Physikalisches Kolloquium

I will begin with a historical introduction starting from Michael Faraday’s discovery of the magneto-optical phenomena and the basic physics behind it. Next, I will present the revolution caused by the advent of lasers in magneto-optics studies and the developments which made the nonlinear magneto-optics one of the most precise measurement techniques. While focusing on hot atomic-vapor samples, I will also present some magneto-optic studies with cold, trapped atoms and colour centers in diamonds and their applications to magnetometry.

In der Nacht vom 7. auf den 8. Februar 1922 gelang es Walther Gerlach und Otto Stern im sogenannten Stern-Gerlach-Experiment SGE, zum ersten Male das magnetische Moment eines Atoms, des Silberatoms, zu messen und den Beweis zu erbringen, dass Arnold Sommerfelds und Pieter Debyes Postulat der Richtungsquantelung von atomaren magnetischen Momenten in einem äußeren Magnetfeld der Wahrheit entsprach. Das Messprinzip des Experimentes als hochauflösendes Impulsspektrometer für einzelne Atome im Vakuum und der historische Weg der Durchführung dieses Experimentes werden dargestellt. Das Ergebnis des SGE zeigte damit auch erstmals, dass auch die inneratomaren Drehimpulse gequantelt sind. Die Bedeutung des SGE für die Entwicklung der Quantenphysik besprochen.

Ice clouds constitute an important component in the Earth-atmosphere system. Like all clouds, they influence the hydrological cycle and the energy budget of the system. Thereby, the partial reflection of incident radiation results into a cooling effect (albedo effect), and the absorption and re-emission of thermal radiation results into a warming effect (greenhouse effect). However, for ice clouds in the tropopause region the net effect (warming or cooling) is unclear, because both opposite effects are of the same order of magnitude. Thus, the net effect depends on further properties of the multiscale system of ice clouds, such as the size and shape of the crystals, as well as the formation of structures within clouds resulting into heterogeneous media. In particular, the formation of structures in (ice) clouds is relatively poorly known so far and requires further investigation. In this talk we investigate processes and phenomena on different scales of ice clouds. We start with single crystals and their properties, as well as methods to measure these particles. To represent the ensemble ice cloud, models have to be developed and further investigated. The formulation of reduced order models leads us to ice clouds as nonlinear oscillators. The interaction on different scales and of different processes finally leads to the formation of characteristic structures. These investigations are current research and are carried out in interdisciplinary collaboration.

Protein pattern formation is essential for the spatial organization of intracellular processes like cell division, and flagellum positioning. A prominent example of intracellular patterns is the oscillatory pole-to-pole oscillations of Min proteins in E. coli whose function is to ensure precise cell division. Cell polarization, a prerequisite for processes such as stem cell differentiation and cell polarity in yeast, is also mediated by a diffusion-reaction process. More generally, these functional modules of cells serve as model systems for self-organization, one of the core principles of life. Under which conditions spatio-temporal patterns emerge, and how these patterns are regulated by biochemical and geometrical factors are major aspects of current research. In this talk I will review recent theoretical and experimental advances in the field of intracellular pattern formation, focusing on general design principles and fundamental physical mechanisms.

Water is ubiquitous and the most important liquid for life on earth. Although the water molecule is seemingly simple, various macroscopic properties of water are most anomalous, such as the density maximum at 4°C or the divergence of the heat capacity upon cooling. Computer-simulations suggest that the anomalous behaviour of ambient and supercooled water could be explained by a two state model of water. An important role in this ongoing debate plays the amorphous forms of water. Since the discovery of two distinct amorphous states of ice with different density (high- and low density amorphous ice, HDA and LDA) it has been discussed whether and how this phenomenon of polyamorphism at high pressures is connected to the occurrence of two distinct liquid phases (HDL and LDL). X-ray free electron laser allow us to investigate metastable states of supercooled water within nano- to microseconds. In my talk I will give an overview on our recent X-ray experiments on supercooled water and amorphous ices.

Optical fields can now be controlled with similar degrees of freedom as microwave fields for many decades already: we can now control not just the pulse envelope but also the optical carrier field. With few cycle laser pulses, this allows steering of electrons in unprecedented ways. I will give an overview over recent experiments we performed mainly with the atomically thin material graphene. Here we can drive the intraband motion of electrons but also interband transitions. For the intense ultrashort fields we employ, these processes become intricately coupled - a hallmark of strongfield physics. In particular, we could observe subsequent coherent Landau-Zener transitions, leading to Landau-Zener-Stückelberg-Majorana interferometry, representing fully coherent electron dynamics in a room-temperature material. In the second part of the talk, we will shine light on the graphene-gold interface and how it will add to the currents we can excite. Because of the different symmetries involved, we can disentangle virtual and real carrier excitations. With these insights, we have recently demonstrated a first Boolean logic gate based on two laser pulses carrying the logic information in the carrier envelope phase, which might bring lightwave or petahertz electronics closer to reality.

Electric manipulation of magnetization is essential for the integration of magnetic functionalities in integrated circuits. Spin-orbit torque (SOT), originating from the coupling of electron spin and orbital motion through spin-orbital interaction, can effectively manipulate magnetization. Symmetry breaking plays an important role in spintronics based on SOT. SOT requires inversion asymmetry in order to have a net effect on magnetic materials, which is commonly realized by spatial asymmetry: a thin magnetic layer sandwiched between two dissimilar layers. This kind of structure restricts the SOT by mirror and rotational symmetries to have a particular form: an “antidamping-like” component oriented in the film plane even upon reversal of the magnetization direction. Consequently, magnetization perpendicular to the film plane cannot be deterministically switched with pure electric current. To achieve all-electric switching of perpendicular magnetization, it is necessary to break the mirror and rotational symmetries of the sandwiched structure.

We have about 200 different types of cell in our body, and each of them has very special mechanical properties. Illustrative examples are contracting muscle cells, migrating immune cells or elastic red blood cells. There intriguing mechanical properties are to a great part determined by the so-called cytoskeleton (the “skeleton of the cell”), a composite biopolymer network composed of three filament systems – intermediate filaments, actin filaments and microtubules – along with cross-linkers and molecular motors. In my talk, I will focus on intermediate filaments, the most flexible and the most extensible ones among the different types of filament, with an intriguing non-linear behavior. It has been shown previously that the presence of intermediate filaments in a cell has an influence on its mechanics. Here we unravel different contributions to network properties and cell mechanics, such as the assembly kinetics and mechanical properties of the individual filaments, filament-filament interactions, and network rheology. To explain our experimental results on molecular grounds, we design models that include the strictly hierarchical build-up of the filaments and non-equilibrium transitions between folded and un-folded states. Taken together, the experiments and the modelling indicate that intermediate filaments serve as “safety belts” and shock absorbers” for the cell, thus avoiding damage at strong and fast impact, while maintaining flexibility (e.g., during cell motility).

In the early 1990s cryptography went into a foundational crisis when efficient quantum algorithms were discovered which could break almost all public key encryption schemes known at the time. Since then, an enormous research effort has been invested into basing public key cryptography, and secure computation in general, on problems which are conjectured to be hard even for quantum computers. This research program has been resoundingly successful, leading to unexpected developments, such as the discovery of fully homomorphic encryption schemes. Furthermore, cryptography research has now moved beyond just "post-quantum security”, i.e. security against quantum adversaries, and investigates cryptographic protocols for a (still hypothetical) quantum world, where not just adversaries, but also honest users have access to scalable quantum computers and quantum communication channels. This enables applications such as quantum money, which are impossible using purely classical information. In this talk I will give an overview of the field and some of the (in my opinion) most challenging open problems.

Photonic quantum simulation and sensing Georg von Freymann1,2 1Physics Department and Research Center OPTIMAS, Rheinland-Pfälzische Technische Universität Kaiserslautern Landau RPTU, 67663 Kaiserslautern, Germany 2Fraunhofer Institute for Industrial Mathematics ITWM, 67663 Kaiserslautern, Germany Applications of quantum technology are highly sought-after and thus supported by public funding agencies. However, the meaning of application varies depending on who you talk to: In the physics community application often means a useful laboratory implementation, while from an industrial perspective, application means solving a measurement problem in production or even creating a saleable product. To address the physics perspective, I will discuss 3D µ-printed photonic quantum simulators based on coupled waveguide system, focusing on topological protection and Floquet (time-periodic) driving. Such experimental model systems allow for studying these phenomena under very well controlled conditions. Examples are periodic driving of topologically protected edge modes in the one-dimensional Su-Schrieffer-Heeger-chain leading to depopulation of the edge mode despite topological protection [1], periodic driving of two-dimensional honey-comb-lattices establishes topological protection in an otherwise topologically trivial model system [2], switching of topological protection via excitation with and without orbital angular momentum of light [3], and establishing higher-order topological insulators using p-orbitals of the waveguides [4]. From the industry perspective I discuss recent results for terahertz quantum-sensing with undetected photons [5] allowing to measure terahertz spectral properties with visible light only, enabling both single-shot layer thickness measurements as well as spectroscopy. [1] Z. Cherpakova, C. Jörg, et al., Limits of topological protection under local periodic driving, Light: Science&Applications 8, 63 (2019). [2] C. Jörg, et al., Dynamic defects in photonic Floquet topological insulators, New J. Phys. 19, 083003 (2017). [3] C. Jörg, et al., Artificial gauge field switching using orbital angular momentum modes in optical waveguides, Light: Science&Applications 9, 150 (2020). [4] J. Schulz, J. Noh, et al., Photonic quadrupole topological insulator using orbital-induced synthetic flux, Nature Communications 13, 6597 (2022) [5] M. Kutas et al, Terahertz Quantum Sensing, Science Advances 6, eaaz8065 (2020)

„I think we have it“ – with these words, the then Director General of CERN, Rolf-Dieter Heuer, commented on July 4th, 2012 the detection of a new elementary particle at the Large Hadron Collider (LHC). The search for the Higgs boson, which had lasted almost 50 years, had reached its goal. With the discovery of the Higgs boson, a new era began at the LHC, the precise measurement of the particle's properties. With the help of these properties, conclusions can be drawn about the fundamental structure of the universe and matter. In this colloquium, I will discuss the latest result and prospects in the quest to decipher the Higgs boson.

An electric dipole moment (EDM) of a fundamental particle would violate time and parity symmetry and by the virtue of the CPT theorem also the combined symmetry of charge conjugation and parity inversion. Searches for EDM are generally considered highly sensitive probes for new physics and might shed light on still unresolved questions in particle physics and cosmology like the origins of matter, dark matter, and dark energy. At the Paul Scherrer Institute in Switzerland, we are setting up an experiment searching for a muon EDM with a sensitivity of 3E-21 ecm using, for the first time, the frozen-spin technique~\cite{Farley2004PRL} in a compact storage ring. This will lay the ground work for a second phase with a final precision of better than 6e-23 ecm. This staged approach to search for a non-zero muon EDM probes previously uncharted territory and tests theories of BSM physics by: i) improving the current direct experimental limit of d < 1.5E-19 ecm (CL 90%) by roughly three orders of magnitude; ii) being a complementary search for an EDM of a bare lepton; iii) being a unique test of lepton-flavor symmetries; and iv) in the case of a null result, will be a stringent limit on an otherwise very poorly constrained Wilson coefficient.

Reactions with short-lived nuclei are key to understand the properties of neutron-rich nuclei and neutron-rich nuclear matter. In recent years, quasi-free scattering experiments have been developed and established for experiments with radioactive beams at GSI and RIKEN. The inverse kinematics of the reaction opens thereby the possibility for a complete characterisation of the final state, which results in an almost background-free measurement. Recent results with stable and radioactive beams will be discussed including the first measurement of short-range correlations in inverse kinematics, the observation of alpha clusters at the surface of heavy nuclei, as well as the observation of a correlated four-neutron state. The perspective for a precise determination of the neutron-neutron scattering length using the 6He(p,p alpha)2n reaction will be discussed as well.