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What is warm dense matter? Why is it worth researching it and what results will it bring? Read an interview with 2024 Lem Prize winner  Tobias Dornheim, PhD from the Helmholtz-Zentrum Dresden-Rossendorf.

Michał Ciepielski: Can you tell us more about the research you are involved in?

Tobias Dornheim, PhD:  Within my group, we develop and utilize advanced computational methods for the description of quantum many-body systems. Our main focus is the accurate description of so-called warm dense matter. It is characterized by high temperatures, densities and pressures and abounds in a great variety of astrophysical objects including giant planet interiors, brown dwarfs and the outer layer of neutron stars. In addition, warm dense matter plays an important role in technological applications, most notably material science and inertial fusion energy (i.e., laserfusion).

A particular feature of warm dense matter is the complex interplay of effects such as Coulomb attraction and repulsion between the electrons and nuclei, partial ionization, quantum degeneracy and strong thermal excitations. The holistic treatment of this state is notoriously challenging even for state-of-the-art methods deriving material properties exclusively from the fundamental equations of quantum mechanicst (so-called ab-initio methods).

This unresolved challenge sparks the need for the development of new methodologies.

You received the Lem Prize 2024 for your work on warm dense matter and high energy density physics. What are the characteristics of this research and what is its purpose?

Due to its importance for the modeling of astrophysical objects and technological applications alike, warm dense matter is nowadays routinely created in large research facilities in Europe (e.g., the European XFEL in Germany), North America (e.g., the National Ignition Facility in Livermore, California, USA) and East Asia (e.g., SACLA in Japan).

Here, a key challenge is given by the rigorous diagnostics, which is rendered difficult not only by the extreme conditions, but also by the ultrafast time-scales (~ns-fs) of corresponding experiments; indeed, even basic parameters such as the temperature could not have been measured directly, but had been inferred indirectly by matching theoretical models with experimental observations.

As a consequence, the inferred experimental conditions have depended on a number of model assumptions and approximations, making their quality unclear.

As part of my research, I have presented a new framework for the model-free diagnostics of warm dense matter using x-ray Thomson scattering measurements. As the result, we can directly infer key parameters such as the temperature from the experiment without the need for any models or approximations.

This is very important for the diagnostics of laserfusion applications, but also laboratory astrophysics and material science.

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In 2022, you were awarded a prestigious ERC Starting Grant. The research conducted under it has the potential to answer many key questions in the field of warm dense hydrogen and other elements. What stage is this project currently at?

During the first two years of the ERC project (3/2023-3/2025), we developed new capabilities for approximation-free path integral Monte Carlo (PIMC) simulations of warm dense matter. In particular, it has become possible to simulate light elements (hydrogen and beryllium have currently been demonstrated) with an unprecedented system size.

These new capabilities have allowed us to present a number of important results, including the first exact results for the density response and exchange—correlation kernel of warm dense hydrogen.

In addition, we have combined the PIMC set-up with our model-free framework for the interpretation of x-ray scattering experiments, which has given important new insights on a spherical implosion experiment conducted at the National Ignition Facility (NIF) in Livermore.

The implications of these findings will be explored further in a dedicated Discovery Science project at the NIF later this year. Starting April 2025, the ERC project has entered its second phase with an additional PhD student and PostDoc joining our group. Specifically, the goal will be to develop a fundamental solution to the most pressing computational bottleneck in PIMC.

This is a challenging high-risk high-gain endeavor, but we are confident that we can make it work.

What do you consider your greatest achievement to date? What are your professional plans for the nearest and more distant future?

Objectively, I consider the application of the „exotic” concept of imaginary-time correlation functions to experimental measurements obtained in the real world---which is at the heart of the model-free diagnostics framework that has been awarded the Lem Prize---as my greatest achievement so far; it certainly has had the highest practical impact, as it has already been adapted at a number of research facilities, and by groups in Europe, the US, and China.

In the near future, I plan to permanently establish my research group as a major contributor to warm dense matter research. In particular, I want to continue to develop new methods, apply these methods extensively to solve current problems, and work closely with relevant experiments.

In the more distant future, I intend to apply our simulation capabilities to a variety of other quantum many-body systems beyond warm dense matter. In this regard, ultracold atoms such as helium come to mind, where our methods can likely help to resolve a number of long-standing problems.

Are you familiar with the literary output of Stanisław Lem? Perhaps you have found some of his works particularly memorable, or useful in your life or academic career?

I have encountered Lem for the first time as a young adult watching a German screen adaption of the Ijon Tichy character, which resonated with me. Since then, I have read two novels by him, which I also liked. I reread „The Invincible” recently, and it is impressive how closely its implications match current debates about potential dangers of artificial intelligence (the infamous „paper clip maximizer” comes to mind).

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Have you been to Poland before or worked with Polish scientists?

Since 2020, I am regularly in Görlitz at the Center for Advanced Systems Understanding (CASUS), which is located a 5-minute walk away from the border with Poland. Consequently, I have many Polish colleagues, and I have been to Poland innumerable times. CASUS has been set up as a German-Polish institute and we have close ties to the University of Wroclaw, which I have also visited a number of times over the last years.

This has led to an ongoing collaboration with Prof. Blaschke from Wroclaw, which has already resulted in published work.

How do you like spending your free time? Does someone who conducts advanced scientific research on a daily basis have time for a hobby?

I genuinely think that taking enough free time from your career is non-optional to preserve the required level of energy and enthusiasm that it takes for academic research. Personally, I am a very passionate reader (so rest assured that a sizable fraction of the prize money will be invested accordingly). In addition, I enjoy hiking and traveling