Semesterübersicht

Recent advances in optical studies of condensed matter have led to the emergence of a variety of phenomena that have conventionally been studied in quantum optics. These studies have not only deepened our understanding of light-matter interactions but also introduced aspects of many-body effects inherent in condensed matter. This talk will describe our recent studies of Dicke cooperativity, i.e., many-body enhancement of light-matter interaction, a concept in quantum optics, in condensed matter. This enhancement has led to the realization of the ultrastrong coupling (USC) regime, where new phenomena emerge through the breakdown of the rotating wave approximation (RWA). We will first describe our observation of USC in a 2D electron gas in a high-Q terahertz cavity in a magnetic field, and definitive evidence for the vacuum Bloch-Siegert shift, a signature of the breakdown of the RWA. Further, we have shown that cooperative USC also occurs in magnetic solids in the form of matter-matter interaction, i.e., spin-magnon and magnon-magnon interactions in rare earth orthoferrites. Particularly, the exchange interaction of N paramagnetic Er3+ spins with an Fe3+ magnon field in ErFeO3 has exhibited a vacuum Rabi splitting whose magnitude is proportional to N1/2 [6]. In the lowest temperature range, these cooperative interactions lead to a magnonic superradiant phase transition. These results provide a route for understanding, controlling, and predict novel phases of condensed matter.

Recent advances in optical studies of condensed matter have led to the emergence of a variety of phenomena that have conventionally been studied in quantum optics. These studies have not only deepened our understanding of light-matter interactions but also introduced aspects of many-body effects inherent in condensed matter. This talk will describe our recent studies of Dicke cooperativity, i.e., many-body enhancement of light-matter interaction, a concept in quantum optics, in condensed matter. This enhancement has led to the realization of the ultrastrong coupling (USC) regime, where new phenomena emerge through the breakdown of the rotating wave approximation (RWA). We will first describe our observation of USC in a 2D electron gas in a high-Q terahertz cavity in a magnetic field, and definitive evidence for the vacuum Bloch-Siegert shift, a signature of the breakdown of the RWA. Further, we have shown that cooperative USC also occurs in magnetic solids in the form of matter-matter interaction, i.e., spin-magnon and magnon-magnon interactions in rare earth orthoferrites. Particularly, the exchange interaction of N paramagnetic Er3+ spins with an Fe3+ magnon field in ErFeO3 has exhibited a vacuum Rabi splitting whose magnitude is proportional to N1/2 [6]. In the lowest temperature range, these cooperative interactions lead to a magnonic superradiant phase transition. These results provide a route for understanding, controlling, and predict novel phases of condensed matter.

Magnetic resonance imaging is an inherently non-invasive technique with biological applications from the cellular to human size-scales. A major technological push has been towards stronger magnetic fields, which can be >20 Tesla for preclinical studies and >10 Tesla for humans, since these increase the signal strength and ultimate imaging resolution. Such systems, however, require advances in hardware design, acquisition sequences and image processing algorithms to achieve optimal performance. The first part of this talk will concentrate on technical challenges and practical approaches for human scanning at 7 Tesla and above. The challenges include B_1 and B_0 inhomogeneities, increased specific absorption rate, and high sensitivity to movement. Neurological and neuroscience applications discussed include ocular and neurological tumours, epilepsy, neuromuscular diseases, glymphatic clearance and mechanistic studies of lithium for bipolar disorders. The second part will discuss the opposite end of the MRI spectrum, ultra-low field systems at ~50 mT which have been designed to address the challenges of global healthcare accessibility. The challenges here are diametrically opposite to those at high field, and topics of system design, characterization and in vivo applications will be highlighted.

The CP violation observed in the hadronic decays of charmed mesons remains a puzzling open question for theorists. Calculations relying on the assumption of inelastic final-state interactions occurring between the pairs of pions and kaons fall short of the experimental value. It has been pointed out that a third channel of four pions can leave imprints on the CP asymmetries of the two-body decays. At the same time, plenty of data are available for rare decays such as \(D^0\to\pi^+\pi^-\ell^+\ell^-\), which provide a promising environment for the search for new physics. With this motivation, we study the cascade topology \(D^0\to a_1(1260)^+(\to \rho(770)^0\pi^+)\,\pi^-\), which has been measured to contribute significantly to the \(4\pi\) decays of the same meson, and estimate its effect on the branching ratio of the rare decays. I will also comment on the possibility of this topology contributing to the decay amplitude of \(D^0\to\pi^+\pi^-\) and by extension to the related CP asymmetry.

Thanks to their extremely weak interaction with matter and neutral electric charge, neutrinos travel vast cosmic distances without deflection, providing a unique and complementary approach to investigating the most energetic events in the Universe. Neutrino telescopes are designed to detect Cherenkov light inferred by neutrino interactions. After more than fifteen years of data collection, the pioneering ANTARES detector has been successfully dismantled, making way for its next-generation successor, KM3NeT, deployed at two sites in the Mediterranean Sea. Near the former ANTARES location, off the coast of Toulon (France), KM3NeT/ORCA is dedicated to studying the intrinsic properties of atmospheric neutrinos through their oscillations within the Earth. Further southeast, off the coast of Sicily, KM3NeT/ARCA is monitoring the high-energy sky in search of cosmic neutrinos. In this presentation, I will highlight the latest insights in neutrino (astro)physics emerging from the depths of the Mediterranean. Particular attention will be given to the recent detection of an ultra-high-energy neutrino event, designated KM3-230213A, by KM3NeT/ARCA. The observed particle is a muon with an estimated energy of 120+110−60 PeV. Its exceptionally high energy and nearly horizontal trajectory suggest that its parent neutrino originated from a cosmic accelerator or could potentially be the first detected cosmogenic neutrino—produced when ultra-high-energy cosmic rays interact with background photons in the Universe. This groundbreaking observation underscores the remarkable capabilities of deep-sea neutrino telescopes in unveiling new astrophysical phenomena. To also view graphic content, follow the link: https://www.thep.physik.uni-mainz.de/files/2025/04/Title_Abstract_Mainz_AK_16.04.2025.pdf
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We aim to advance antimatter research through tabletop experiments that operate independently of accelerator infrastructure, allowing for much lower noise levels and freedom from beamtime schedules. Our approach involves Dual RadioFrequency Traps (DRFTs) to confine the constituents of antihydrogen: positrons and antiprotons. Due to the different charge-to-mass ratios, each species primarily couples to a separate RF field. Unlike other traps, DRFTs naturally allow two species, even those of opposite charge, to be brought close together so that high production rates of antihydrogen can be achieved. Additionally, their open geometry is advantageous for laser spectroscopy. In our pioneering study, we develop a DRFT for co-trapping electrons and calcium ions which act as stand-ins for positrons and antiprotons. We demonstrate seperate storage times of up to a second and are developing an improved DRFT to extend this. In parallel, we are developing a low-energy positron source which will allow us to study bound positron-atom systems while other groups work on developing tools to transport antiprotons for future studies on antihydrogen.

Muon conversion — the process of a bound muon decaying into an energetic electron — is one of the best probes of charged lepton flavor violation. The experimental limit is soon expected to improve by four orders of magnitude, thus calling for precise predictions on the theory side. Equally important are precise predictions for muon decay-in-orbit, the main background for muon conversion. While the calculation of electromagnetic corrections to the two processes above the nuclear scale does not involve significant challenges, it becomes substantially more complex below that scale due to multiple scales, bound-state effects and experimental setup. In this talk, I present a systematic framework that addresses these challenges by resorting to a series of effective field theories. Combining Heavy Quark Effective Theory (HQET), Non-Relativistic QED (NRQED), potential NRQED, Soft-Collinear Effective Theory I and II, and boosted HQET, I derive a factorization theorem and present the renormalization group equations.

Neutrinos, though nearly massless and weakly interacting, play a central role in modern physics—from the origin of mass and the nature of matter–antimatter asymmetry to the search for physics beyond the Standard Model. Yet one of the main obstacles to fully realising their potential lies in our limited understanding of how neutrinos interact with matter. These interactions are complex, often involving nuclear effects that are difficult to model and challenging to measure. As a result, they introduce significant systematic uncertainties in precision experiments, including those aiming to determine mixing parameters and explore CP violation. This talk will provide an accessible overview of why neutrino interaction physics is both essential and challenging, and how it connects nuclear and particle physics. I will outline current experimental limitations and discuss the key requirements for future progress: well-characterised neutrino beams, dedicated measurements, and new experimental strategies. These advances are not only crucial for interpreting results from current and future experiments, but also for enabling discoveries that may reshape our understanding of fundamental physics. As an illustrative example, I will introduce nuSTORM—a proposed facility based on stored muons—as a next-generation platform for precision neutrino scattering and searches for new physics.
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Optical atomic clocks with eighteen significant digits are the most accurate measurement devices available to us with applications ranging from tests of fundamental physics to height difference measurements in relativistic geodesy. The uncertainty in trapped-ion clocks is limited by systematic frequency shifts and quantum projection noise. In my presentation, I will show how quantum engineering techniques can overcome these limitations. Quantum algorithms provide access to new clock species such as highly charged ions with reduced systematic shifts and high sensitivity to searches for new physics, including hypothetical fifth forces, variation of fundamental constants and dark matter candidates. Dynamical decoupling and entangled state spectroscopy in a multi-ion frequency reference offer suppression of systematic shifts, while improving the signal-to-noise ratio of the clock and thus the required averaging time to reach a certain resolution. These developments will pave the way towards a next generation of quantum-enhanced clocks that enter the 10-19 relative frequency uncertainty regime.

Life is physics and chemistry in action. While molecular simulations of systems as complex as whole cells are now within reach, predicting chemical reactivity on relevant time and length scales remains a challenge. I will present our recent work towards bringing action – here: chemistry – to classical simulations and molecular design through machine learning. Among others, we substitute costly quantum mechanical calculations with a graph neural network-based emulator. Our framework can deal with the plethora of life’s chemistry amidst the ‘jiggling and wiggling’ of biomolecules. Importantly, we also uncover unexpected biomolecular processes that we in turn put to test in experiments. Finally, I will demonstrate how we harness a flow-matching model to predict biomolecular dynamics. Our method paves the way for generating novel flexible and functional proteins.

Top-quark pair production in association with a jet is a key process at the LHC. Its high sensitivity to the top mass and the increasing experimental precision call for the QCD corrections to be computed at the next-to-next-to-leading order (NNLO). In this seminar, I will present the computation of the two-loop Feynman integrals required to obtain NNLO QCD predictions in the leading colour approximation. These integrals are characterised by significant algebraic complexity—stemming from the multi-scale, five-particle kinematics—as well as analytic complexity, due to the appearance of nested square roots and elliptic functions. I will discuss modern methods for tackling multi-scale integrals in a way that is suitable for phenomenology, and outline first steps to extend these techniques to cases involving elliptic functions.

Quantum field theory (QFT), a framework that combines special relativity and quantum mechanics, is the language used to describe the fundamental particles and interactions of our universe. One of the most powerful tools in QFT is perturbation theory, which has given rise to some of the most precise predictions in science. There are QFTs, however, where perturbation theory does not work, the most prominent being the theory of the strong force, Quantum Chromodynamics. In this talk, I will describe some recently developed tools used to better understand non-perturbative QFTs. One of these tools is called supersymmetry, which allows one to treat the spherical cow of QFTs. Finally, I will explore whether these tools could be applicable to the real world strong force.
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By employing the Bloch-sphere formalism, I will present a novel class of unstable qubits, which are called Critical Unstable Qubits (CUQs). The characteristic property of CUQs is that the energy-level and decay-width Pauli vectors, E and Γ, are orthogonal to one another, and the key parameter r = |Γ|/(2|E|) is less than 1. A remarkable feature of CUQs is that they exhibit atypical behaviours like coherence-decoherence oscillations when analysed in an appropriately defined co-decaying frame of the system. In the same frame, I will show how a unit Bloch vector b describing a pure CUQ sweeps out unequal areas during equal intervals of time, while rotating about the vector E. These phenomena emerge beyond the usual oscillatory pattern due to the energy-level difference of a standard two-level quantum system. I will illustrate how these new features are relatively robust and persist even for quasi-CUQs, in which the vectors E and Γ are not perfectly orthogonal to each other. I discuss potential applications of our results to quantum information and to unstable meson-antimeson and other systems.

Neutrinos are the least known particles in the Standard Model. In particular, whether they are Dirac or Majorana particles is an important open question. Theoretically, it is also possible that they are pseudo-Dirac, which are fundamentally Majorana fermions, but essentially act like Dirac fermions in most experimental settings, due to extremely small active-sterile mass splitting. We will discuss how such tiny values of mass splitting can be accessed via oscillations over astrophysical baselines. We use the recent multi-messenger observations of high-energy neutrino sources to probe hitherto unexplored values of mass splitting, which improve the reach of terrestrial experiments by more than a billion. Finally, we will present an intriguing hint for nonzero active-sterile mass splitting in the recent IceCube data.

Currently, pulsed parametric down-conversion (PDC) in waveguides is one of the most promising platforms for non-classical light sources. PDC provides not only an efficient way to generate biphoton pairs and squeezed states but also can be used in more advanced circuits to prepare non-Gaussian states of light. However, waveguides typically possess surface imperfections that may cause significant scattering of the guided light, providing multimode mixed output states. In this talk, we discuss the properties of the multimode PDC in lossy waveguides and show how critical these losses are for the generation of biphoton, squeezed, and entangled states ([Quantum 9, 1621 (2025)] and [arXiv:2501.08917]).

Electroweakly interacting stable particles in the (1 − 10) TeV mass range can be a dark matter candidate with rich testability. In particular, gamma-ray line-like features are expected to be a smoking-gun signature for indirect detection. In this framework, one fundamental question follows: How can we distinguish DM spin among DM candidates with the same electroweak interaction by contrasting their predictions? A straightforward but crucial effort is to derive annihilation spectra with higher accuracy. However, one encounters complexities due to non-perturbative corrections following the mass hierarchy between heavy DM and electroweak mediators. In this talk, we present how to construct Soft-Collinear Effective field Theory (SCET) to systematically resum large Sudakov logarithmic corrections for spin-0, 1/2, and 1 DM, and achieve accurate prediction. We focus especially on spin-1 DM, which is the last piece to complete all the possible predictions. After specifying the leading operators to describe heavy DM annihilation in SCET, we discuss how to extract spin-dependence from the predicted gamma-ray spectrum.

I will introduce our IBP package NeatIBP, which automatically generates small-size integration-by-parts (IBP) identities for Feynman integrals. Based on the syzygy and module intersection techniques, the generated IBP identities’ propagator degree is controlled and thus the size of the system of IBP identities is shorter than that generated by the standard Laporta algorithm. Resently, we updated NeatIBP with some new featrues, such as, spanning cut and the reduction interface of NeatIBP and Kira. I will also give some powerful applications of NeatIBP.

Precision measurements involving nuclei are at the cutting edges of nuclear physics and testing the Standard Model (SM) of physics. For instance, precision beta decay measurements have the potential to constrain beyond SM physics at TeV scales. To interpret these experiments, it is crucial to have comparably accurate theoretical predictions of relevant quantities along with an accurate understanding of the underlying nuclear dynamics. In this contribution, I will overview recent calculations of electroweak processes with quantum Monte Carlo (QMC) computational methods used to solve the many-body Schr ̈odinger equation. The QMC approach retains the complexity of many-nucleon dynamics and provides highly accurate results for light nuclei. I will discuss calculations of observable quantities with readily available data–such as beta decay and electromagnetic reactions–used to validate models of nuclear many-body interactions and electroweak currents. I will present QMC calculations of predicted quantities relevant to on-going beta decay experiments and discuss how these results will impact experimental determinations of beyond standard model physics.
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We introduce quantum magnetic J-oscillators that operate at zero magnetic field by exploiting nuclear spin-spin J-coupling transitions in molecules. This is achieved by coupling in situ hyperpolarized samples to a programmable digital feedback system that digitizes, delays, and amplifies the sample-generated magnetic field before feeding it back to the sample. Due to the insensitivity of the J-couplings to magnetic field drifts, we achieved coherent J-oscillations lasting over 3000 s, with a linewidth of 337 μHz limited primarily by acquisition time, reaching the Cramér-Rao lower bound in estimating error in frequency measurement [1]. The ability to control the feedback delay and gain enabled us to resolve overlapping resonances, making possible on-demand spectral editing. Application of quantum oscillators was demonstrated on a diverse range of molecules (nitriles, heterocycles, organic acids). The J-oscillators produce highly resolved, sharp spectra, reveal hidden transitions, and may allow distinction of complex mixtures that conventional zero-field NMR [2] cannot resolve. As a result, this approach can expand the scope of zero-field NMR for analytical chemistry, biomolecular characterization, and fundamental physics. [1]. S. Fleischer, S. Lehmkuhl, L. Lohmann, S. Appelt, Approaching the Ultimate Limit in Measurement Precision with RASER NMR. Appl. Magn. Reson. 54 (11), 1241–1270 (2023). [2]. D. A. Barskiy, et al., Zero- to Ultralow-field Nuclear Magnetic Resonance. Prog. Nucl. Magn. Reson. Spectrosc. (2025).

Symmetries play a key role in many areas of modern physics. For example, the symmetry-breaking paradigm describes how various phases of matter emerge. In magnetism, spontaneous symmetry breaking leads to well-known phases of ferromagnets and antiferromagnets. Ferromagnets have a net magnetization, while antiferromagnets have atomic magnetic moments that cancel out. Surprisingly, recent research shows this magnetic dichotomy, developed in the 1930s, is incomplete [1-5]. In this talk, we introduce our recently developed classification of magnetic phases based on spin-lattice symmetries. These are pairs of operations in spin and lattice space. This unorthodox perspective has led us to identify two unconventional magnetic phases: altermagnets [4] (see figure) and antialtermagnets [5]. Like antiferromagnets, both have compensated magnetic order and thus no net magnetization. But unlike antiferromagnets—and similar to ferromagnets—they induce spin polarization in the electronic structure. The key distinction between altermagnets and antialtermagnets lies in their behaviour under time-reversal symmetry. Altermagnets break time-reversal symmetry in their electronic structure, resulting in features like d-wave spin order [2]. In contrast, antialtermagnets preserve time-reversal symmetry and exhibit properties such as p-wave spin order [5,6]. We’ll also explore how the concept of altermagnetism was inspired by our earlier theoretical prediction[1-2] and experimental observation of an unconventional spontaneous Hall effect[7]. Additionally, we’ll highlight recent photoemission experiments that have confirmed altermagnetic order in materials like MnTe and CrSb [8]. Finally, we’ll discuss the broader implications of altermagnetism and spin symmetries. These findings have potential applications in areas such as spintronics, magnonics, topological materials, 2D materials, and multiferroics [9]—all of which could lead to faster, smaller, and more energy-efficient AI-era information technologies [1,9].

Precise numerical evaluation of Feynman integrals plays a central role in particle physics. While some methods focus on individual phase-space points, others exploit differential equations to efficiently propagate results across kinematic regions. LINE is a novel open-source program that integrates both strategies within a single tool, computing boundary values and propagating them accordingly. Written primarily in C, it is designed for performance and scalability, enabling large-scale computations on clusters without relying on proprietary software. In this talk, I will introduce the core ideas behind LINE and present a few representative applications.

The elements of the CKM matrix are fundamental parameters of the Standard Model. Substantial experimental and theory efforts are under way, aimed at determining them ever more precisely and accurately. Interestingly, some tensions between different approaches have persisted over many years, in particular, between determinations from inclusive and exclusive semi-leptonic meson decays, respectively. This talk addresses new ideas aimed at resolving these tensions. In particular, I will discuss how fundamental principles like analyticity and unitarity can be used to improve existing, and enable new computations in lattice QCD. The ideas will be demonstrated for the analysis of simulations of exclusive as well as inclusive semileptonic Bs-meson decay. In the former case, the use of Bayesian inference allows for formulating a model- and truncation independent parameterisation of hadronic form factors. In the latter case, the properties of the QCD transfer matrix are used to tackle the computation of the inclusive decay rate for the first time.
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Synthetic antiferromagnets are magnetic multilayers consisting of two or more ferromagnetic layers that are coupled antiferromagnetically. They play an important role in spintronic devices, e.g., as field sensors, and as synthetic materials for fundamental explorations. In this talk, I will highlight the use of synthetic antiferromagnets for quantum information science with spin waves, i.e., for quantum magnonics. Examples that are discussed are unidirectionally-coupled magnetic layers that give rise to magnon quantum amplification, and new ways to entangle magnons between two ferromagnetic layers. Both these examples rely on the possibility to engineer both the interactions between the layers, and the interactions of the magnetic layers with the environment. This tunability highlights the potential of synthetic antiferromagnets for quantum magnonics.

This talk explores the rapidly evolving field of quantum technologies, with a particular focus on semiconductor quantum dots (QDs) and their potential in quantum communication and distributed quantum computing. Quantum dots are excellent sources of single and entangled photons and offer significant tunability in their optical properties through variations in material composition, shape, and confinement. Our work focuses on epitaxially grown GaAs/AlGaAs QDs that emit near the optical transitions of rubidium or the zero-phonon line of silicon-vacancy centres [1]. There is strong potential for such QDs in hybrid systems, which are vital for future quantum repeaters and extended quantum networks [2]. Several milestone experiments have been realized, incorporating single quantum dots. These include entanglement swapping between photon pairs [3] and quantum key distribution (QKD) experiments between Hannover and Braunschweig [4]. In order for QDs to realise their full potential in quantum technology applications, it is imperative to realize a seamless integration of QDs into photonic devices, with the objective of enhancing the efficiency of photon extraction and optimising the coupling to fibre networks. This necessitates the development of innovative strategies in the fields of optical positioning, photonic design and fabrication. A calibration model is introduced with the objective of enhancing the accuracy of wide-field optical positioning for the alignment of solid-state single photon emitters within photonic nanostructures. This is expected to result in a significant increase in the yield of high performance quantum photonic devices. Furthermore, the development of hybrid circular photonic crystal gratings for the generation of entangled photon pairs at telecom wavelengths represents a promising advancement for direct coupling efficiency into single-mode fibres [5]. [1] X. Cao, J. Yang, T. Fandrich, Y. Zhang, E. P. Rugeramigabo, B. Brechtken, R. J. Haug, M. Zopf, and F. Ding, Nano Letters 23, 6109 (2023). [2] P. van Loock, W. Alt, C. Becher, O. Benson, H. Boche, C. Deppe, J. Eschner, S. Höfling, D. Meschede, P. Michler, et al., Advanced Quantum Technologies 3, 1900141 (2020), https://advanced.onlinelibrary.wiley.com/doi/pdf/10.1002/qute.201900141. [3] M. Zopf, R. Keil, Y. Chen, J. Yang, D. Chen, F. Ding, and O. G. Schmidt, Phys. Rev. Lett. 123, 160502 (2019). [4] J. Yang, Z. Jiang, F. Benthin, J. Hanel, T. Fandrich, R. Joos, S. Bauer, S. Kolatschek, A. Hreibi, E. P. Rugeramigabo, et al., Light: Science & Applications 13, 150 (2024), ISSN 2047-7538. [5] C. Ma, J. Yang, P. Li, E. P. Rugeramigabo, M. Zopf, and F. Ding, Opt. Express 32, 14789 (2024).

In the ΛCDM cosmological model the Universe is assumed to be isotropic and homogeneous when averaged on large scales. That the Cosmic Microwave Background has a dipole anisotropy is interpreted as due to our peculiar (non-Hubble) motion because of local inhomogeneity. There must then be a corresponding dipole in the sky distribution of sources at high redshift. Using catalogues of radio sources and quasars we find that this expectation is rejected at >5σ, i.e. the distribution of distant matter is not isotropic in the 'CMB frame’. This calls into question the standard practice of boosting to this frame to analyse cosmological data, in particular to infer acceleration of the Hubble expansion rate using Type Ia supernovae, which is then interpreted as due to a Cosmological Constant Λ. We find that the inferred acceleration is anisotropic (in the direction of the CMB hotspot) and likely illusory because of our being embedded in a coherent bulk flow, rather than due to dark energy.

Gravitational wave (GW) observations have opened new avenues for probing Beyond Standard Model (BSM) physics. While most detections involve typical black hole (BH) or neutron star (NS) mergers, events like GW190814, GW190425, and GW230529 observed by the LIGO-Virgo-KAGRA (LVK) collaboration include at least one compact object whose nature—either a binary neutron star (BNS) or a low-mass black hole (LMBH)—remains uncertain. One possible formation channel for such LMBHs involves dark matter (DM) capture and accumulation in NSs, leading to a collapse-induced transmutation of the NS into a BH of comparable mass. I will discuss how DM capture in NSs can lead to the formation of LMBHs, highlighting the relevant parameter space that governs this process. I will also show how current GW observations can constrain portions of this DM parameter space. In the second part, I will focus on the gravitational waveforms of BNS mergers versus LMBH mergers, examining their distinguishing features and assessing the capability of current and future detectors to differentiate between these two scenarios.

The quest to probe the Standard Model more deeply and to search for physics beyond its current limits continues to drive the development of future particle colliders. Numerous ambitious proposals, both linear and circular designs, and lepton and hadron colliders, aim to push the frontiers of energy and luminosity to unprecedented levels. This seminar will present an overview of the main collider concepts, examining their scientific motivations, design strategies, and individual strengths. In addition, the technological challenges these proposed machines face will be discussed, along with potential synergies and complementarities among the different proposals.
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Fundamental questions in astrophysics and cosmology such as the matter to antimatter asymmetry (baryon asymmetry) in the universe or the existence of dark matter are thought to be closely linked to particle physics. For example, the baryon asymmetry of the universe can be explained by models of leptogenesis, which require special properties of neutrinos. And the as yet unknown dark matter presumably consists of particles that require physics beyond the standard model of particle physics. This question can be investigated using search and precision experiments in (astro)particle physics at low energies. In this colloquium, this will be illustrated by three examples: the search for neutrinoless double beta decay, the direct search for the neutrino mass and the direct search for dark matter. These searches will be explained using specific experiments, such as the KATRIN and XENONnT experiments including their recent results, as well as the respective perspectives for the future possibilities

The cross section for an infrared safe observable in hadron-hadron collisions can be written as the convolution of parton distribution functions (PDFs) for the two hadrons and a perturbatively calculable hard scattering function, with power suppressed corrections. The arguments that this PDF factorization works at all orders of perturbation theory involves the cancellation of what are called Glauber singularities from soft gluons that interact with both hadrons. I illustrate the arguments used in a 1988 paper with Collins and Sterman by applying these arguments to graphs at the lowest order in which the Glauber problems appear. We can see in this simple example how the contributions from Glauber singularities disappear. This sort of analysis is perhaps useful for the analysis of "non-global" logarithms in certain observables.

Integrated Quantum Optics, Department Physics, Institute for Photonic Quantum Systems (PhoQS,), Paderborn University, 33098 Paderborn, Germany Quantum technologies promise a change of paradigm for many fields of application, for example in communication systems, in high-performance computing and simulation of quantum systems, as well as in sensor technology. Current efforts in photonic quantum science target the implementation of practical devices and scalable systems, where the realization of quantum devices for real-word deployment and controlled quantum network structures is key for many applications. Here we present our work on three different fields in this area: non-linear integrated quantum optics, pulsed temporal modes and photonic quantum computation.

In a world where a single company like Google consumed more energy than a country with population of about 20 million [1] in 2019 and this is growing exponentially, it is essential to find energy efficient approaches to make our computing needs sustainable. One potential solution is the use of nanoscale magnetic computing devices. Towards this end, energy efficient approaches based on electrical field control of nanoscale magnetism are pursued in our group: (i) strain mediated switching of the magnetization of nanomagnets [2]; (ii) creation and annihilation of magnetic skyrmions using direct voltage control of magnetic anisotropy (VCMA) [3]; and more recently magnetoionic control [4]. Such nanoscale magnetic devices have application to non-volatile memory [3], hardware AI [4, 5, 6] and quantum control of spins [7,8,9]. We will discuss skyrmion mediated voltage control of nanoscale magnetization that has potential for extremely energy efficient non-volatile memory [3] and are robust to switching errors in the presence of thermal noise, material and device inhomogeneities, while scaling to lateral dimensions of 20 nm and below [3]. Furthermore, energy efficient AI hardware can be realized with nanomagnetic devices. Multi-state nanoscale domain wall racetracks can be used as highly quantized synapses in deep neural networks [5] and convolutional neural networks, with overall improvement in area, energy, and latency by 13.8, 9.6, and 3.5 times respectively [5] compared to purely CMOS implementations. Additionally, interacting nanomagnets can be used for analog [6] and digital reservoir computing [6] and long-term prediction of temporal data [6]. We will specifically discuss experimental implementation of reservoir computing with magnetoionic devices [4] that do not need conversion of signals to GHz unlike when Spin Torque Nano Oscillators (STNOs) are used. In quantum computing, in addition to energy efficiency, one significant problem is implementing qubits in a scalable manner at temperatures of a few Kelvin. We argue that ensemble spin qubits may offer such a possibility [7]. Furthermore, by driving the magnetization of nanomagnets electrically, highly confined microwaves can be generated at the Larmor precession frequency of proximally located spins [8]. This can implement single-qubit quantum gates with fidelities approaching state-of-the-art in a scalable manner. Further confinement of microwaves using convergent-divergent skyrmion devices can implement even more localized and low footprint quantum control of spins [8]. New experimental and simulation results in these directions will be discussed [9]. References [1] FORBES Editor’s pick, Oct 21, 2020,04:26pm EDT [2] Nano Letters, 16, 1069, 2016; Nano Lett., 16, 5681, 2016; Appl. Phys. Lett. 121, 252401, 2022; https://arxiv.org/abs/2501.00980 [3] Nature Electronics 3, 539, 2020; Scientific Reports,11, 20914, 2021; Scientific Reports, 14, 17199, 2024 [4] https://arxiv.org/abs/2412.06964 [5] Nanotech. 31 145201, 2020, IEEE Access, 10, 84946, 2022; IEEE Trans. on Neural Networks and Learning Sys, 36, 4996, 2024. [6] Appl. Phys. Lett. 121, 102402, 2022; Comm. Phys. 6, 215, 2023; Neuromorph. Comput. Eng. 2 044011; IEEE Access, 11,124725, 2023 [7] https://arxiv.org/abs/2503.12071 [8] Communication Physics 5, 284, 2022; Physical Phys. Rev. Applied 22, 06407, 2024. [9] https://arxiv.org/abs/2407.14018

In this talk, we derive the coupled dynamics between the bubble wall and the plasma from first principles using nonequilibrium quantum field theory. The commonly used equation of motion of the bubble wall in the kinetic approach is shown to be incomplete. In the language of the two-particle-irreducible effective action, the conventional equation misses higher-loop terms generated by the condensate-particle type vertices (e.g.,~ φϕχ2, where φ is the background field describing the bubble wall, ϕ the corresponding particle excitation and χ another particle species in the plasma). From the missing terms, we identify an additional dissipative friction which is contributed by particle production processes from the condensate-particle type vertices. We also show how other transmission processes beyond the 1-to-1 elementary transmission studied in the literature for ultrarelativistic bubble walls, e.g., 1-to-1 mixing and 1-to-2 transition radiation, can be understood from the kinetic approach.

Centuries after Newton described a gravitational force, Einstein revolutionised our understanding of space, time and gravity in his general theory of relativity. Another 100 years later marked the beginning of a new era of astronomy, in which spacetime oscillations (a.k.a. gravitational waves) are used to detect the inspiral and merger of the most compact objects in the Universe. In my talk, I will review the methods and results of 10 years of gravitational-wave astronomy that saw already about 300 detections, most of them of black-hole mergers. I will discuss some insights and open questions that emerged from those observations and illustrate that Einstein's picture still holds for these most violent cosmic collisions.

In this talk, I will review two device concepts based on nonlinear dissipative magnetic dynamics. First, we revisit the problem of spin superfluidity, which has been predicted to facilitate coherent spin transport. We propose to both exhibit and exploit this elusive transport phenomenon via the "spin-superfluid quantum interference device" (spin SQUID) — inspired by its superconducting (rf SQUID) and superfluid helium (SHeQUID) counterparts. In particular, we discuss its potential electric-field sensing functionality based on the microwave response of the simplest pertinent structure: a magnetic ring with a single weak link. In the second part of the talk, we systematically address the pseudo-Hermitian physics of dynamically-coupled magnetic macrospins, with a focus on non-Hermitian mode hybridization and its potential utility as a scalable building block for dynamic Ising machines.

As high-performant sources of single photons, epitaxial quantum dots can be considered as a semiconductor launchpad for quantum photonic technologies. There is still a variety of challenges to tackle on the road to an ideal source of single or entangled photons for quantum photonic applications. Here, we present an overview of recent developments in our group on the engineering of single-photon sources for quantum photonic applications made from III-V semiconductor quantum dots grown by molecular beam epitaxy. By integrating InAs/InP quantum dots into circular Bragg grating resonators, Purcell-enhanced single-photon emission with a Purcell factor of ≈7 in the telecom C-band was achieved. Low multi-photon emission probabilities are obtained, and Hong-Ou-Mandel two-photon interference is demonstrated. By further improving the epitaxial growth, photonic design and excitation schemes, we were able to obtain record two photon interference visibilities. We furthermore report on recent progress made in our group towards deterministic generation of one-dimensional photonic cluster states directly in the telecom C-Band as a resource for quantum repeaters and quantum computers.