Julian Wienand: Chaotic quantum systems under the microscope

The physicist completed his PhD under the supervision of Monika Aidelsburger.

In his dissertation, Julian Wienand studied chaotic quantum systems and how to descibe them. His work demonstrated for the first time that such systems can be understood macroscopically as diffusion processes. The model of fluctuating hydrodynamics (FHD), which simplifies the description of complex systems, is therefore also applicable to quantum processes. The results could significantly simplify the analysis of quantum systems and open new perspectives for studying complex quantum phases.

To describe the flow behaviour of water, it is not necessary to know the movement of each individual water molecule – macroscopic flow equations are sufficient. However, when observing small particles in water, they not only show the expected flow motion but also erratic, jerky movements: the so-called Brownian molecular motion. These fluctuations can be described as white noise, turning “hydrodynamics” into “fluctuating hydrodynamics” (FHD). The FHD theory allows the entire behaviour of a system under certain conditions to be determined by a single number: the diffusion constant.

Whether this approach also works for chaotic quantum systems was one of the topics of Julian Wienand's dissertation. Using a quantum microscope, he observed the behaviour of a chaotic quantum many-body system with ultracold atoms in optical lattices. The high-resolution imaging allowed him to measure not only the average density of the atoms but also their fluctuations. By letting the system evolve from an unfavourable initial state towards thermal equilibrium, he was able to track how density correlations and fluctuations developed over time. The results show that FHD describes the system both qualitatively and quantitatively. This suggests that even chaotic quantum systems can be surprisingly simple to understand macroscopically as diffusion processes – much like Brownian motion.

Julian Wienand started in 2018 as a master's student at LMU and continued his PhD there after a research internship at Stanford University. A highlight of his time was the 2023 Summer Academy of the German National Academic Foundation in Leysin, which he co-designed and led as a lecturer. “It was a fantastic feeling to share our field of research and conduct experiments with the participants.”

What’s next?

After successfully defending his dissertation, Julian Wienand will continue his research for a while before moving on to new challenges – possibly as a trainee at the EU Commission: “The PhD has shown me how diverse laboratory work can be. But I also see exciting opportunities outside of academic research,” says Julian Wienand.