Research project Locality vs. topology in quantum matter

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)

This year's summer school runs from 9 to 22 June at Högberga konferensgård on Lidingö outside Stockholm. About fifty participants will come there to deepen their knowledge. It is a summer school organized for PhD students and postdocs, both theoretical and experimental, in all aspects of quantum limits. Nobel Laureate Frank Wilczek, Professor at Fysikum and Professor Antti Niemi, Nordita are the initiators of the summer school. Since 2016, Quantum Connections Workshops and Summer Schools have been organised on the initiative of Frank Wilczek in collaboration with Antti Niemi from Nordita. Both have made the Summer School what it is today - a well-organised activity for national and international students involved in quantum mechanics. The Quantum Connections event is jointly organised by Fysikum and Nordita (with Stockholm University, KTH Royal Institute of Technology and Uppsala University as hosts), together with the TD Lee Institute and the Wilczek Quantum Center at Shanghai Jiao Tong University.   Professor Frank Wilczek's position at the Department of Physics extended until 2030 In 2004, Frank Wilczek, who since 2016 has also worked at Fysikum, received the highest honour in science - the Nobel Prize in Physics - for his discovery of asymptotic freedom and the development of the theory of quantum chromodynamics (QCD). "Last year, we celebrated 50 years of Quantum Chromodynamics (QCD) at the Quantum Connections Summer School. It was also 50 years since we came up with the theory that gave us the Nobel Prize and my wife and I celebrated 50 years of marriage. 2023 Summer School therefore featured several Nobel Laureates giving lectures. We also organised a separate Nobel symposium on anyons. Unlike ordinary particles, which are categorised as fermions or bosons, anyons can exhibit statistical properties that lie between the two", says Frank Wilczek. Every year, around 50 students participate in our Quantum Connections Summer School, selected from around 250 applications. During the lab day, the students get an opportunity to get to know research at the Physics Centre.  This year they have also been able to see each other's areas of activity through a poster exhibition. "Three of my students from MIT (Massachusetts Institute of Technology) are attending this year's summer school. On the research side, there are new opportunities to observe the behaviour of quantum particles in space and time through quantum mechanics and quantum computing. My appointment as a professor at the Department of Physics has been extended until 2030, and I also plan to publish a new book in the spring of 2025," says Frank Wilczek.

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.