Born in Vorarlberg, a state in western Austria, he grew up in Lustenau, just 200 meters from the Swiss border, and therefore speaks both Vorarlberg dialect and Swiss German – "or whatever you speak there," Stefan adds with a barely perceptible smile. He went to ETH Zurich to study physics. The decision for the renowned university was initially a rather pragmatic one: it was the closest university geographically, Stefan says. "It was a welcome side effect that the university is also pretty good".

Coming from a family of linguists and literary scholars, he was a little out of line with physics, the 31-year-old says. "When I started studying, I thought I was interested in astronomy, but I quickly realized that wasn't my thing." In the end, it was quantum information theory that he found most interesting, especially the mathematical framework behind it. For his Ph.D., he moved from Zurich to Munich to the chair of mathematical physics at the Technical University. The problem he dealt with in his dissertation, namely how much information can be transmitted via a photonic channel, was motivated by physics. However, the main work – "It has its appeal to play around with functions and inequalities" – was relatively detached from the original question.

Learning how to manage big computers

After completing his Ph.D., the scientist initially took a position at the LRZ that was not directly related to his previous studies. "I did everything that had to do with server operation," says Stefan. At the time, there were no postdoctoral positions available in the research groups which he would have found interesting to continue his research, and he had always been interested in working with computers: "Since I was 15 years old, I have only used Linux on my personal computers." Before he seriously considered returning to research, it was already clear that there would be a quantum department at the LRZ. "And in the 1.5 years in between, it didn't hurt to learn how to manage big computers.“

With the SuperMUC-NG, a particularly big computer has to be managed at the LRZ. The rows of black server racks with the many interconnected computing units, the blinking LEDs and the roaring cooling system require their own floor in the so-called computing cube of the LRZ. Quantum computers, on the other hand, are still small – not necessarily in terms of spatial dimensions, but in terms of the number of qubits. "To my friends I always say that I play with big and exotic computers," Stefan says. Integrating quantum computers into classical supercomputers is another goal of the quantum department at the LRZ. "There are problems that we assume can be broken down in such a way that a large part can be calculated on a supercomputer and only a small part is outsourced to a quantum computer," the mathematician explains. There is therefore hope that for certain problems something can already be achieved with NISQ computers – that is, small and error-prone quantum computers – by using them as accelerators in classical supercomputers.

Although he is only 31 years old, Stefan is one of the old hands in his department. "I was the first member of the quantum group at the LRZ," says the scientist. It was only later that the group became an independent department. Stefan now has more than 30 colleagues working with him to realize the service “quantum computer” at the LRZ. He is a reliable contact person for his team – even when it comes to shedding light on the administrative jungle: "Since I've been working here for almost five years, I've already seen through the thicket of bureaucracy a bit, which is why the others often come to me for such things." When asked whether this bothers him, he shrugs: "That’s ok."

Karate and cubes

Stefan is also an old hand at his karate club, where he has been a member since he started at the age of six. He recently celebrated his 25th anniversary as a member. "Silver wedding, so to speak," he adds. As a teenager, he trained with the Austrian national youth team; today, he is involved in the club's management and is a coach himself. Every weekend, he travels from Munich to his old hometown of Lustenau to train a group of adults. He was lucky that it was the first sport he tried, says Stefan – "and I kept it”. What he likes about it: "The fact that karate is so versatile and there is something for all kinds of people.” During his studies he took a longer break from training. During this time, the time he had previously devoted to karate was given over to another hobby, which Stefan took no less seriously: "I participated in Rubik's Cube competitions.” Rubik's Cube is a kind of  cube-shaped puzzle in which all pieces of the same color must end up on one side of the cube. Solving the Rubik's Cube faster than his competitors filled his weekends back then. "I traveled a lot and even flew overseas to world championships," says Stefan, who at one point was ranked ninth in the world. Since he started doing karate every weekend again, he no longer actively participates in competitions. But of course he still solves Rubik's Cubes.

After a weekend dedicated to his personal hobbies, his head is free again to focus on his work at the LRZ. In the future, Stefan will spend more time advising scientists on which computing resources, such as which specific quantum computer platform, make the most sense for their particular project. What he particularly likes about this, he says, is the opportunity to see what is happening in his old field of research. "But not just in my sub-subfield, but more broadly." In Munich, also thanks to Munich Quantum Valley, there is now a whole range of groups working in the field of quantum information processing, Stefan notes, and one can exchange ideas with researchers and industry representatives from a wide range of backgrounds and specializations. "I like to keep an overview and see what is being researched in the various sub-areas."

For now, Stefan will stay with his Q-Crew, as they call themselves at the LRZ. "A lot is happening in this field at the moment, and a lot is happening quickly," says the mathematician. In a few years, we'll see if and how the field consolidates, and then we'll look further – "but it's not boring yet.”

Published 28 June 2024; Interview 30 April 2024