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As new machine learning technologies are conceived, endless possibilities for breakthrough applications in science open up. We wonder how far we are from developing an artificial scientist that acts like a human one. It would be capable of making observations, analyzing data, building a model, deriving hypotheses, and designing experiments to falsify them. After an overview of common machine learning techniques, we review recent work that attempts to achieve some of these goals. We then present the main ideas that we believe are necessary for future artificial scientific discovery. In particular, we take the perspective of a physicist, and show examples from physical domains that could benefit from the presented approaches. We develop these ideas in various works. First, we focus on learning to make a high-level description of a physical system, e.g. by extracting collective variables. Therefore, we present the idea of using information theory to describe them and introduce the concept of “Renormalized Mutual Information”. Second, we study the problem of experimental design and investigate how an artificial scientist should choose which experiments to perform. We show applications of a deep learning approximation technique to quantum many-body systems. We then generalize our discussion to the problem of scientific exploration, diving into reinforcement learning techniques for intrinsic motivation as a framework for creating “curious” artificial scientists. Furthermore, we imagine ways in which an algorithm could learn concepts from experience and reuse them in different settings. We adopt the program synthesis approach and explore applications to the synthesis of quantum circuits given the associated unitary matrix. Finally, we discuss additional proof-of-concept examples and future developments in the field.

Hybrid quantum systems that couple different physical degrees of freedom for optimal functionality are essential for developing new quantum information platforms at the nanoscale.
A new class of hybrid systems based on magnonics has recently emerged, such as optomagnonic systems where light couples coherently to collective excitations in magnetically ordered systems. In this thesis, we focus on numerical methods to model these systems.
Most efforts so far, both on the theoretical and experimental level, focused on optomagnonic systems where a non-optimal overlap between the optical and magnetic modes is achieved, resulting in a relatively low coupling.
In the first part of this thesis to tackle the issue of improving the optomagnonic coupling strength, we put forward the concept of an optomagnonic crystal: a periodically patterned photonic structure at the microscale based on a magnetic dielectric, which can co-localize photon and magnon modes simultaneously.
This co-localization in a small volume can lead to considerable values of the optomagnonic coupling at the single quanta level, making these systems a suitable candidate for quantum information processing and quantum conversion schemes.
We analytically and numerically study a simple geometry consisting of a 1D array of holes with an abrupt defect, considering the ferrimagnet Yttrium Iron Garnet (YIG) as the basis magnetic material.
We show that both magnon and photon modes can be co-localized at the defect, and choosing an optimal pair of modes using symmetry arguments leads to maximized coupling values in the kHz-range.
We also discuss possible optimization routes to improve the coupling strengths and optical losses.
Since the developed simulation framework disregards effects originating from the interaction between light and the magnetization, we propose performing electromagnetic simulations incorporating the material's magnetic properties.
We therefore extend the Finite-Difference Time-Domain method to treat dispersive magnetic media by incorporating magneto-optical effects through a frequency-dependent permittivity tensor. For benchmarking the extended method, we consider the light scattering on a magnetic sphere in the Mie regime.
We first derive the analytical scattering expressions, which predict a peak broadening in the scattering efficiency due to the atomic energy level splitting in the presence of a magnetic field, together with an additional rotated part in the scattered field profile due to the Faraday rotation.
We show that our numerical method can capture the main scattering features to a remarkable extent and discuss its limitations and possible improvements in accuracy.
To reduce computational costs, especially in micromagnetic simulations for magnetic ground state prediction, we explore an artificial neural network approach to accelerate the ground state prediction of nonlinear time evolutions.
As a proof of principle, we consider a nonlinear wave evolution governed by the nonlinear Schrödinger equation in a model-free framework solved by the imaginary time evolution method.
We develop a suitable training strategy and network model to train a residual neural network based on convolutional layers to predict the evolved wave function at a desired time given the initial wave function and a random parameter set as input.
We show that the neural network approach can predict the desired evolved states for long evolution times in an accelerated fashion and discuss possible optimization routes for a better network performance.

This thesis with the title ‘Assembly of photonic nanostructures by AFM-based pick-and-place handling of individual nanoparticles inside an SEM’ was concerned with the development, setting-up, and application of a nanomanipulation system for the fabrication of particle patterns composed from individually selected and arranged nanoparticles.
This nanomanipulation system permits to collect single nanoparticles from samples, here referred to as ‘reservoir’ samples, and to transfer them individually to a new target location and even to a diﬀerent sample if required. The exchange of the samples is performed in an automated fashion whereby previously stored sample positions can be changed within seconds across centimeter distances and with a repositioning accuracy in the low single digit micrometer to sub-micrometer range. The AFM-based nanomanipulations are performed by a human operator who can interactively control the movements of the AFM tips with a 3D-controller input device with nanometer precision and under the visual observation from the SEM. Additionally, the nanomanipulation setup permits to make use of two independently movable AFM tips in a shared volume of operation in a cooperative fashion to perform nanomanipulations. This is particularly helpful for manipulating higher-dimensional nano-objects than particles, such as wires and ﬂakes, as was also demonstrated during the work on this thesis.
The AFM-based nanomanipulation setup utilizes diﬀerent commercial positioning components and was designed to integrate these components into a commercial scanning electron microscope (SEM) to form a coherent and operable nanomanipulation setup. Electronic signal ampliﬁers to measure the mechanical bending motions of piezoresistive AFM cantilevers during these nanomanipulations were realized with the support of the electronics workshop at the Max Planck Institute for the Science of Light (MPL) in Erlangen. The required software control architectures for inter-operating the constituting components of the nanomanipulation system were developed and integrated. The procedures how to align the system for nanomanipulations with automatic sample switching and how to perform pick-and-place handling of nanoparticles are described in detail.
Besides working with gold and silica nanoparticles for test purposes, nanoparticle patterns from gold and silica nanoparticles were prepared for investigating directional Kerker scattering, optical chirality in heterogeneous particle assemblies, and mode sorting in inhomogeneously sized dimers in cooperation with colleagues at the MPL. Similarly, an optical displacement sensor for nanometer-precise detection of position shifts was realized by functionalizing a photonic crystal waveguide architecture with a precisely placed silicon nanoparticle.
Diﬀerent problems encountered during the development and work with this nanomanipulation approach such as charging and carbon contaminations in the SEM environment and the local modiﬁcation of optical properties in thin ﬁlms of transparent conductive oxides (TCOs) are described. Explorative steps that were undertaken to develop pre-characterization strategies for selecting nanoparticles based on their optical properties are presented. The thesis concludes by considering potential mitigation strategies for relieving the encountered problems and for improving the nanomanipulation system itself, and by discussing prospective applications for what nanoparticle-based structures could be addressed with good prospects for future research.

We present a Raman study of MoS2 powders and MoS2 individual layers covalently functionalized with organic molecules. In MoS2 powders, the defect-induced “LA” Raman mode shows evidence for successful functionalization. Increasing temperature induces oxidation of both functionalized and nonfunctionalized MoS2 into MoO3. In contrast, mechanically exfoliated individual MoS2 layers do not transfer into MoO3 under the same conditions. Instead, the Raman spectra show that the procedure of covalent functionalization leads to a partial transition from the 2H into the 1T' crystallographic phase in few-layer MoS2. We support the identification of the 1T' phase by DFT calculations of the corresponding vibrational modes in the mono- and bilayer 1T'-MoS2.

Monte Carlo simulations of extensive air showers, their Cherenkov emission, and the
telescope response to this emission are essential to many modern Imaging Air Cherenkov
Telescope (IACT) experiments and their event reconstruction techniques. Therefore, the
validity of these simulations is of critical importance for all scientific analyses performed
with the reconstructed data. One such Imaging Air Cherenkov Telescope (IACT) array
is the High Energy Stereoscopic System (H.E.S.S.), featuring four small (CT1-4) and
one central large-mirror telescope (CT5). A recent analysis of CT5 Crab data revealed
a mismatch of ≈46 % in the reconstructed spectral norm compared to results using
CT1-4, found to be caused by an incorrect telescope response simulation. CORSIKA is
used for the air shower and Cherenkov emission simulation and sim telarray to simulate
the telescope response. This work presents the author’s significant contributions to
this Monte Carlo validation effort. Multiple discrepancies between observations and
simulations are identified and include the size of the optical Point Spread Function
(PSF), the atmospheric transmission profile, the telescope trigger thresholds, the mirror
reflectivities, and the Night Sky Background (NSB) settings. The observed deviations
are reduced to satisfactory levels, resulting in a better match between the simulated
and observed telescope trigger rates. Post-validation, the deviation of the reconstructed
spectral norm is reduced to 11 % and now matches previous results within systematic
errors.
A 3D analysis of Crab data using the ABRIR method for improved background rejection
is presented and compared to the results obtained with the existing BDT approach. No
statistically significant difference is found for the derived spectral and spatial parameters.
Whilst the significance of calculated flux points below 10 TeV is reduced, it rises again
for higher energies when the ABRIR method is used.
Background events can also be rejected by an array of telescopes that issue an event-wise
veto. A candidate for such a telescope is the IceACT telescope. It is a low-cost (<10 ke)
and compact IACT conceived initially as part of the surface detector of IceCube. This
work presents a simulation of a modified version of this telescope with TARGET, based
on CORSIKA and sim telarray. The simulation implementation is discussed extensively
and validated on data acquired during a joint measurements campaign with the H.E.S.S.
array in early 2019. The determined proton energy threshold Ep
th = (11.3 ± 0.5) TeV is
in agreement with previous estimates.

Designer quantum materials represent a modern branch of solid state physics with the aim to control or specifically design the fundamental properties of quantum materials. This can be achieved by both (i) the controlled modification and functionalization of existing materials or (ii) the synthesis of entirely new systems which exhibit the desired properties. The present thesis demonstrates the aptness of epitaxial graphene on SiC as a designer quantum material in the spirit of both the above approaches. This is mediated by means of intercalation of other chemical elements at the graphene/SiC
interface. In the sense of concept (i), it is the electronic structure of graphene itself and the induced, extremely high charge carrier density that could be tuned over a wide range across the van Hove singularity on the basis of ytterbium intercalation. This might pave the way towards the first realization of exotic ordered ground states in a true graphene monolayer. In view of approach (ii), the graphene/SiC interface was further shown to enable the large-area synthesis of an otherwise unstable, unique monolayer of intercalated silver atoms with semiconducting character. The detailed electronic properties of the created van der Waals heterostructure were subsequently modified by depositing an ordered alkali metal monolayer on top – hence closing the loop back to concept (i). Fabricated samples have been characterized by established surface science techniques: x-ray photoelectron spectroscopy, low-energy electron diffraction, and – most importantly – (synchrotron-based) angle-resolved photoemission spectroscopy for a detailed study of the electronic band structure.

We study the competition between two different topological orders in three dimensions by considering the X-cube model and the three-dimensional toric code. The corresponding Hamiltonian can be decomposed into two commuting parts, one of which displaying a self-dual spectrum. To determine the phase diagram, we compute the high-order series expansions of the ground-state energy in all limiting cases. Apart from the topological order related to the toric code and the fractonic order related to the X-cube model, we found two new phases which are adiabatically connected to classical limits with nontrivial sub-extensive degeneracies. All phase transitions are found to be first order.

Historically, thermal radiation is related to 3D cavities. In practice, however, it is known that almost any hot surface radiates according to Planck’s law. This approximate universality roots in the smooth electromagnetic mode structure of free space, into which the radiation is emitted. Here, we study the effect for a strongly patterned mode structure and use quasi-transparent point-like thermal light emitters as a probe. As such, we choose current-driven graphene nanojunctions for which the emission into free space obeys Planck’s law. Placed in front of a mirror, however, this process is highly sensitive to a node/antinode pattern of light modes. By varying the distance, we can sample the latter with atomic precision, and observe a deep imprint on the observed spectrum. The experiment allows an unprecedented view on thermal radiation in a spatially/spectrally patterned electromagnetic environment.

Fingerprints of magnetoinduced charge density waves in monolayer graphene beyond half filling
(2022)

A charge density wave is a condensate of fermions, whose charge density shows a long-range periodic modulation. Such charge density wave can be principally described as a macroscopic quantum state and is known to occur by various formation mechanisms. These are the lattice deforming Peierls transition, the directional, fermionic wave vector orientation prone Fermi surface nesting or the generic charge ordering, which in contrast is associated solely with the undirected effective Coulomb interaction between fermions. In two-dimensional Dirac/Weyl-like systems, the existence of charge density waves is only theoretically predicted within the ultralow energy regime at half filling. Taking graphene as host of two-dimensional fermions described by a Dirac/Weyl Hamiltonian, we tuned indirectly the effective mutual Coulomb interaction between fermions through adsorption of tetracyanoquinodimethane on top in the low coverage limit. We thereby achieved the development of a novel, low-dimensional dissipative charge density wave of Weyl-like fermions, even beyond half filling with additional magneto-induced localization and quantization. This charge density wave appears both, in the electron and the hole spectrum.

This article reports the findings of a qualitative study that aimed to explore the ideas of 22 preservice physics teachers regarding astronomy concepts both within and beyond the solar system, as well as their understanding of the Nature of Science in the context of astronomy. The study employed a combination of open-ended vignette tasks and ranking tasks, which were adapted from previous research. The results reveal that while the majority of the preservice teachers held appropriate conceptions of astronomy concepts, some of their ideas lacked depth and were superficial. Moreover, their understanding of the Nature of Science in the context of astronomy was found to be limited. This study highlights the importance of providing further education and training in this area, as well as the need to develop and test effective teaching strategies to enhance preservice teachers’ and physics education students’ understanding of astronomy concepts and the Nature of Science.

Abstract
Broadband optical frequency combs are extremely versatile tools for precision spectroscopy, ultrafast ranging, as channel generators for telecom networks, and for many other metrology applications. Here, we demonstrate that the optical spectrum of a soliton microcomb generated in a microresonator can be extended by bichromatic pumping: one laser with a wavelength in the anomalous dispersion regime of the microresonator generates a bright soliton microcomb while another laser in the normal dispersion regime both compensates the thermal effect of the microresonator and generates a repetition-rate-synchronized second frequency comb. Numerical simulations agree well with experimental results and reveal that a bright optical pulse from the second pump is passively formed in the normal dispersion regime and trapped by the primary soliton. In addition, we demonstrate that a dispersive wave can be generated and influenced by cross-phase-modulation-mediated repetition-rate synchronization of the two combs. The demonstrated technique provides an alternative way to generate broadband microcombs and enables the selective enhancement of optical power in specific parts of a comb spectrum.

Abstract.
Active matter systems often are well approximated as overdamped, meaning that any inertial momentum is immediately dissipated by the environment. On the other hand, especially for macroscopic systems but also for many mesoscopic ones particle mass can become relevant for the dynamics. For such systems we recently proposed an underdamped continuum model which captures translationally inertial dynamics via two contributions. First, convection and second a damping time scale of inertial motion. In this paper, we ask how both of these features influence the collective behavior compared to overdamped dynamics by studying the example of the active phase field crystal model. We first focus on the case of suppressed convection to study the role of the damping time. We quantify that the relaxation process to the steady collective motion state is considerably prolonged with damping time due to the increasing occurrence of transient groups of circularly moving density peaks. Finally, we illustrate the fully underdamped case with convection. Instead of collective motion of density peaks we then find a coexistence of constant high and low density phases reminiscent of motility-induced phase separation.
Graphical abstract

Abstract
As light propagates along a waveguide, a fraction of the field can be reflected by Rayleigh scatterers. In high-quality-factor whispering-gallery-mode microresonators, this intrinsic backscattering is primarily caused by either surface or bulk material imperfections. For several types of microresonator-based experiments and applications, minimal backscattering in the cavity is of critical importance, and thus, the ability to suppress backscattering is essential. We demonstrate that the introduction of an additional scatterer into the near field of a high-quality-factor microresonator can coherently suppress the amount of backscattering in the microresonator by more than 30 dB. The method relies on controlling the scatterer position such that the intrinsic and scatterer-induced backpropagating fields destructively interfere. This technique is useful in microresonator applications where backscattering is currently limiting the performance of devices, such as ring-laser gyroscopes and dual frequency combs, which both suffer from injection locking. Moreover, these findings are of interest for integrated photonic circuits in which back reflections could negatively impact the stability of laser sources or other components.

Abstract
We investigate the frequency-resolved intensity noise spectrum of an Yb-doped fiber amplifier down to the fundamental limit of quantum noise. We focus on the kHz and low MHz frequency regime with special interest in the region between 1 and 10 kHz. Intensity noise levels up to ≥60 dB above the shot noise limit are found, revealing great optimization potential. Additionally, two seed lasers with different noise characteristics were amplified, showing that the seed source has a significant impact and should be considered in the design of high power fiber amplifiers.

We consider the ideal Fermi gas of indistinguishable particles without spin but with electric charge, confined to a Euclidean plane R2 perpendicular to an external constant magnetic field of strength B>0. We assume this (infinite) quantum gas to be in thermal equilibrium at zero temperature, that is, in its ground state with chemical potential μ≥B (in suitable physical units). For this (pure) state we define its local entropy S(Λ) associated with a bounded (sub)region Λ⊂R2 as the von Neumann entropy of the (mixed) local substate obtained by reducing the infinite-area ground state to this region Λ of finite area |Λ|. In this setting we prove that the leading asymptotic growth of S(LΛ), as the dimensionless scaling parameter L>0 tends to infinity, has the form LB−−√|∂Λ| up to a precisely given (positive multiplicative) coefficient which is independent of Λ and dependent on B and μ only through the integer part of (μ/B−1)/2. Here we have assumed the boundary curve ∂Λ of Λ to be sufficiently smooth which, in particular, ensures that its arc length |∂Λ| is well-defined. This result is in agreement with a so-called area-law scaling (for two spatial dimensions). It contrasts the zero-field case B=0, where an additional logarithmic factor ln(L) is known to be present. We also have a similar result, with a slightly more explicit coefficient, for the simpler situation where the underlying single-particle Hamiltonian, known as the Landau Hamiltonian, is restricted from its natural Hilbert space L2(R2) to the eigenspace of a single but arbitrary Landau level. Both results extend to the whole one-parameter family of quantum Rényi entropies. As opposed to the case B=0, the corresponding asymptotic coefficients depend on the Rényi index in a non-trivial way.

Das Verstemmverfahren ist durch hohe Prozesskräfte gekennzeichnet, welche den Einsatz des Fügeverfahrens stark eingrenzen. Die Überlagerung mechanischer Füge- und Umformprozesse mit Ultraschall zeigt großes Potential Fügekräfte zu reduzieren und damit Prozessgrenzen zu erweitern. Mangelnde Prozesskenntnisse verhindern jedoch bislang die industrielle Anwendung ultraschallüberlagerter Verfahren. Im Rahmen dieser Arbeit wurde ein grundlegendes Prozessverständnis für den ultraschallüberlagerten Verstemmprozess an hochlegiertem Edelstahl erarbeitet und die kraftreduzierenden Wirkmechanismen der Schwingungsüberlagerung identifiziert. Durch die Ultraschallüberlagerung konnte dabei im Experiment die Prozesskraft um bis zu 59 %, bei vergleichbarer Verbindungsfestigkeit gesenkt werden. Die maßgeblichen Mechanismen der Kraftreduzierung konnten auf dynamische Einflüsse über das Superpositionsprinzip sowie tribologische Einflüsse durch eine verringerte Oberflächenreibung zurückgeführt werden. Überdies führt eine leichte Taumelbewegung des Umformstempels, bedingt durch die dynamische Anregung, für eine zusätzliche Reduzierung der Prozesskraft. In Kombination mit der vergleichbaren Verbindungsfestigkeit birgt die reduzierte Verstemmkraft ein beträchtliches Potential zur Erweiterung der Prozessgrenzen des Fügeverfahrens.

We consider an extended phase space formulation for cosmological and spherically symmetric models in which the choice of a given μ-scheme can be implemented dynamically. These models are constructed in the context of the relational formalism by using a canonical transformation on the extended phase space, which provides a Kuchař decomposition of the extended phase space. The resulting model can be understood as a gauge-unfixed model of a given μ-scheme. We use this formalism to investigate the restrictions to the allowed μ-scheme from this perspective and discuss the differences in the cosmological and spherically symmetric case. This method can be useful, for example, to obtain a μ-scheme in a top-down derivation from full LQG to symmetry-reduced effective models, where, for some models, only the μ0-scheme has been obtained thus far.

Introduction: Accurate diagnosis and personalized treatments involving site-targeted cancer localization, drug delivery, therapeutic strategy, and disease pathways identification, rely on a precise understanding of biomarker kinetics, drug pharmacokinetics, and mechanistic behaviour of functionalized tracers through in vitro and in vivo studies. X-ray fluorescence (XRF) computed tomography (XFCT) offers a potential alternative to current 3D imaging techniques for spatiotemporal localization of nanoparticle-tracers with high spatial resolution and sensitivity. In this work, the applicability of a benchtop cone-beam system with a polychromatic X-ray source was examined with regard to physical constraints of engineered tissue models.
Methods: A tissue engineering approach based on a decellularized scaffold was used to establish a 3D breast cancer model with MDA-MB-231 cells in co-culture with primary human fibroblasts. The 3D breast cancer system, in combination with small-animal-sized phantoms, was used to demonstrate the novel integrated pre-clinical imaging approach to perform in vitro surrogate investigations and non-destructive analysis on biophantoms. These models are adopted to evaluate the functionality and optimize the setup for high-spatial-resolution, fast, and fully-3D quantitative imaging. Polychromatic X-rays from a microfocus source are used for XRF stimulation from conventional Gadolinium (Gd) and nanoparticle-based Molybdenum (MoNPs) contrast agents.
Results and Discussion: The intestinal scaffold allowed the invasion of the breast cancer cells over this barrier and therefore provides a valuable tool to study metastasis formation of tumor cells from epithelial origin. The breast cancer model was well suited for the development and validation of the proposed XRF imaging, with spatial resolution under <2 mm and contrast dose in the order of a few 100 μg/mL (∼0.3 mg/mL for Gd and ∼0.5 mg/mL for MoNPs), radiation dose in the order of a few 100 cGy (280 cGy for Gd and 94 cGy for MoNPs, with a possible reduction of an order of magnitude for Gd and 67% for MoNPs), and imaging time in the order of 10 min for Gd (33 min total) and 100 min (2.8 h total) for MoNPs, approaching in vivo conform conditions for pre-clinical studies. High-resolution XFCT for tissue-engineered cancer models would be of significant interest in biomedical research and diagnostic imaging, e.g., for an increased mechanistic understanding of molecular processes in tumor formation or early cancer detection.

Abstract
Graphene‐based materials are among the most promising candidates for studying superconductivity arising from reduced dimensionality. Apart from doping by twisted stacking, superconductivity can also be achieved by metal‐intercalation of stacked graphene sheets, where the properties depend on the choice of the metal atoms and the number of graphene layers. Many different and even unconventional pairing mechanisms and symmetries are predicted in the literature for graphene monolayers and few‐layers. However, those theoretical predictions have yet to be verified experimentally. Here, it is shown that potassium‐intercalated epitaxial bilayer graphene is a superconductor with a critical temperature of Tc = 3.6 ± 0.1 K. By scanning‐tunneling microscopy and angle‐resolved photoelectron spectroscopy, the physical mechanisms are analyzed in great detail, using laboratory equipment. The data demonstrate that electron–phonon coupling is the driving force enabling superconductivity. Although the consideration of an s‐wave pairing symmetry is sufficient to explain the experimental data, evidence is found for the existence of multiple energy gaps. Furthermore, it is shown that low‐dimensional effects are most likely the cause of a gap ratio of 6.1 ± 0.2 that strongly exceeds the Bardeen‐Cooper‐Schrieffer (BCS) value of 3.52 for conventional superconductors. These results highlight the importance of reduced dimensionality yielding unusual superconducting properties of K‐intercalated epitaxial bilayer graphene.

Zusammenfassung
Ultrakurze und intensive Laserimpulse ermöglichen es, Elektronen in Festkörpern auf der Attosekunden‐Zeitskala zu lenken. In Graphen‐Gold‐Strukturen lassen sich dabei lichtfeldinduzierte Ströme unterscheiden, die aus der Anregung von realen oder virtuellen Ladungsträgern resultieren. Mithilfe der zeitlichen Symmetrie der genauen Laser‐Wellenform lassen sich die zwei Arten von Ladungsträgern individuell ansprechen. Das hat es uns ermöglicht, ein potenziell lichtschnelles logisches Gatter zu realisieren.