Research group Quantum and Complex Systems

Researchers from Stockholm University and the Indian Institute of Science Education and Research (IISER) Mohali have reported a practical way to spot one of physics’ strangest predictions: the Unruh effect, which says that an object speeding up (accelerating) would perceive empty space as faintly warm. But, trying to heat something up by accelerating it unimaginably fast is a non-starter in the lab. The team shows how to convert that tiny effect into a clear, timestamped flash of light.

Oscar is the recipient of Magnus Axelsson Prize 2025. "My main goal will be to bring more physics into the problem-solving process so that students can better understand the meaning behind the abstract calculations," says Oscar. He is in charge of the tutorials for the course Advanced Quantum Mechanics. Oscar completed his bachelor's at the University of Barcelona and his master's at Uppsala University. In 2022, he got a PhD position at Fysikum and became part of Professor Emil Bergholtz's research group, Quantum and Complex Systems.

In the rapidly evolving field of quantum materials, theorists at Stockholm University are pushing the frontier of what kinds of exotic matter can exist. Two new works—one just published in Nature Communications and another selected as an Editors’ Suggestion in Physical Review Letters—reveal strikingly unconventional states of electrons that could pave the way for future quantum technologies.

In their latest publication in Nature Communications, researchers Hui Liu, Raul Perea-Causin, and Emil J. Bergholtz demonstrate that Fibonacci parafermions—exotic quasi-particles which have so far evaded experimental realization—can emerge in experimentally accessible moiré structures formed by overlaying two-dimensional materials with a twist.

Moiré superlattices, structures that arise when two layers of two-dimensional (2D) materials are overlaid with a small twist angle, have been the focus of numerous physics studies. This is because they have recently been found to host novel fascinating unobserved physical phenomena and exotic phases of matter. (Phys.org)

Emil Johansson Bergholtz is Professor of Theoretical Physics at Fysikum. He has been appointed both a Wallenberg Academy Fellow and a Wallenberg Academy Scholar. "It was a coincidence that I got into the subject of physics. I liked solving problems and things that gave resistance. But without the research branch, it is not certain that I would have continued with physics. It was fun and inspiring with the advanced problem solving in the program, says Emil Bergholtz.

Maria Zelenayová, PhD student at Fysikum, and her supervisor Professor Emil Bergholtz looked into localization phenomena in non-Hermitian systems that challenge traditional beliefs about states within the energy continuum and those in the gap. The understanding of non-Hermitian extended band gaps and bound states holds significant implications across various domains. In quantum mechanics, this opens up new possibilities for controlling and manipulating the properties of materials. In technology, these insights can be utilized to develop more efficient semiconductors and other advanced materials.

We present a class of systems in which a particle - antiparticle pair cannot annihilate each other after they have moved along a loop, and instead form a new type of composite particle. This occurs in so-called non-Hermitian systems; classical metamaterials or "open" quantum systems that are coupled to the rest of the universe. Lukas Königis a PhD Student at Fysikum and is part of the research group Quantum and Complex Systems.

The fundamental postulates of quantum mechanics rest upon the fact that the Hamiltonian, which quantifies the energy of the system, is given by a Hermitian operator. This means that the complex conjugate and transpose of the Hamiltonian should be equal to itself. Almost a century after the formulation of quantum mechanics, researchers are now trying to break free from this constraint — non-Hermitian Hamiltonians, which do not satisfy the aforementioned property, are now a new frontier of exploration in both theoretical and experimental physics. Paolo Molignini is part of the project and is a Postdoctor in Condensed Matter and Quantum Optics at Fysikum.

Mohamed Bourenanne is part of the National Quantum Communication Infrastructure in Sweden (NQCIS) project, which received SEK 30 million from Vinnova and is coordinated by KTH. His research group works in the field of quantum communication and their laboratory is located at AlbaNova. - All eavesdropping attempts can be revealed thanks to the basic theories of quantum mechanics, thus increasing information security.