08 Fakultät Mathematik und Physik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9
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Item Open Access Self-organized structures and excitations in dipolar quantum fluids(2024) Hertkorn, Jens; Pfau, Tilman (Prof. Dr.)Quantum many-body phenomena at a macroscopic scale, such as superfluidity and superconductivity, are rooted in the interplay between microscopic particles, governed by the laws of quantum mechanics. Exploring how this interplay leads to quantum behavior at a large scale allows us to gain a deeper understanding of nature and to discover new quantum phases. An elusive quantum phase in which the frictionless flow of superfluids and the crystal structure of solids coexists - the supersolid - was recently realized with quantum droplets in dipolar Bose-Einstein condensates. In this thesis we investigate self-organized structures, their formation mechanism, and excitations in dipolar quantum fluids created from such Bose-Einstein condensates. We show that the supersolid formation mechanism is driven by density fluctuations due to low-energy roton excitations, leading to a crystal structure of quantum droplets that are immersed in a superfluid background. These roton excitations split into a Goldstone mode and a Higgs amplitude mode, associated to the broken translational symmetry in the supersolid. We investigate the symmetry breaking of dipolar quantum fluids in a range of confinement geometries and establish a comprehensive description of elementary excitations across the superfluid to supersolid droplet phase transition. The droplets are stabilized by an interplay between interactions and the presence of quantum fluctuations. We show how this interplay can be used to find regimes where droplets are immersed in a high superfluid background, allowing for frictionless flow throughout the crystal. Moreover we show that towards higher densities beyond the quantum droplet phase, this interplay leads to several new self-organized structures in the phase diagram of dipolar quantum fluids. We theoretically predict new supersolid honeycomb, amorphous labyrinth, and other phases in oblate dipolar quantum fluids. Finally, we present a new experimental setup for the exploration of self-organized phases in dipolar quantum fluids and which also lays the foundation for the implementation of a quantum gas microscope. The results of this thesis present a complete framework for understanding and creating exotic phases in dipolar quantum fluids. The versatile structure formation, governed by a competition of controllable interactions and the presence of quantum fluctuations, positions dipolar quantum fluids as a model system for exploring self-organized equilibrium in weakly-interacting quantum many-body systems.Item Open Access Cavity QED based on room temperature atoms interacting with a photonic crystal cavity : a feasibility study(2020) Alaeian, Hadiseh; Ritter, Ralf; Basic, Muamera; Löw, Robert; Pfau, TilmanThe paradigm of cavity QED is a two-level emitter interacting with a high-quality factor single-mode optical resonator. The hybridization of the emitter and photon wave functions mandates large vacuum Rabi frequencies and long coherence times; features that so far have been successfully realized with trapped cold atoms and ions, and localized solid-state quantum emitters such as superconducting circuits, quantum dots, and color centers Reiserer and Rempe (Rev Modern Phys 87:1379, 2015), Faraon et al. (Phys Rev 81:033838, 2010). Thermal atoms, on the other hand, provide us with a dense emitter ensemble and in comparison to the cold systems are more compatible with integration, hence enabling large-scale quantum systems. However, their thermal motion and large transit-time broadening is a major bottleneck that has to be circumvented. A promising remedy could benefit from the highly controllable and tunable electromagnetic fields of a nano-photonic cavity with strong local electric-field enhancements. Utilizing this feature, here we investigate the interaction between fast moving thermal atoms and a nano-beam photonic crystal cavity (PCC) with large quality factor and small mode volume. Through fully quantum mechanical calculations, including Casimir-Polder potential (i.e. the effect of the surface on radiation properties of an atom), we show, when designed properly, the achievable coupling between the flying atom and the cavity photon would be strong enough to lead to quantum interference effects in spite of short interaction times. In addition, the time-resolved detection of different trajectories can be used to identify single and multiple atom counts. This probabilistic approach will find applications in cavity QED studies in dense atomic media and paves the way towards realizing large-scale, room-temperature macroscopic quantum systems aimed at out of the lab quantum devices.Item Open Access Vibrational quenching of weakly bound cold molecular ions immersed in their parent gas(2020) Jachymski, Krzysztof; Meinert, FlorianHybrid ion–atom systems provide an excellent platform for studies of state-resolved quantum chemistry at low temperatures, where quantum effects may be prevalent. Here we study theoretically the process of vibrational relaxation of an initially weakly bound molecular ion due to collisions with the background gas atoms. We show that this inelastic process is governed by the universal long-range part of the interaction potential, which allows for using simplified model potentials applicable to multiple atomic species. The product distribution after the collision can be estimated by making use of the distorted wave Born approximation. We find that the inelastic collisions lead predominantly to small changes in the binding energy of the molecular ion.Item Open Access Continuous wave Doppler-free spectroscopy on the 𝐀 𝟐𝚺+ ← 𝐗 𝟐𝚷𝟑/𝟐 transition in thermal nitric oxide(2022) Kaspar, Patrick; Pfau, Tilman (Prof. Dr.)The framework of this thesis is given by the development of a laboratory prototype for a new kind of gas sensing scheme to detect smallest quantities of nitric oxide in a large background of other gases. This thesis presents the underlying concepts of the sensing principle and some of the technical aspects developed for its investigation. In addition, the technique of Doppler-free saturated absorption spectroscopy, which is a standard technique in atomic physics, was applied to thermal nitric oxide molecules. It enabled direct resolution of the hyperfine structure and the determination of the corresponding hyperfine constants for the involved excited state. The results show the capabilities of the application of this technique to thermal molecules and in addition prove that saturation on the investigated transition is possible. This is an important result in terms of the overall goal the development of the trace gas sensor prototype.Item Open Access Nonlinear optics in dense atomic vapors(2022) Christaller, Florian; Pfau, Tilman (Prof. Dr.)In this thesis, two building blocks for our next generation single-photon source, based on thermal rubidium Rydberg atoms, have been investigated. One of these is the coherent excitation of atoms with lasers in a four-wave mixing process on the nanosecond timescale. The second effect is the light-induced atomic desorption, where the atomic density is increased by an off-resonant laser pulse in a micrometer sized vapor cell on the nanosecond timescale.Item Open Access Laser cooling of barium monofluoride(2024) Rockenhäuser, Marian; Pfau, Tilman (Prof. Dr.)In this work, the first implementation of direct laser cooling of barium monofluoride (BaF) molecules using sub-Doppler forces is presented. This species is a promising candidate for parity violation measurements, the search for the electron’s permanent electric dipole moment and ultracold chemistry. However, due to its large mass, comparatively narrow linewidth and potential branching losses through an intermediate electronic state, this molecular species is notoriously difficult to cool. To achieve laser cooling, first, spectroscopic measurements of the relevant optical transitions were performed. This allowed for an improvement of the molecular constants by one order of magnitude. Next, Doppler-free spectroscopy was conducted on the cooling transition, which revealed a resolved hyperfine splitting in the excited state. Previous molecular laser cooling experiments employed sinusoidal sideband modulation to address such hyperfine structure states. Here, serrodyne modulation was used to create optimized optical spectra, resulting in significantly improved laser cooling. Finally, a Raman cycling scheme was implemented to achieve background-free imaging of the resulting cold molecular beam. In conclusion, an intense and transversally cold molecular beam of 138BaF was prepared, which paves the way for precision tests of fundamental symmetries using BaF.Item Open Access An approach to quantum physics teaching through analog experiments(2022) Aehle, Stefan; Scheiger, Philipp; Cartarius, HolgerWith quantum physics being a particularly difficult subject to teach because of its contextual distance from everyday life, the need for multiperspective teaching material arises. Quantum physics education aims at exploring these methods but often lacks physical models and haptic components. In this paper, we provide two analog models and corresponding teaching concepts that present analogies to quantum phenomena for implementation in secondary school and university classrooms: While the first model focuses on the polarization of single photons and the deduction of reasoning tools for elementary comprehension of quantum theory, the second model investigates analog Hardy experiments as an alternative to Bell’s theorem. We show how working with physical models to compare classical and quantum perspectives has proven helpful for novice learners to grasp the abstract nature of quantum experiments and discuss our findings as an addition to existing quantum physics teaching concepts.Item Open Access Electrical detection of Rydberg interactions in nitric oxide at room temperature(2023) Munkes, Fabian; Pfau, Tilman (Prof. Dr.)In this work I will present measurements of Rydberg states in nitric oxide (NO) at room temperature. The detection of the Rydberg states is realized by measuring the current of free charges resulting from collisions of the excited molecules. All measurements are performed using continuous-wave (cw) lasers in a sub-Doppler configuration, which together with a stabilization setup yield a frequency error of only 2𝜋 × 2.5 MHz. The full width at half maximum (FWHM) of a typical Rydberg state is only about 2𝜋 × 130 MHz. We take a look at the necessary theory of diatomic molecules first. Afterward, a thorough walkthrough of the experimental setup is given. The heart of our setup is a custom-designed measurement cell, which features readout electronics based on a transimpedance amplifier (TIA). As such I will also give an overview on the basics of operational amplifiers (OpAmps). When all prerequisites are introduced, we will take an in-depth look on the Stark effect in Rydberg states. To our knowledge, the presented resolution is unmatched, and may enable us to give a more precise value to the g–quantum defect in NO in the future. In a final experimental section I show the collisional broadening and shift of Rydberg states of NO due to an increasing background gas density. Such measurements have a long history in alkalis, yet to our knowledge, no such measurements in NO exist. The overall experiment is performed in the context of a trace-gas sensor for NO in a medical application. This work gives suitable density and electric field ranges for such a sensor.Item Open Access Coherent interaction of atoms with a beam of light confined in a light cage(2021) Davidson-Marquis, Flavie; Gargiulo, Julian; Gómez-López, Esteban; Jang, Bumjoon; Kroh, Tim; Müller, Chris; Ziegler, Mario; Maier, Stefan A.; Kübler, Harald; Schmidt, Markus A.; Benson, OliverControlling coherent interaction between optical fields and quantum systems in scalable, integrated platforms is essential for quantum technologies. Miniaturised, warm alkali-vapour cells integrated with on-chip photonic devices represent an attractive system, in particular for delay or storage of a single-photon quantum state. Hollow-core fibres or planar waveguides are widely used to confine light over long distances enhancing light-matter interaction in atomic-vapour cells. However, they suffer from inefficient filling times, enhanced dephasing for atoms near the surfaces, and limited light-matter overlap. We report here on the observation of modified electromagnetically induced transparency for a non-diffractive beam of light in an on-chip, laterally-accessible hollow-core light cage. Atomic layer deposition of an alumina nanofilm onto the light-cage structure was utilised to precisely tune the high-transmission spectral region of the light-cage mode to the operation wavelength of the atomic transition, while additionally protecting the polymer against the corrosive alkali vapour. The experiments show strong, coherent light-matter coupling over lengths substantially exceeding the Rayleigh range. Additionally, the stable non-degrading performance and extreme versatility of the light cage provide an excellent basis for a manifold of quantum-storage and quantum-nonlinear applications, highlighting it as a compelling candidate for all-on-chip, integrable, low-cost, vapour-based photon delay.Item Open Access On the development of activating teaching materials in theoretical physics(2024) Scheiger, Philipp; Nawrodt, Ronny (Prof. Dr.)Die Arbeit beschreibt die Entwicklung und Testung von interaktiven und kognitiv aktivierenden Lehrmethoden für die Theoretische Physik. Basierend auf aktuellen Studienabbruchraten und fachdidaktischer Forschung wird hier ein Handlungsbedarf ausgemacht. Aktivierende Materialien und Lehrkonzepte für fortgeschrittene Physikvorlesungen im deutschsprachigen Raum können hierbei helfen. Die Arbeit adressiert diese Lücke, indem sie Materialien und Methoden entwickelt, die den Bedürfnissen der Studierenden entsprechen und einfach von anderen Dozenten und Tutoren übernommen werden können. Probleme wie mangelnde Mathematikausbildung und Schwierigkeiten beim Verständnis physikalischer Konzepte werden durch verschiedene didaktische Ansätze angegangen, einschließlich der Peer Instruction und der didaktischen Rekonstruktion. Ein Schwerpunkt liegt zudem auf der Ausbildung von Lehramtsstudierenden und der Verbindung zwischen Theoretischer Physik und Schulphysik. Dies beinhaltet die Entwicklung neuer Seminare, die den Nutzen der Theoretischen Physik für angehende Lehrkräfte betonen und auf den diskutierten Methoden aufbauen.