Between lenses and lasers

The vacuum chamber containing the atoms is located between tons of optics and cables, which are piled up on several levels on the main table. “The main table is really, really full!” David laughs, looking with enthusiasm at what appears to outsiders as tangle. He finds it impressive how generations of Ph.D. students have built up the experiment over the years, constantly adding new elements and upgrades. It is one of the older experiments at the MPQ.

The setting was quite different during his master’s thesis at the Ludwig-Maximilians-Universität (LMU). There he was in a laboratory that was still under construction. “It was a very technical work. I characterized our quantum gas microscopes.” These quantum gas microscopes play a central role in experiments with cold atoms, including in David's current laboratory at the MPQ. “To be able to see the individual atoms on their lattice sites, we basically need a very well-functioning microscope,” the physicist explains. These microscopes are complex custom-made products, very expensive and are usually ordered from highly specialized companies. Therefore, a laboratory must thoroughly check whether the supplied microscope meets all the specified values. Others have already faced this task, too, but David's master's thesis was also about systematically documenting in detail the necessary steps and best suitable methods. “This led to an appendix of around 80 pages with lots of pictures and descriptions,” says the doctoral student, laughing a little. “If I want to eliminate this aberration, then I have to move this lens. If I want to eliminate that aberration, then I have to rotate that lens,” he says, providing an example to make it clearer. Ultimately, his work was honored with the MCQST Master's Award. Even more than the award, however, he was pleased to hear from colleagues who used his work as a reference.

By the end of his school years in Gröbenzell, northwest of Munich, David knew he wanted to study physics. However, deciding what to specialize in for his master's degree was not so easy. “I voluntarily extended my master at LMU by a year because I wanted to attend more lectures.” In the end, it was the lectures on quantum optics and ultracold quantum gases that particularly impressed him – not only from a physics perspective, but also the way in which the content was taught. “I had the feeling that good teaching was really important to people.” There are so many exciting fields in physics, and it wasn't easy for him to decide, but in the end it also depends on who can teach you things better, thinks David. This semester, he is a tutor himself: “I want to pass on not only the knowledge to the students, but also the enthusiasm for these topics.”

Perhaps the enthusiasm will spread all by itself. “There are many phenomena in quantum physics that we don't know from our everyday world. But they are tangible for us every day in the experiment!” David enthuses. The formulas that physics students learn in their bachelor's program are applied daily by researchers at the MPQ, he adds. In addition, there is also the technical aspect, which fascinates him: “The fact that we can control the atoms in almost every respect is really impressive.”

Controlled disorder

This high degree of control can also be used to create disorder. The physics of so-called disordered systems can be studied particularly well with the help of cold atoms in optical lattices, David explains. Experiments in the field of disorder physics are also possible with the setup in his laboratory at the MPQ. It is an area that particularly interests the young physicist. To investigate the behavior of disordered systems, the atoms are no longer placed in a uniform “egg carton landscape”. Instead, they are placed in a kind of wild, hilly landscape. This landscape can also be precisely adjusted with the existing setup in David's laboratory: “We can create hills of different heights. We have very good control over this.” It's actually a bit like at home, David says with a laugh, in a spontaneous attempt to motivate the abstract topic. “If it's messy, then everything moves more slowly. It's the same in quantum systems – to put it simply.” In an ordered system, particles have “the need” to move, to interact with each other, to explore their environment, the physicist continues. Ultimately, the particles want to distribute themselves evenly, just as milk distributes itself in coffee. “If it's messy, then that no longer happens,” says the doctoral student. “The particles get stuck, so to speak.” David emphasizes that this is not a classical effect: “It's not that the particles have too little energy to move. The reason for this localization, as we call it, is a quantum mechanical interference effect.” The wave function that describes the atom does spread out. However, due to the disorder surrounding the atom, its wave function interferes destructively everywhere. The behavior of many interacting particles in such a disordered background remains an unresolved research question: “We don't know to what extent localization takes place for interacting particles or how robust it is.”

David first studied disordered systems and the effects of localization in Cambridge. After completing his master's degree at LMU, the well-known English university city was his next stop. During his research stay, the young physicist wanted to determine whether, and if so, at what kind of experiment he would like to do his doctorate. At LMU, he learned a lot about what it is like to work in a laboratory that is being set up, says David. But he lacked the experience of working on an experiment that had already existed for some time. “I wanted to go abroad and work in a lab that was up and running,” says David, summarizing his motivation for the year in Cambridge, which – without that having been the original intention – also led to a second master's degree.

“You get the feeling that the whole city is a university”

The year in Cambridge brought the hoped-for clarity. “With a new experiment, you have an overview right from the start, whereas with an older experiment it is sometimes a bit mysterious how and why things work – or don't work,” says David with a laugh. But being closer to physics was ultimately the deciding factor in his decision to pursue his doctorate on a more advanced setup. Apart from the technical aspects, David's time in Cambridge was also very enriching. He was particularly impressed by the city's atmosphere and sense of community: “You get the feeling that the whole city is a university.” The college system reinforces this sense of community as well, says David, as you spend your time with a diverse group of people, whether in the dining hall or during extracurricular activities.

Apart from playing the drums, David's favorite hobby, he spent his free time in Cambridge mainly dancing Ballroom and Latin. At first, he only participated in the “more relaxed practice sessions”, as he describes them. Just as he had already done in Munich. But then others from the practice sessions convinced him to join the university's dancesport team, with whom he then went to numerous competitions. Rehearsing choreographies and performing them was a cool experience, says David. What he remembers most is the England-wide tournament in Blackpool, where all English universities compete: “Being able to dance our choreography in front of so many people in this beautiful, large hall – that was very surreal.” It was an experience that showed him he would love to do it again. That’s why he is now attending a dance club in Munich that provides the necessary training.

While still in England, David applied for both, a Munich Quantum Valley doctoral fellowship and a doctoral fellowship for Cambridge. In the end, he was accepted for both. “The decision was really, really difficult,” says the doctoral student. There are so many aspects to consider: the physical topic and research environment, friends, how much you like the city, he names a few examples. “I initially created a big Excel spreadsheet to work it out,” David says, laughing. However, he discarded the Excel sheet and ultimately reduced his decision criteria to the physics he wanted to do and the opportunities offered by the lab. “And as much as I liked Cambridge – the opportunities you have here, the variety of labs, and also the networking. I think it's a once-in-a-lifetime opportunity to do a Ph.D. here.”

In his lab at the MPQ, doctoral students typically begin with a technical contribution to the experimental setup. This also helps to get to know the experiment better, says David. Then, they move on to physical projects. Again, a decision that is not easy for the young physicist. Besides the work on disordered systems, there are many other exciting projects in his lab that interest him. But no matter which physical question David will ultimately tackle in his Ph.D. project, he will not have to give up the enthusiasm that research with ultracold atoms always evokes in him: “The fact that it is possible to really see the individual atoms in our experiment as they sit in a standing wave of light is still incredibly fascinating to me. I don't know if this will become something ordinary one day – I almost hope not!”
 

Published 30 Mai 2025; Interview 24 April 2025