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From quantum optics to ultracold gases and quantum information, Maciej Lewenstein’s career is a journey through some of the most fascinating areas of theoretical physics. Born in Warsaw in 1955, he began his path with a passion for mathematical physics, only to be unexpectedly steered toward quantum optics by an unforeseen academic decision. Since then, he has collaborated with some of the most influential figures in the field, including Nobel laureates such as Anne L’Huillier, Roy Glauber, and Eric Cornell, contributing to the development of fundamental models in high-order harmonic generation and quantum simulators.
Since then, he has collaborated with some of the most influential figures in the field, including Nobel laureates such as Anne L’Huillier, Roy Glauber, and Eric Cornell, contributing to the development of fundamental models in high-order harmonic generation and quantum simulators.
His research is marked by a multidisciplinary approach and an insatiable curiosity, leading him around the world—from Colorado to Paris—to explore unexpected connections between physics and music, even studying randomness in jazz improvisation.
Today, as the head of a prestigious research group at ICFO, the Institute of Photonic Sciences in Castelldefels, near Barcelona, he continues to push the boundaries of knowledge, critically examining the potential of quantum computers while also strongly committed to training the next generation of physicists.
We met him at the Department of Physics at the Politecnico di Milano, retracing the key moments of his career—from his early experiences in Poland and Germany to his latest projects—to understand what it truly means to conduct research at the highest levels and what challenges lie ahead for the future of quantum physics.
Your research spans several fields, what initially attracted you to theoretical physics and how has your research evolved over time?
When I was a student in Warsaw, Poland, I was actually interested in mathematical physics. There was this sort of unhealthy snobbery in the department of physics and all the best students wanted to study mathematical physics, quantum field theory and things like that.
But when I was about to graduate, the department dean said that it wasn’t possible for all the best students to go into mathematical physics, and he declared that no one could specialise in mathematical physics any longer.
Since I couldn’t pursue a Master’s degree in that field, I and my closest friend (who is now also a professor of theoretical physics in Warsaw) decided to join the quantum field theory group or something like that. There, they told us we should work on quantum optics. I said: ‘But we want to study quantum field theory!’ And they told us: ‘Quantum optics is applied quantum field theory.’ And that’s how I started with quantum optics.
Which leading figures in physics did you work with?
I did my Master’s degree with Kazik Rzążewski, a very important figure in quantum optics. My friend, on the other hand, studied with Krzysztof Wódkiewicz (now deceased), another a leading figure in quantum physics. In 1981 I received a six-month scholarship from the Deutsche Akademische Austausch Dienst (DAAD). I started in Germany and went to Essen, where I met the man who would later become my supervisor. I had started under Rzążewski in Poland, but then moved to Essen and finished my doctorate there with Fritz Haake.
Why didn’t you return to Poland?
Strikes by the Solidarity union had broken out in Poland and a state of emergency was imposed. I was afraid to return because I would have been drafted into the army for two years. So I stayed in Germany until the end of my thesis, which I completed in 1983. I returned to Poland in early 1984.
When I was finishing up, my supervisor, Fritz Haake, told me something very important: ‘Maciej, quantum optics is not separate from theoretical physics. You need to study more. Study statistical mechanics, condensed matter physics and so on.’
What did you learn from this professor?
In a way, he shaped my way of thinking. He taught me that the best thing is not to know everything about one thing, but rather to know something about everything. And it would be even better to know everything about everything! I would say that his most important teaching was this open, multidisciplinary view, not confining yourself to one field, but exploring, learning new things and having fun doing it. If I got bored with one topic, I could always move on to another.
In 1986, at just 30 years old, you obtained a teaching position in Poland
Yes, at the time, my work was still mainly focused on quantum optics, but I was already beginning to take an interest in statistical mechanics, spin glass and neural networks. In the 1980s, neural networks were already drawing great interest. Everyone said they would revolutionise everything, it’s just that there weren’t enough powerful computers at the time to prove it.
In late 1986, I received an offer from Roy Glauber, who won the 2005 Nobel Prize in Physics. He’s considered the inventor of modern quantum optics and the description of the quantum coherence of light. So I went to Harvard towards the end of 1986. My work was still centred on quantum optics, but with a new twist: I began to work closely with experimentalists, particularly Tom Mossberg, a physicist who was at Harvard at the time but then moved to Oregon.
In the 1980s, I also started working on high-intensity and ultra-fast lasers, a field that was emerging at that time, and I wrote a few articles on the subject.
Another important thing is that I also started working on modelling cognitive processes. I was interested in neural networks, so I looked for a collaborator in Poland and found a sociologist-psychologist with no formal mathematical or physics training, but who was very skilled in computer simulations. Together we wrote several articles on the patterns of public opinion in societies based on concepts such as the influence some people hold over others. We probably already predicted the rise of Donald Trump at the time, but unfortunately we didn’t pay attention!
In 1992, I worked for six months in Paris, at the Saclay science and technology hub where Anne L’Huiller also worked. That was when I started to work on the intensity of these harmonics, which underlie all the physics done by the group around Sato, who is much appreciated by people in this department, such as Mauro Nisoli and Caterina Vozzi.
That was when I wrote what I consider to be my most influential article, which I believe received around 5,000 citations. It deals with the generation of high-order harmonics in low-frequency laser fields. This concept had actually already been introduced by Paul Kirkham and Ken Kulander.
Can you explain it to us in simple terms?
The idea is that a laser pulse hits an electron and ejects it from the area near the nucleus via the so-called tunnelling process. The electron crosses the barrier created by the Coulomb force of the nucleus and the laser. It is ejected outside, but can still be accelerated in the laser field. It then returns to the nucleus, or the ion, releasing high-harmonic photons in the process. This is exactly the mechanism for which Anne L’Huillier, Pierre Agostini and Ferenc Krausz received the Nobel Prize in Physics in 2023.
This is exactly the mechanism for which Anne L’Huillier, Pierre Agostini and Ferenc Krausz received the Nobel Prize in Physics in 2023.
At some point, then,I think in 1993, I moved to JILA, the Joint Institute for Laboratory Astrophysics in Boulder, Colorado.
Peter Zoller, a renowned theorist in quantum optics and related disciplines, was working there. As soon as I arrived, I was told that I should organise the Bose-Einstein Condensation seminar. So, for a year, I ran this seminar with experimentalists and theorists.
The experimentalists included Eric Cornell and Carl Wieman, winners of the Nobel Prize in 2001. There was also John Hall, who won the Nobel Prize in 2005 — the same year as Roy Glauber — for his studies on laser spectroscopy and optical frequency measurements.
It was an extraordinary environment, and it was there that I started to deal with ultracold atomic gases and many-body systems. So my interest in the subject just kept growing.
You now head the quantum optics theory research group at ICFO. What is the focus of your research and who is working with you?
Well, a bit of everything, really. It is quite a big group, and so far I have been really lucky with funding. I received three grants from the European Research Council (ERC). These funds are very prestigious and highly competitive. I was awarded three ‘Advanced Grants’ for senior researchers like me. We are talking about funding of €2.5 million for five years, so our work is quite stable and well funded. My group has between 20 and 30 people.
Do you also have young female researchers in your group?
Yes, of course. We have an active policy to try to attract more women to research. I think women make up about a quarter to a third of my team, which is a good result for physics.
The large team allows us to work on many projects simultaneously. We have quantum optics projects, but we generally work on ultracold gases, quantum information and quantum simulators, particularly systems that can reproduce interesting phenomena from condensed matter or high-energy physics. We use atoms and ions, systems that can be controlled with low-energy physics.
When did your interest in quantum information begin?
During my time at JILA, the Joint Institute for Laboratory Astrophysics in Colorado, Peter Zoller became interested in the subject. I witnessed the development of a famous article on quantum logic gates with ions written by Cirac and Zoller in 1993 and 1994. These logic gates form the basis of modern quantum computers.
On the other hand, I wrote one of the first articles on the quantum ‘perceptron’, which relates to quantum neural networks and quantum machine learning. At the time, however, no one was interested in these topics, so my work remained in the shadows. Only recently has it started to be cited, since it was one of the first studies of its kind.
Since then, I have been working on quantum information from both a theoretical point of view, studying the mathematics of quantum correlations, and a more applied point of view, working on quantum simulators.
What do you think about quantum computers?
I am not against quantum computers, but I have my reservations. They are excellent as simulators, but I doubt that they can be actually useful for solving practical problems better than classical computers.
You have worked with some of the world’s leading physicists, including many Nobel Prize winners. Can you tell us about a particularly significant moment from these collaborations?
When I arrived at Saclay, no one really understood the three-stage model for generating high harmonics, that is, the mechanism whereby an electron is extracted by tunnelling, accelerated by the laser field and then reabsorbed by the nucleus with the emission of high-harmonic photons.
At a conference in Belgium in early 1993, Paul Corkum and Ken Kulander first presented the classical model to explain this process.
It was a turning point. Soon after, we realised that we could describe the same phenomenon quantum mechanically using the language of Feynman’s path integrals and quantum field theory. We quickly wrote our first article on the topic, and it was strongly influenced by that conference in Belgium.
A funny anecdote from that conference: There was a flood in the area, so you couldn’t drink tap water because it was contaminated. The hotel staff told us that we could drink anything else available, so of course everyone chose beer! Which, fortunately, is found in abundance in Belgium. It was a very ‘brilliant’ conference.
Quantum mechanics is often viewed as a complex and abstract field. What is one concept from your research that might fascinate or surprise the audience?
Well, the most surprising aspect of quantum mechanics is probably what is called ‘nonlocality’. I will try to explain this property, although it’s not easy to do. The property has been shown experimentally, and Alain Aspect, Anton Zeilinger and John Clauser were awarded the 2022 Nobel Prize for doing so.
The idea is related to the concept of very strong quantum correlations, called ‘entanglement’, but nonlocality is stronger in a way.
Let us imagine that we have Alice and Bob. I’ll use an entirely classical example: you are Alice and I am Bob. Suppose we have two balls, one white and one black. We put them in two bags randomly, without knowing which one is in which bag. You take one bag and I take the other, then I go to the moon, because Elon Musk is paying for my trip! When you open your bag and find the white ball, you immediately know that I have the black one. If you find the black one, you know I have the white one. This is a classic correlation and not at all surprising.
The problem with quantum mechanics is that the colours are not defined until they are measured. If the two balls are in an entangled and anti-correlated state and if Alice measures white, she can be sure that Bob will measure black. But if she measures 30% grey, then Bob will measure 70% grey complementarily.
Einstein argued that there should be a ‘local reality’ for every property of a particle. But in quantum mechanics, we need to accept that colour is only defined at the moment it is measured. This is paradoxical because it implies an instantaneous correlation between particles, even those separated by enormous distances. However, information cannot be transmitted faster than light, because everything is governed by causality.
This is one of the most surprising aspects of quantum mechanics. Some people are looking for alternative interpretations, such as the many-worlds theory, but I prefer to accept quantum mechanics as it is: a powerful computational tool.
You are a big jazz and are said to have at least 9000 vinyl records. What connections do you see between jazz and physics?
Yes, in the last few years I have mainly been interested in avant-garde jazz and free improvisation. In Italy, some of the most important jazz musicians are Andrea Centazzo, a drummer who often works in the United States, and Walter Prati, a professor at the Como Conservatory, who also plays improvised music. Ennio Morricone was also a pioneer of improvisational and avant-garde music.
In free improvisation, there are no fixed rules, which makes it interesting from a physical and mathematical point of view. If you play alone, just follow your instincts. But if you play with someone else, you try to communicate with them, creating a kind of musical ‘entanglement’. I have always wondered how unpredictable free improvisation really is. For example, we can predict Beethoven’s music or even John Coltrane’s jazz with statistical models or artificial intelligence. But how predictable is free improvisation?
I tried to get a grant from an American foundation to study randomness in improvised music using quantum random number generators to compare the level of randomness in music. I didn’t receive that funding, but with other funds I hired a composer at ICFO, Reiko Yamada, who is still with us, following a programme called ‘sonification of quantum mechanics’. We turned quantum data into numbers and these into sounds and tried to see if we could ‘hear’ quantum effects in music. We also played concerts at a festival in 2021 and recorded an album and we now have two Master’s students and a PhD candidate who are both physicists and musicians working on it. In my opinion, contemporary music — by John Cage, for example — is not predictable, even though no one has measured it yet.
In my opinion, contemporary music — by John Cage, for example — is not predictable, even though no one has measured it yet.
You have worked in several countries and at different research institutions. What are the advantages of an international approach to science?
You learn a lot — not only languages (I speak Italian, English, French, Catalan Spanish, German, Russian and Polish), but also different cultures. This makes you more flexible, adaptable and able to solve problems in new ways.
I believe it is essential for young researchers not to stay in the same place, but to travel and work in different environments. It can be difficult for family life, of course, but it is an invaluable experience for personal and professional growth.
Many students find quantum physics fascinating but are often frightened by it. What advice would you give to someone who wants to pursue a career in theoretical physics?
It is true that studying physics is difficult, but the right attitude is crucial. If you really like it, you will succeed.
When choosing a supervisor for your thesis or PhD, it is also important to look at the personal aspect. Ask yourself if this a person you can work with well for the next few years. The research team and the environment are also important. You have to create good chemistry.
If you could talk with a physicist from the past, who would you choose and why?
The obvious answer would be Einstein, to better understand why he did not accept quantum mechanics. But I might choose Aristotle, like an Indiana Jones adventure!
What are you doing at the Politecnico di Milano?
I am on sabbatical. I normally work in Catalonia, but my assignment there is coming to an end and I have decided to slow down a bit and work with new research groups.
I work at the Politecnico two days a week with Mauro Nisoli and Caterina Vozzi from the Department of Physics. After March I will go to Kraków for a while. I am looking to start new collaborations with research groups here.
What are you currently working on?
I am exploring the connection between quantum mechanics and quantum electrodynamics, an area that has not been studied much. In normal experiments, light can be described well using classical physics, but quantum effects emerge under certain conditions.
Applications could include quantum cryptography, measurement or new types of quantum spectroscopy. The idea is that quantum light could reveal more information about materials than classical methods. This is a very new research, so we will see where it leads!
Do you still enjoy research?
Absolutely! I would stop if I were no longer having fun.