16.04.2026          Antrittsvorlesungen | Tobias Heindel und Alexander Mook

Vortragsankündigung Heindel [PDF]

The Coin-toss Check - How cheating can be avoided in the future quantum internet

Prof. Dr. Tobias Heindel, Department für Quantentechnologie

Imagine you want to decide who can use the family e-bike at the next day with the help of a coin toss, but you cannot observe the result of the toss on your own, as you need to communicate via phone. What is so simple on the soccer field, turns out to be extremely challenging at the distance if the communicating parties are far apart and cannot involve a third person as referee. Both parties would be tempted to communicate not the true result, heads or tails, but rather the most advantageous for themselves. But how does this connect to quantum technologies and the future quantum internet?

In my lecture, I will check for you how the tossing of coins in the quantum world can enhance the security in the future quantum internet. In this context I will present the first steps of my research team in the field of quantum cryptography beyond quantum key distribution. Using a quantum light source emitting single photons at the push of a button, we implement a coin flipping experiment in the quantum world and comparatively analyse its performance relative to alternative and classical realizations [1]. On this way we will explore, how single photons can be produced at high speed using experimental quantum technologies, how single photons can be used for ultra secure data encryption, and why this is not enough to realize all functionalities needed in a future quantum internet. Not least, we will learn about the award-winning science communication project QuanTour [2], in which a quantum light source as used in our experiments is travelling across Europe and the world.

[1] D.A. Vajner et. al., Nature Communications 17, 2074 (2026)

[2] R. Sender, The travels of a quantum light source, Optics & Photonics news 26, December 2025

Symmetry Lost, Magnetism Gained: Altermagnetism at Surfaces 

Vortragsankündigung Mook [PDF]

Prof. Dr. Alexander Mook, Institut für Festkörpertheorie

Magnetism is often introduced through familiar examples such as ferromagnets, where all magnetic moments align, or antiferromagnets, where they cancel each other. In recent years, a new and less intuitive form of magnetic order - altermagnetism - has been identified [1]. It combines features of both: although the total magnetization vanishes, the electronic excitations can still distinguish between opposite spin directions, leading to observable effects typically associated with ferromagnetic materials. This makes altermagnets particularly promising for spintronics, where one aims to use the electron’s spin to store and process information - potentially allowing for devices that are both fast and robust without producing stray magnetic fields.

In this talk, I will show that such unconventional magnetism does not only depend on the properties of the bulk material, but can also emerge at its boundaries [2]. Surfaces naturally break some of the symmetries present in the interior of a crystal. As a result, they can fundamentally alter the behavior of electrons - and, as I will demonstrate, even create altermagnetic features in systems that are not altermagnetic in the bulk.

Starting from basic symmetry principles, I will introduce the key ideas behind altermagnetism and explain how surfaces modify them. I will then discuss simple physical pictures as well as theoretical results that illustrate how spin-dependent effects arise at surfaces, and how they could be detected experimentally.

This perspective highlights a broader lesson: by looking at familiar materials in new ways - focusing on symmetry, geometry, and reduced dimensionality - we can uncover unexpected phenomena. In this sense, some of the most interesting physics does not happen deep inside a material, but at its very edges.

[1] Libor Šmejkal, Jairo Sinova, and Tomas Jungwirth, Phys. Rev. X 12, 031042 (2022)

[2] Colin Lange, Rodrigo Jaeschke-Ubiergo, Atasi Chakraborty, Xanthe H. Verbeek, Libor Šmejkal, Jairo Sinova, Alexander Mook, arXiv:2602.08773 (2026)