Optimal transport (OT) is an ubiquitous optimization problem in mathematics, machine learning, and natural sciences since it induces a family of geometrically intuitive, robust distances on the space of probability distributions. It is relevant for modern physics for two main reasons: First, many data samples can be represented as (probability) distributions on suitable feature spaces, such as histograms, point clouds, image densities, et cetera. In a more abstract way, whole datasets can be interpreted as distributions on sample space. OT distances provide the natural language for comparing and interpolating between data, for instance in the context of generative modelling. Second, OT directly emerges in physical modelling or approximations, for instance in density functional theory or in statistical physics in the form of the Schrödinger bridge problem. In this talk we give a gentle introduction to optimal transport, the induced distances, and its applications to physics.
Biological membranes undergo dramatic shape transformations during vesicle release, organelle division, and intracellular trafficking. At the microscopic level these events require a change in membrane topology: the transformation of one continuous membrane into two separate ones. Such transitions occur on nanometre length scales where lipids must rearrange into highly curved, non-bilayer structures while still preserving the membrane’s barrier function.
In cells, specialized proteins such as dynamin catalyze these topological transitions. In this talk I will show how theoretical modelling and simulations can reveal the mechanisms and the control knobs that make them fast, reliable, and sometimes leaky. Using self-consistent field theory, coarse-grained molecular simulations, and phenomenological models, we map the energy landscapes governing membrane fusion and fission and identify the transient intermediates that dominate them, including hemifusion diaphragms, stalks, rim pores, and worm-like micelles.
A central message is that small changes in membrane composition or membrane–protein interactions can strongly reshape the energy landscape, producing large changes in the free-energy barriers and therefore in the rates and outcomes of membrane division. I will also discuss how multiple-membrane geometries, such as mitochondria and endoplasmic-reticulum contact sites, open additional pathways to membrane fission through transient inter-membrane fusion events.
My lab exploits optical resonances — from thin-film cavities to microscopic lasers inside living cells — to build new tools for science and medicine. Here I present three recent developments.
First, by monitoring resonance shifts of deformable optical micro-cavities and microscopic lasers, we resolve cellular forces in the picoNewton range and record contraction-induced refractive index changes down to 10⁻⁵ refractive index units, providing a new window onto the mechanical activity of cells and tissue.
Second, I will show how we have optimised OLED (organic light-emitting diode) technology for biomedical implants. OLEDs — the technology of choice for modern smartphones and TVs — can be integrated on a much wider range of substrates than conventional LEDs, making them well suited for neurostimulation and optogenetics, that is light-based control of genetically engineered neurons).
Finally, by driving the interaction between molecular excited states and optical cavity modes into the strong coupling regime, we obtain a new set of parameters for cavity tuning. We exploit these to develop thin-film optical filters and LEDs with angle-independent emission spectra — a key requirement for example for compact fluorescence-based sensing devices.
Warum fällt vielen Studienanfänger*innen der Einstieg in die Physik so schwer – und wie lassen sich ihre Lernprozesse besser verstehen und gezielter unterstützen? Trotz bekannter Hürden in der Studieneingangsphase wird die universitäre Physiklehre in der deutschsprachigen fachdidaktischen Forschung bislang nur vereinzelt untersucht. Der Vortrag stellt ein Forschungsprogramm vor, das diese Lücke systematisch schließt. Im Mittelpunkt stehen Studien zum Verständnis physikalischer Repräsentationen – etwa von Vektorfeldern und vektoriellen Differentialoperatoren – sowie zu typischen Lernschwierigkeiten im Umgang mit diesen Konzepten. Eye-Tracking-Experimente erlauben dabei Einblicke in visuelle Aufmerksamkeitsmuster, Denkprozesse und Fehlvorstellungen. Die Ergebnisse fließen in die Entwicklung neuer, vorlesungsbegleitender Aufgabenformate ein, die in der Lehre erprobt und nach dem Prinzip der evidenzbasierten Medizin evaluiert werden. So entsteht ein enger Forschungs-Lehr-Zirkel, der zeigt: Hochschuldidaktik Physik ist kein Randthema, sondern ein Schlüssel zur Weiterentwicklung universitärer Lehre – vorausgesetzt, sie bekommt den Raum, den sie verdient.
Superconducting qubits have emerged as a leading platform for quantum computing, offering scalability and excellent coherence properties. In this colloquium, I will discuss the fabrication techniques employed to realize quantum processors based on superconducting qubits, highlighting advances in materials towards highest qubit coherence and integration strategies towards large-scale processors. I will illustrate the building blocks of a quantum processor using a 17-qubit system, which we are currently operating in our laboratory. Furthermore, I will explore how this quantum processor can be leveraged for quantum applications, particularly the efficient generation of entangled states based on well-controllable simultaneous couplings between two or more qubits.
The rise of two-dimensional (2D) materials has ignited a revolution in modern physics. These atomic sheets offer a unique playground for studying transport, optics, and magnetism, where properties can be stacked, strained, and twisted to create entirely new materials not found in nature. By relying on weak but precise van der Waals forces to hold these layers together, we can now engineer electronic behavior at the smallest possible scale. In this talk, I will explore the frontier of two-dimensional Spintronics, a field that aims to transcend the "power wall" of traditional electronics. Instead of simply moving electrical charges, spintronics harnesses the electron's spin to process and store information, promising devices that are faster and more energy-efficient than today’s silicon chips. I will discuss how one can "indoctrinate" a material like graphene with magnetic or spin-orbit properties through the spin proximity effects, where the wavefunction hybridizes with a distinct neighbor to inherit new physical identities. I will present recent experimental and theoretical advances in this spin van der Waals engineering, demonstrating how tunable interfacial interactions can lead to both Hamiltonian engineering to discover fundamental phenomena, but also potential applications.
Topological pumps provide a powerful method for transporting particles with remarkable precision by slowly and cyclically modulating a lattice potential. This transport is topologically protected - a feature it shares with the quantum Hall effect - making it inherently robust against noise and experimental imperfections.
In this talk, I will introduce a novel paradigm for this concept: moving beyond the transport of individual particles to the pumping of qubits carrying quantum information. Our experiments, which employ ultracold fermions in dynamical optical lattices [1,2], demonstrate the coherent transport of not only single atoms but also entangled atom pairs over hundreds of lattice sites. This capability allows us to perform fundamental quantum computations during transport, including high-fidelity two-qubit gates. I will show how we can chain these operations together to build non-local quantum circuits and to assemble entangled states for quantum computation and quantum simulation purposes.
[1] Zhu et al. PRX (2025) “Splitting and connecting singlets in atomic quantum circuits”
[2] Kiefer et al. Nature (2026) “Protected quantum gates using qubit doublons in dynamical optical lattices”
When I first studied physics in Göttingen in the mid-1990s, the idea that I would one day spend my career watching zebrafish hearts beat and marine worms develop would have seemed improbable at best. Yet the tools that physics provides — optics, engineering, quantitative thinking — turn out to be exactly what modern biology needs most. In this talk, I argue that the most important contribution a physicist can make to the life sciences is not a new theory, but a new instrument — and a new way of sharing it. Just as telescopes in astronomy and accelerators in particle physics have become community resources that have transformed entire fields, the microscope must evolve from a personal laboratory tool into a shared, open infrastructure. This is the idea behind Flamingo: a modular, open light-sheet fluorescence microscope designed to travel between labs, to be built and modified by its users, and to bring state-of-the-art imaging to biologists who would otherwise never have access to it. I will show what becomes possible when physicists and biologists work in genuine exchange — from imaging the developing embryo across scales of space and time, to uncovering the mechanics of the beating heart. The microscope, it turns out, is not just an instrument. It is an argument about how science should be done.
When the interaction with a polar lattice is strong, charge carriers in semiconductors create their own potential well by polarizing the crystal. This formation process of so-called polarons was first described by Pekar in 1946 and later on more quantitatively by Fröhlich. This birth of a polaron has now been directly measured in time-resolved photoemission electron microscopy (TR-PEEM) experiments performed in order to understand the unique photocatalytic properties of BiOI nanoplatelets. I will highlight further examples of colloidal nanocrystals, where we have found interesting functional properties related to unique solid state physics concepts.
21cm cosmology presents a unique opportunity to map the early Universe, bridging the vast observational gap between the Cosmic Microwave Background (CMB) last scattering surface and the high-redshift galaxies observed by JWST and ALMA. Produced by neutral hydrogen atoms in the intergalactic medium, the 21cm signal is highly sensitive to radiation from early sources of light, offering a pristine window into the signatures of metal-free Population III stars, early stellar remnants, and accreting black holes. In this talk, I will review the latest theoretical developments, existing observational constraints, and prospects for navigating this faint cosmic frontier. The formation of the first stellar generations and the subsequent emergence of X-ray binaries drove a fundamental phase transition in the early Universe. Due to immense observational challenges, the precise physical properties of these primordial light sources remain highly uncertain. The cosmological 21cm signal contains unique information about these first generations of UV and X-ray sources, as well as their feedback on the surrounding intergalactic environment. Currently, state-of-the-art radio telescopes are establishing increasingly informative upper limits, placing new constraints on early star formation and X-ray source properties. These observations are synergistic with direct information on high-redshift galaxies obtained by the JWST. In my talk, I will discuss multi-wavelength synergies which, alongside observations with the upcoming Square Kilometre Array (SKA), will open a transformative window into the cosmic dawn. This combined approach will not only allow us to characterize the properties of Pop III stars and early galaxies, but will also enable novel tests of standard cosmology and dark matter physics across an entirely unexplored redshift regime.
Ionic interactions provide a powerful and tunable means to direct polymer phase behavior, with applications in solid-state batteries and polymer compatibilization. This talk explores two key systems—neutral-charged block copolymers and ion-functionalized polymer blends—using an electrostatic fluctuation-augmented self-consistent field theory. For AB block copolymers with partially charged A-blocks, we demonstrate that ion correlations induce a "chimney-like" phase diagram, but dielectric contrast between blocks weakens the "chimney-like" feature. For sparse charge distributions, ion clustering, lead to hierarchical nanostructures (e.g., alternating layers, concentric cylindrical shells, and spherical shells) in lamellar, cylindrical, and spherical phases. These findings are in general agreement with literature data on neutral–charged diblock copolymers and salt-dope diblock copolymers. In the second part, we examine end-charged diblock copolymers and polymer blends. The strong curvature preference of the ion clusters along with the large electrostatic correlation results in stable frustrated P phase and single-gyroid phase. For polymer blends with opposite end charges, strong ion correlations effectively link the polymers, inducing phase behavior resembling that of neutral block copolymers. However, the disordered phase is not uniform but rather consists of spatially organized ion clusters. Consequently, the order-disorder transition occurs at a significantly lower critical χN than in neutral systems. These findings highlight the critical role of ionic interactions in tailoring polymer self-assembly for advanced materials.
Photo-active (bio-)materials are usually characterised by molecular photochemical processes originating from elementary steps, such as intra- and intermolecular electron- and proton-transfer processes in photocatalysis, or in photochromic reactions involving cis-trans isomerization, hemolytic bond cleavage or cyclization reaction with the formation or break of a bond. These reactions happen first in the excited state at the sub-picosecond to nanosecond timescale followed by the formation of different photo-products in the ground state that relax and also evolve in the micro-millisecond timescale to the final photo-active functional product. Revealing crucial species that control the photo-activity requires the ability to probe dynamics over multiple timescales, ranging from hundreds of femtoseconds to seconds. Historically, short and long timescales have been addressed using different experimental techniques, detrimental to a unified description of the entire multiple timescale dynamics, as it often leads to different excitation conditions in terms of lights characteristics and sample environment. Fortunately, this situation has evolved significantly over the last two decades due to the advent of new high frequency femtosecond Yb laser and development of multiple timescale spectroscopy. I will discuss here the potential of this new method to decipher the photomechanism of photo-active bio-molecules.
