The sound of quanta

Anja Metelmann is a physicist and quantum computing expert. She teaches and conducts research as the first bridge professor at the Karlsruhe Institute of Technology (KIT) and the University of Strasbourg. Bridge professorships are cross-border professorships at the European Campus - the network of the universities of Basel, Freiburg, Mulhouse, and Strasbourg as well as the KIT - and are unique in this form in Europe. At the Institute for Condensed Matter Theory at KIT, Metelmann conducts research on artificial quantum systems; in Strasbourg, she works at the newly founded European Quantum Center.

Dr. Metelmann, as a bridge professor, you teach and do research at two different locations: At KIT in Karlsruhe and at the University of Strasbourg. Is this constant change between two countries a big adjustment for you?

Not really, because science generally only works across borders. And especially my field of research, quantum physics, is very international. I have been exchanging ideas with colleagues all over the world for years, have done research in the USA and Canada, and meet experts at conferences or for research projects. Nevertheless, this intensive collaboration with Strasbourg is something special.

Why is that?

Because the European Quantum Center, where quantum science is taught and researched, opened there in October 2023. There, international scientists will work closely together to pursue new and exciting research directions in quantum physics. I try to be there at least one day a week because this exchange of ideas is very inspiring for my research, and also because my colleagues in Strasbourg are researching a promising quantum system that is different from ours in Karlsruhe. There they are using natural atoms as a platform for processing quantum information.

What does that mean?

The basic unit of quantum information is the qubit, the quantum mechanical variant of the classical bit. The energy levels of atoms can be used to realize such qubits. Of interest here are the Rydberg atoms, which have proven to be relatively stable and long-lived. They could therefore offer a promising approach to the development of quantum computers, i.e., computers for extremely complex calculations. However, it is not yet possible to say whether the Rydberg atoms will actually be used in quantum computers, since different concepts are possible.

At KIT, for example, artificial atoms are being researched.

Yes, exactly. From a physical point of view, these are not atoms, but cleverly assembled superconducting circuits consisting of a large number of atoms. At very low temperatures, however, they behave very much like atoms, which is why we sometimes call them artificial atoms. As a theorist, I develop concepts and protocols for assembling them into quantum systems, which are also comparatively easy to control. The bridge professorship gives me an insight into two different, promising systems and allows me to check which models are transferable. Another focus is the selection and signal processing of such quantum systems, for example, we want to develop extremely low-noise amplifiers: I am leading a new EU-funded research project in this area.

What do amplifiers do in quantum technology?

They read qubits with an extremely weak microwave signal to minimize the feedback effect on the quantum system. However, current realizations of amplifiers used for this purpose do not yet achieve all the required technical characteristics. Together with researchers from France, Sweden and Austria, we want to design a new generation of quantum amplifiers. Many other fields of research are also working with low-noise microwave amplifiers: Some teams are using them to search for dark matter, others want to use them in medicine. This is important basic research that could improve our everyday lives in the future, whether through faster Internet, more accurate medical diagnoses, or eavesdropping-resistant communications.

To most people, however, quantum physics sounds very abstract.

And rightly so. That's why we try to make it at least a little bit tangible: At the Berlin Science Week, we collaborated with sound artists to create a performance that uses electronic music to bring the concepts of quantum physics to the general public.

A concert about quantum physics?

Yes, indeed. The compositions performed were inspired by the sounds of quantum labs, experimental data transformed into sound, and quantum physics concepts. For example, data from quantum jumps and the sounds of refrigerator valves were transformed into sounds, and the sound of the chalk I use to write my concepts on the blackboard during lectures was captured.

Sounds crazy!How did the audience react?

Most of the audience was fascinated, even though they might not otherwise have any contact with the subject. Even after the concert, they probably won't be able to explain exactly how quantum research works. But they got an idea of what aspects are important, of the work, and the thinking behind it. This experience was very enriching for me.