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Fabian Grusdt, one of the first MCQST START Fellows, has received the prestigious Starting Grant from the European Research Council (ERC). Science journalist Hubert Filser has interviewed Fabian on his passion for physics, career, and future plans. *

### Fabian Grusdt: I’ve just always been passionate about this one thing, about physics.

Fabian Grusdt doesn’t waste any time. He swipes a couple times on the screen of his tablet and shows what he is working on right now. You can see pages of formulas, mathematical symbols and brackets, sequences of vectors, operators and calculations that look complicated to a layperson. It is a crash course in theoretical quantum physics. Grusdt laughs as if to demonstrate that he understands it is not easy for a layperson to grasp the material he has shown you. The formulas and descriptions reflect a rapidly developing, new field of research: it involves the very complex behavior of highly correlated quantum systems and how you can understand this by using novel quantum simulators. In the meantime, Grusdt records his theories solely on the tablet. In the past – and that cannot be all that long ago for a 30-year-old – he wrote down his calculations on wave functions with pencil and paper, he says.

Now the quantum physicist has received the prestigious Starting Grant from the European Research Council (ERC). He will also implement his project “SimUcQuam” (Simulating Ultracold Correlated Quantum Matter: New Microscopic Paradigms) as part of the Munich Center for Quantum Science and Technology (MCQST), the Munich-based cluster of excellence. Specifically, Grusdt will be searching for fingerprints of the microscopic constituents of strongly interacting electrons in complex quantum systems. In the long run, it may be possible to understand even fundamental phenomena such as high temperature superconductivity, a field in which many Nobel prizes have already been awarded. “Their microscopic origin is only insufficiently understood,” says Grusdt.

These are big goals for a young physicist. His career reads like the biography of a born genius. In school, Grusdt skipped the 11th grade. While preparing for the Abitur (German higher education entrance qualification) at the Tegernsee Gymnasium, he was at the same time remotely studying physics and mathematics at the TU Kaiserslautern. As a young physicist, he has already three science and three nature publications, his colleagues often citing his work. He does not think much of those who call him a high-flyer. He shakes his head and says: “I’m not a genius. I’ve just always been passionate about this one thing, about physics.” That is a difference, according to him.

And indeed, Fabian Grusdt can speak about physics for hours. He talks about his enthusiasm, which took shape in his youth, when he was learning astronomy or the “physics of putting out a candle and its oscillations” or about the first tentative experiments as an undergraduate and then as a doctoral candidate in Kaiserslautern, and finally later as a postdoctoral student at the renowned Harvard University.

##### “With the best undergraduates, graduates and post-doctoral students, you can implement your ideas more quickly.”

And he has many ideas. They revolve around the inner structure of complex quantum systems. “Just as the discovery of atoms and their inner structure could explain the diversity of our matter to a large extent, I now want to find previously unknown parts in the quantum systems and explain their behavior.” This involves fairly abstract constructs which nobody has observed before, but which may possibly be proven with new quantum simulators of ultracold atoms, as are being built at the Munich Cluster. Atoms are locked into crystal lattices of laser beams and cooled to temperatures just above absolute zero. Model systems are created by using complicated experimental arrangements and allow researchers to take precise measurements, for example, mimicking solid-state systems and investigate theoretical phenomena such as high temperature superconductivity.

When you listen to Grusdt, such theoretical considerations quickly become less abstract. He can easily explain complex connections, calmly listen to questions and never loses sight of the big picture in his responses. He is not a physics nerd who gets caught up in the minutia of his expertise. Rather, you sense a person who wants to work on big things. There is this story from Harvard when he presented his most important basic idea at that point. It involved a hole in a high temperature superconductor whose theoretical foundations are still not understood. Grusdt used work from the 1960s to formulate an idea that a moving hole leaves a trace on its way through an antiferromagnet, a kind of memory. Spins of particles are reversed. Grusdt calls this memory path a string. At one end of the path is the hole; at the other end is a kind of spin excitation with a large effective mass; a spinon, as the quantum physicist calls it. Both are connected via the string, like a dog on a leash to its owner, Grusdt explains. “All in all, this is obviously about collective effects.”

##### One of those Eureka moments

It was a theoretical idea at the time of the lecture, formulated in terms of the operators mentioned in the introduction. However, his colleagues in the group led by Harvard professor Eugene Demler were curious. This was exactly what Grusdt’s post-doctoral colleague Daniel Greif had proven in quantum simulators, those new systems that are also now being used in Munich. It was one of those eureka moments that are often written about. “It was very special,” says Grusdt. “As a researcher, you always dream that theoretical predictions will coincide with reality.”

That is what he plans to continue working on in Munich with his ERC grant. “We have ideal conditions here, an ideal triad of theory, numerics and quantum simulators,” says Grusdt. The Munich cluster offers excellent opportunities for exactly this. Grusdt will be collaborating with experimental groups at the LMU and improve his theories by using numerical simulations. “There is a huge number of possibilities in Munich now,” says Grusdt. “Not only can we make specific theoretical predictions for the highly complex behavior of quantum systems and do numeric approximations, but we can also check the proposed structures in experiments. That is fantastic.” His idea of strings was only the first step, according to him. Now he wants to study how and when they break up again and what happens when several strings come together. “Maybe this will lead to the big explanation of high temperature superconductivity, who knows.”

On this long journey, he explains, it is also important to have an atmosphere like the one in Munich – a feeling of community. “Expertise is not the only factor; people actually grow closer here,” says Grusdt. You sense the tremendous persuasion of physicists, from colleagues such as Immanuel Bloch or Ulrich Schollwöck. “It is important that young physicists grasp the big picture,” says Grusdt. He would also like to share this enthusiasm with his students. Fabian Grusdt is 30 years old, not a born genius, but a researcher who is fascinated by his field and does not waste any time. And those are not the worst conditions for new discoveries.