Researchers at the Würzburg site of the Cluster of Excellence ctd.qmat have successfully transferred the topological quantum Hall and spin Hall effects to a hybrid light-matter system using targeted material design.
Back in 1980, Nobel laureate Klaus von Klitzing, then working in Würzburg, first demonstrated topological charge transport with the quantum Hall effect. In 2006, Professor Laurens Molenkamp at Julius-Maximilians-Universität Würzburg (JMU) provided the world's first experimental evidence of the quantum spin Hall effect as an intrinsic property of a topological insulator. Both phenomena protect electrons from scattering.
Now, Sebastian Klembt from the Chair of Applied Physics, a Principal Investigator at the Würzburg-Dresden Cluster of Excellence ctd.qmat - Complexity, Topology and Dynamics in Quantum Matter and recently appointed Professor of Experimental Physics I at JMU, has transferred these effects to a hybrid quantum material together with an international team. To achieve this, the researchers used polaritons - a hybrid of light (photons) and matter (excitons). These form in 'micropillars' - tiny semiconductor structures in which light and matter interact strongly.
The experiments were carried out at the Chair of Applied Physics at JMU led by Simon Widmann. The theoretical framework was developed in collaboration with Ronny Thomale - also a Principal Investigator at ctd.qmat and Professor of Theoretical Physics I - as well as researchers from Nanyang Technological University, Singapore.
The Breakthrough: Pseudospin Through Targeted Material Design
"Our microstructures are much smaller than the diameter of a human hair. We engineered them in the cleanroom to give our laser light unique properties. The topological light transport we demonstrated - and the underlying effect - open new possibilities for topological polariton lasers and optical information processing," explains Sebastian Klembt.
The Würzburg researchers engineered gallium arsenide (GaAs) into a chain of elliptically shaped micropillars. When laser light hits the sample, photons interact with excitons to form hybrid polaritons. Mirror layers confine these particles within the micropillars, where they behave like electrons in topological transport: "The elliptical shape of the micropillars and the angles at which they are coupled generate what is known as an artificial gauge field. Much like a magnetic field acting on electrons, this gauge field determines the behavior of our polaritons," Klembt adds.
In this hybrid material system, the geometry causes light to become either left- or right-circularly polarized - meaning the electric field rotates clockwise or counterclockwise. These two polarizations propagate along opposite paths, forming an optical analogue of the quantum spin Hall effect. "The circular polarization of light acts as a pseudospin," says Klembt.
Hybrid Light-Matter Particles as a Key to New Technologies
The findings, published in Nature Communications, open up new possibilities for applications such as topological polariton lasers, spin-based transistors, and optical information processing. In this context, the polarization of light can also serve as an information carrier.
Publication
Artificial gauge fields and dimensions in a polariton hofstadter ladder. Simon Widmann, Jonas Bellmann, Johannes Düreth, Siddhartha Dam, Christian G. Mayer, Philipp Gagel, Simon Betzold, Monika Emmerling, Subhaskar Mandal, Rimi Banerjee, Timothy C. H. Liew, Ronny Thomale, Sven Höfling & Sebastian Klembt, Nature Communications 17, 1586 (2026), DOI: 10.1038/s41467-026-68530-0
The Cluster of Excellence ctd.qmat
The Cluster of Excellence ctd.qmat - Complexity, Topology and Dynamics in Quantum Matter - at Julius-Maximilians-Universität Würzburg and Technische Universität Dresden explores and develops novel quantum materials with tailored properties. Around 300 researchers from over 30 countries work at the interface of physics, chemistry, and materials science to lay the foundations for tomorrow's technologies. In 2026, the cluster entered the second funding period of the German Excellence Strategy of the Federal and State Governments - with an expanded focus on the dynamics of quantum processes.
