete energy levels and energy states of electrons in matter and atoms and then making them usable. Now we are talking about technologies that take advantage of individual quantum systems, individual ions, atoms, even small mesoscopic systems that behave like a single atom, to exploit the laws of quantum mechanics that apply to these quantum states of these systems in order to solve problems or tasks more efficiently, measure more accurately or measure more sensitively than is possible with conventional technologies. This is undoubtedly something that is already successful and will be successful. For example, metrology, which uses quantum states, entangled states, squeezed states that allow gravitational waves to be detected, an effect that was predicted by Einstein and for which mankind has been searching for a long, long time.

In other words, there are already examples of applications where quantum technology 2.0 is the prerequisite and made this possible in the first place. There are also quantum communication systems in which quantum effects can be used to ensure tap-proof communication, which is conceptually on a different level to the normal tap-proof communication that we have when we have https at the top of the browser line. Because we can be sure, based on the laws of physics, that no eavesdropping on our communication has taken place or is taking place. That also works. What is still outstanding at the moment is that we are not yet in a position to implement these amplification stages in practice. We know conceptually how it should work, but the technical implementation is an enormous challenge, and large consortia are working on realizing it.

What about quantum computers, how do they come into play?

This is perhaps the ultimate goal or ambitious goal that is being worked towards. The quantum computer promises to solve certain problems that cannot be solved with classical computers in a reasonable amount of time, because the classical algorithms would simply take too long to perform these calculations. Fantastic progress is being made in this area at the moment. However, it is not easy to predict whether we will one day have a quantum computer that will actually solve problems that we cannot solve with mainframe computers. A lot of basic research is still needed to actually implement the existing concepts technically. Then we can say: but that's a big risk, if we don't know whether it will work, should we invest so much time and money in it?

However, it can be said that this effort, in which hundreds of working groups and now also start-ups and large companies and really the big tech players are involved, will also lead to unexpected results. We are entering a terrain where we are manipulating quantum states in a complexity that has never before been possible in humanity or in nature. Whether in the end this goal, which is now the motivation, i.e. the quantum computer that cracks classical cryptography, will be achieved. Even if this were not the case, very, very valuable discoveries would be made along the way, which could possibly lead to completely different applications than the ones we initially have in mind.

So it's also a lottery factor. Some comparable major undertakings have produced by-products that were enormously important for future technologies. One example is the moon landing. This was supposedly one of the best investments of mankind. Computer technologies had to be developed for this moon landing. This brought the USA a long way forward and we can see where the greatest expertise and market power in computer technology lies today.

One of the applications, for example, where it is now being shown that there can be a quantum advantage, would also be machine learning. This means that data centers emit just as much CO2 as all air traffic worldwide. This raises the question: Are there perhaps more efficient methods of information processing? This is a tangible problem. By surfing the Internet, we now consume large amounts of data and ultimately also energy. At some point, we have to start thinking about this. It may be possible to calculate energy more efficiently at some point. You have to be very careful here; on a small scale, it may look as if quantum computers are efficient because quanta, i.e. the smallest possible energies, are used to manipulate states. However, the technical overhead is still incredibly high. You have a huge machine that has to be cooled to low temperatures, large laser systems are required and so on. The energy input for what the machines can do today is horribly inefficient. But in principle, it is conceivable that we could work more energy-efficiently.

What are the challenges and opportunities in turning quantum concepts into practical applications?

We have already shed some light on the opportunities. But what are the challenges? You have to imagine that in order to be able to use a quantum system and measure it extremely sensitively, it also means that this system is extremely sensitive to interference. Getting a grip on the interferences so that the processing of quantum information, for example on a quantum computer, does not get messed up is an enormous challenge.

You can start by saying that we are now cooling the entire system to very low temperatures, etc. Even then, there are still fundamental problems. There has been a breakthrough, known as quantum error correction, which makes something that previously seemed impossible possible. The ability to detect and correct errors caused by such disturbances. That sounds simple again, so in the computers we are using right now, this happens all the time, that information is stored redundantly and then, if individual bits flip, it is possible to detect and correct this error. But in the quantum world, this is not easily possible. In quantum error correction, there are now the first very encouraging results where it has actually worked to calculate with error-corrected quantum bits.

This is still an enormous challenge because it makes everything much, much more complex. You don't work to encode a zero or one, or as in quantum mechanics, zero and one, so you don't work with a single ion or a single atom, but you need a dozen or more, which together encode a so-called logical one. This makes the whole thing enormously complex and the more complex the quantum states become, the more fragile they become. You can imagine that in a quantum calculation, the quantum computer in the worst case goes through states that are comparable to a Schrödinger cat. This is an example that Schrödinger used to illustrate the discrepancy between the quantum world and the macroscopic world that surrounds us - in other words, that we don't see cats that are dead and alive at the same time. But a quantum computer has to maintain such states, where it has one state and another at the same time, but which are very different, almost like dead and alive. Whether this is possible on a scale where it becomes technically interesting and relevant to actually carry out calculations, i.e. digital quantum calculations, has yet to be shown. But what is certainly an exciting field and does not suffer so much - in quotation marks - from this susceptibility to error are so-called quantum simulations. A distinction must now be made: Quantum calculations and quantum simulations. They are not the same thing. You can compare it to a digital computer or a wind tunnel in the classical world. Why did they invent the wind tunnel or the flow channel for boats? Because at that time there were no computers available or the computers were not yet powerful enough to solve the hydrodynamic equations necessary to calculate the flow of water around a tanker or the flow on an airplane. It simply wasn't possible. And what do you do then? You make a simulation, i.e. you make a model and then you look at how the flow flows in a very controlled system in a wind tunnel. Richard Feynman came up with this idea and this was perhaps the birth of quantum information processing. If you like, in order to understand quantum effects that are relevant to us, such as superconductors for example, in detail, you have the problem that these quantum effects take place in a material that I cannot change. In other words, you can look at it, but the investigations I can carry out on it are limited.

If I am now in a position to assemble a quantum system from individual constituents that can be accessed and which then behave in their dynamics, like this solid body in which superconductivity takes place, then I have the opportunity to understand what is happening there on a microscopic level and also to vary parameters, which I cannot do so easily with normal materials.

You can perhaps change the composition of alloys or something similar, but you can't change a specific parameter in situ in a controlled way. A quantum simulator like this, where you recreate a complex system, but in such a way that you have much more control over it and can change parameters and measure them in detail - that is an approach that is certainly promising for me and will bring us further insights and progress.

Berlin has become increasingly recognized as a center for quantum research. How does the Berlin ecosystem promote progress in this field and how does the Berlin Quantum Alliance contribute to collaboration in quantum research?

Well, I haven't been here that long. I moved to Berlin in 2019 and had an offer from Berlin that convinced me, and part of what convinced me was the scientific landscape that exists there. And it's just that in Berlin, especially where the Institute of Physics at Humboldt University is now located in Adlershof, there is a large technology park with many, many companies that are very strong in optical technologies and small and medium-sized companies that produce and develop high-tech, and this location for optical technologies is very, very strong in Berlin.

And optical technologies are the prerequisite for quantum technology. So you can perhaps manage without light on a small scale, but as soon as you want to communicate or work, i.e. link computers, quantum computers and chips, you need light. So on the technology and industry side and on the university side, there are also many groups working on controlling the interaction of light with matter or on the theoretical level with the description of the dynamics of these interaction processes and also the development of algorithms or benchmarks for quantum objects. And that is certainly a very, very good prerequisite for creating something bigger than is already there. And we are now very lucky that the state of Berlin has placed its trust in us and given us the money to set up the Berlin Quantum Alliance.

And the aim is really to pool these skills. So you shouldn't underestimate how big the hurdle is to do something together. You simply have to say that. Berlin may have three universities, but the distance is no less than if I were to travel from Bonn to Cologne to get from one university to another and have a structure that makes it possible to bundle these competencies and also create a meeting place to create this Deep Quantum Hub. This will hopefully enable us to achieve more together than before. And we also want to use these funds to bring new minds to Berlin, there are funds for new appointments to close gaps. The fact is that there is still no experimental platform in Berlin that could really carry out quantum simulation or quantum computing.

And there are exciting approaches and we would very much like to have a working group in experimental physics that closes this gap and contributes a platform like this. And then also a working group in theory that deals with quantum software.

We have talked about Berlin. The next question relates to Germany. How do you think Germany is positioned compared to other countries in terms of quantum research and development?

Okay, so first of all, Germany is at the forefront of quantum research. We have a few centers in Germany that have global visibility and are beacons of science and are really at the forefront of their respective fields. You can say that, you can say that. But now the next step is to turn this into applications, but then in conjunction with steps such as spin-offs, and when it comes to founding start-ups and trying to turn a new idea into a product, I don't think Germany is at the forefront.

I can see that in other countries... I don't know whether they are more willing to take risks, whether the structures are better? I have to say that I'm not that familiar with the subject matter. I myself am not and will not be involved in any spin-offs in the future. For good reason. But for me it's simply because I've seen what it means to be self-employed.

My father had his own company and I am very happy that I have the freedom that I have as a professor but am also a civil servant. But when I compare that with Israel, for example, they are worlds apart. I have contacts in Israel and I can see that the research landscape there is very different, much, much more flexible, more active when it comes to spin-offs. In other words, we still have some catching up to do, and that is perhaps also a cultural thing, I can hardly say.

In other words, we are actually in a very, very good position. One huge advantage that Germany has over other countries in Europe is that research and participation - including in politics - are highly valued. In other words, we simply know that funding basic research is something that is essential for future prosperity and if cuts are made, then we have to say that the Alexander von Humboldt Foundation has just been cut. But first of all, it's not as if the research budget is simply, I don't know, cut in half, as has happened in other countries. So the predictability that we have in Germany and the reliability of research funding as well as the appreciation of basic research, I see that as a great advantage.

Let's stay with an international view of the field: what joint projects or initiatives are there between Germany and other countries to strengthen the global network for quantum research?

First of all, we have very, very powerful research funding within the European Union. And there have long been framework programs in which research that was more applied was funded. But since the establishment of the European Research Council, basic research has also been funded by the European Union. This is a very, very important means of promoting cooperation across borders. And I have actually always participated in this and benefited from it. When it comes to research collaborations across European borders, it's no longer so easy. There are actually closer research collaborations with Israel, because there are binational programs that are funded by the German research community, but also by the "German Israel Foundation", in order to promote cooperation on a scientific level, also in the context of reconciliation and reparation. I have already collaborated with researchers from Israel in the past and have applications in progress.

In my view, such cooperation, not only with Israel, but also with other countries, is not only fruitful because it expands the number of people or the number of real collaborations, but also because different cultures come together.

I just said that with Israel, which has a completely different approach to the utilization of research results than we do. And I did my doctorate in France and also studied in England, and I've just realized that these systems all have their own peculiarities and that if you join forces, you can achieve more than each one on its own.

Finally, let's look at the next generation: for prospective researchers interested in experimental quantum physics, what advice would you give them and what resources would you recommend to deepen their understanding in this field?

You always transfer something of what you have experienced yourself or how it has worked out for you to the general public. And on the one hand, I have the feeling that it makes a lot of sense to look for an area in which there is still great potential for development and the complexity of experimental, constructive physics is not yet so advanced that it will be difficult to enter this field yourself if you become independent.

In the field of ultracold atoms, for example, there are now experimental setups of such complexity that even a call to a university would not bring in enough money to set up such an experiment. And I myself have been lucky enough to have achieved something by observing the state of the art and then by bringing together two technologies that were just emerging, in other words I helped to bring a new field of research into being and that is of course the best thing that can happen to you.

The problem is that it's difficult to plan something like this and you can't deny that there is an unpredictability in an academic career like this. So it's just that I was very lucky to be in the right place at the right time and then I was also lucky in the realization of the first experiments. It's only in hindsight that you realize that: So it worked, but then you realize that if you do it differently, it doesn't work. You were just lucky that you did it that way. And that is, of course, the charm of research, but on the other hand it is also a risk if you want to build your future on it. And that's why I would say that this sounds almost negative, but I always say it that way anyway. I think you should only embark on an academic career if you can't imagine doing anything else.

In other words, if the great aspects of the academic world seem so important and enriching that you are prepared to accept the disadvantages of the uncertainties and the work and flexibility that is demanded of you.

Thank you very much for your time and the fascinating insights, Professor Rauschenbeutel.

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