Hello Prof. Dr. Eisert, thank you very much for taking the time to answer our questions. Please introduce yourself.
The pleasure is mine. I've been a professor at Freie Universität Berlin since 2011. Before that, I was also a professor in Potsdam and a lecturer - that's a kind of assistant professor - at Imperial College London. I am also affiliated with the Heinrich Hertz Institute and the Helmholtz Center Berlin. I studied in the USA and in Freiburg.
I am a theorist, i.e. a "dry swimmer". Since around 1900, it has proven to be the case that physicists either do experiments or boil down theories. This is because experiments have become difficult and theories very mathematical, and this division of labor has proved its worth. The fact is that even though I think quite mathematically, I am often pragmatically motivated and also have a lot to do with experiments and enjoy working with experimental colleagues.
I am a quantum information scientist from the early days and have already completed my doctorate in this field. In that sense, I'm not too old a "veteran of the field".
Can you briefly explain your background in quantum many-body theory, quantum information theory and quantum optics? How did you become interested in these areas of research and what prompted you to get involved in them?
I was fascinated by quantum mechanics early on, actually as soon as I came into contact with it. On the one hand, I was attracted by its mathematical beauty and the profound statements about the description of nature - and at the same time, it seemed to me to be unintuitive. Things can be in different states at the same time, the measurement changes the object, there is absolute chance. I found that exciting.
When I then learned about quantum information - a field in which fundamental questions came together with those of modern technological development - a great love was born. You can ask what holds the world together at its foundations, not too far removed from philosophical questions. And at the same time think about future technologies.
In a way, I see these two topics as one thing: for me, quantum computers are particularly controlled many-body systems. And many-particle systems, especially uncontrolled quantum computers. There is some truth in these thoughts. And they also show why the midfield is so exciting. For people who work as methodically as we do, it's attractive.
At the beginning of last year, you received an ERC Advanced Grant for research into quantum computers. What specific project are you tackling with it?
I would like to understand where exactly the boundary is between classical and quantum physics. Up to what point can quantum computers still be simulated classically efficiently with high-performance computers because they are too noisy and faulty in practice? At what point does this no longer work, and where do real quantum advantages come into play? This is the third ERC grant for me and it was of course a great honor to win such an award.
What are the main goals of your research and what potential applications do you see for your findings?
I want to understand what quantum computers are good for. What they can really do. What practical applications there are, and what can be said about them, precisely, rigorously and without hype. And I want to understand phases of the matter.
Quantum technologies have been a much-discussed topic in recent years. How important do you think quantum physics is for shaping future technologies and scientific breakthroughs?
It is already very important. In the first quantum revolution, the foundations were laid for semiconductors such as transistors and lasers - things that are now in almost every high-tech product. Now applications in quantum computers, quantum simulators, metrology and secure communication are on the table.
I myself have been working a lot on the question of what computational applications there can be that go beyond academic contexts. The focus here was on questions of machine learning and optimization, but also on simulation. We have had great success in each of these areas and have been able to demonstrate quantum advantages in the first two areas, for example, which is important for the field. Much of this will only be applicable in ten years' time, but it is in the nature of future technologies that you have to look ahead a little. And in the latter, to explore new applications in the simulation of exotic situations of quantum fields, such as those reminiscent of general relativity - in studies that are already possible and interesting now.
What are the challenges and opportunities in turning quantum concepts into practical applications?
Some, such as secure communication, are almost ready for the market. Applications in metrology, i.e. the science of precise measurement, are also obvious. Quantum computing and simulation are more difficult, but perhaps also more exciting. A key question is what new applications there are that are not only of conceptual relevance, but also of practical industrial relevance. Then there is the question of the extent to which errors are really a problem. There are ideas about what can be done with noisy quantum computers. There are some indications that error correction will be necessary, but this will be accompanied by a surplus of necessary resources. The situation in the scientific landscape right now is incredibly exciting. There are also a few practical challenges in terms of attracting funding or finding talent, but that's nothing that can't be overcome.
Berlin has become increasingly recognized as a center for quantum research. How does the Berlin ecosystem foster progress in this field, and what unique advantages does the city offer researchers like you?
Berlin is a scientific center in several respects. First of all, it is a wonderful research location with a large number of bright minds. Then there are also a number of formats that support the Berlin ecosystem. The Einstein Research Unit on Quantum Devices is promoting the topic as part of the Excellence Initiative. It is also part of the DFG Cluster of Excellence MATH+. Above all, the Berlin Senate is funding the ecosystem as part of Berlin Quantum.
How does the Berlin Quantum Alliance contribute to collaboration in quantum research?
In fact, the initiative is now called Berlin Quantum to avoid confusion with the Quantum Alliance, the network of clusters of excellence in Germany, of which we are also a member. Berlin Quantum promotes basic research, which is still extremely important in this field - it's not about finished technologies that just need to be implemented. But it is also the interface to the world of start-ups and industry. Berlin is also richly blessed with non-university institutes that make their contribution here.
How do you rate Germany's position compared to other countries in terms of quantum research and development? What about international cooperation?
Germany is excellently positioned internationally in this field and is a technology leader in some sub-areas, such as quantum simulation. This is not only due to the fact that the German government recently spent two billion euros on the field, but also to a great deal of stamina and a long tradition in the field. Germany is also very active and visible in the European context and plays an important role, for example in the European Quantum Flagship. But networking with North America, Asia and Australia is also intensive and close.
How do you see future developments or trends in experimental quantum physics? How do you envision the landscape of quantum technologies in the next ten years, and what breakthroughs do you hope to see?
The recent history of the field has shown that development is often erratic and disruptive. The idea of quantum computers is not new: Feynman, Deutsch and Benioff were already thinking out loud in the 80s about whether completely new types of computers could be realized with quantum systems. The theoretical bombshell burst in the mid-1990s when the Shor algorithm was presented, an algorithm that is practically relevant and can efficiently solve an important problem that cannot be solved efficiently using classical methods. This was extremely stimulating as a theoretical development. Quantum simulators have indeed been around for some time.
But large-scale quantum computers have only been built in the last five years. This creates an extremely exciting dynamic. These computers are not yet perfect and have hundreds, not yet thousands, of quantum bits. But until recently, such devices were still dreams of the future. An important role will be played by whether it is possible to correct errors that are unavoidable. Just a few weeks ago, surprisingly strong results were presented that showed the logical manipulation of 48 error-corrected quantum bits. This took everyone by surprise, and it showed that development can go much faster than expected.
There are good reasons to be and remain optimistic. You have to steer clear of the hype - which undoubtedly exists in such a future-oriented field - and keep doing your good work. As I like to say, quantum computing is also interesting if you only say things that are true.
Thank you very much for the interview.