Transport spectroscopy of disordered graphene quantum dots etched into a single graphene flake

We present transport measurements on quantum dots of sizes 45, 60 and 80 nm etched with an Ar/O2-plasma into a single graphene sheet, allowing a size comparison avoiding effects from different graphene flakes. The transport gaps and addition energies increase with decreasing dot size, as expected, and display a strong correlation, suggesting the same physical origin for both, i.e. disorder-induced localization in presence of a small confinement gap. Gate capacitance measurements indicate that the dot charges are located in the narrow device region as intended. A dominant role of disorder is further substantiated by the gate dependence and the magnetic field behavior, allowing only approximate identification of the electron-hole crossover and spin filling sequences. Finally, we extract a g-factor consistent with g=2 within the error bars.Comment: 5 pages, 4 (color) figure

Spin-stress and spin-strain coupling in diamond-based hybrid spin oscillator systems

Hybrid quantum systems, which combine quantum-mechanical systems with macroscopic mechanical oscillators, have attracted increasing interest as they are well suited as high-performance sensors or transducers in quantum computers. A promising candidate is based on diamond cantilevers, whose motion is coupled to embedded Nitrogen-Vacancy (NV) centers through crystal deformation. Even though this type of coupling has been investigated intensively in the past, several inconsistencies exist in available literature, and no complete and consistent theoretical description has been given thus far. To clarify and resolve these issues, we here develop a complete and consistent formalism to describe the coupling between the NV spin degree of freedom and crystal deformation in terms of stress, defined in the crystal coordinate system XYZ, and strain, defined in the four individual NV reference frames. We find that the stress-based approach is straightforward, yields compact expressions for stress-induced level shifts and therefore constitutes the preferred approach to be used in future advances in the field. In contrast, the strain-based formalism is much more complicated and requires extra care when transforming into the employed NV reference frames. Furthermore, we illustrate how the developed formalism can be employed to extract values for the spin-stress and spin-strain coupling constants from data published by Teissier et al..Comment: 14 pages, 3 figures; SOM available for download under https://quantum-sensing.physik.unibas.ch/publications/research-articles.htm

Music and Democracy

Music and Democracy explores music as a resource for societal transformation processes. This book provides recent insights into how individuals and groups used and still use music to achieve social, cultural, and political participation and bring about social change. The contributors present outstanding perspectives on the topic: From the promise and myth of democratization through music technology to the use of music in imposing authoritarian, neoliberal or even fascist political ideas in the past and present up to music's impact on political systems, governmental representation, and socio-political realities. The volume further features approaches in the fields of gender, migration, disability, and digitalization

Algebraic Properties of Lattice Polytopes Coming From Graphs

Die Arbeit besteht hauptsächlich aus zwei Teilen: einer Zusammenfassung kombinatorischer und algebraisch-geometrischer Themen (Gitterpolytope, torische (Gorenstein-)Varietäten, und Matroide), und einem Ergebnisteil. Letzerer besteht aus zwei Teilen. Im ersten Teil wird eine konstruktive Klassifikation von Multigraphen, deren graphisches Matroid ein Basispolytop erzeugt, das die Gorenstein-Eigenschaft erfüllt, erarbeitet. Im zweiten Teil wird ein Satz rekursiver Formeln, die die Ehrhartpolynome von symmetrischen Kantenpolytopen, die aus vollständig-biparten Graphen hervorgehen, zueinander in Beziehung stellen, vorgestellt. Außerdem wird Algorithmus, mit dem man solche Formel erzeugen kann, aufgezeigt.:1. Introduction 2. Notation 3. Preliminaries 3.1 Lattice Polytopes 3.2 Toric Varieties 3.3 Matroids 3.4 Gorenstein Toric Varieties 4. Results 4.1 Gorenstein Matroids 4.2 Recursive Formulas of Symmetric Edge Polytope

Spins, disorder and interactions in GaAs and graphene

This thesis describes experiments on semiconductor spin physics under the influence of diverse disorder and carrier-carrier interaction. Motivated by recent observations of GaAs spin qubit coherence limited by hyperfine coupling to nuclear-spin en- semble fluctuations, we started out to find ways to study the electron-spin nuclear-spin coupling or to avoid the nuclear spin bath altogether. This can be done in several different ways and here we pursued two fairly different approaches. One is the investigation of the dynamics of nuclear spin polarization in GaAs and the other aims at spin-related effects in graphene na- nostructures which possibly have negligible nuclear spin contributions due to the natural abundance (about 99 %) of zero- spin isotopes. The experiments on GaAs are performed using a non-local spin injection device with Fe ferromagnetic contacts on a degene- rately n-doped epilayer. At low temperatures, where the injected spin polarization allows dynamic polarization of the nuclear spins via hyperfine interaction, distinct spin signals are used to study the dynamics of the nuclear spin system both in presen- ce and absence of net electron spin polarization. The nuclear spin-lattice relaxation in an unpolarized environment reveals an unexpected breakdown of the Korringa law of nuclear spin relaxation otherwise valid for metallic systems. This is manifested in the observed deviation from a linear tem- perature dependence of the nuclear T_1 time and is interpreted as a result of hyperfine coupling to conduction electrons which are influenced by the interplay of disorder and carrier-carrier interaction. This finding therefore gives important insight into the strong influence of intimate coupling between the electron and nuclear spin sub-systems. Transport experiments on lithographically defined graphene quantum dots are performed at low temperatures. Three graphe- ne quantum dots of different nanometer sizes fabricated on a single graphene flake allow a detailed investigation of the size dependence of the Coulomb interaction, the energy spectra, and the influence of disorder within the nanostructures. The onset of Landau quantization in perpendicular magnetic fields reveals signatures of the electron-hole crossover reflecting the bandstructure symmetry of graphene. Suppression of orbital effects by applying external magnetic fields parallel to the sam- ple plane allows to address spin effects of the charge transitions in the quantum dots. The observed field dependence of Cou- lomb blockade peak splittings is not inconsistent with the Zeeman splitting proportional to an expected g-factor of 2. The transport data evidence strong influence of disorder supposably induced by both charged impurities in the close vicinity of the quantum dots and by edge disorder as a result of the fabrication process lacking precise control of the edge structures

Dressed states of a strain-driven spin in diamond

Emerging quantum technologies, such as quantum information processing and quantum metrology, require quantum systems that provide reliable toolsets for initialization, readout, and coherent manipulation as well as long coherence times. The coherence of these systems, however, is usually limited by uncontrolled interactions with the surrounding environment. In particular, innovations building on solid-state spin systems like the Nitrogen-Vacancy (NV) center in diamond ordinarily involve the use of magnetic field-sensitive states. In this case, ambient magnetic field fluctuations constitute a serious impediment that shortens the coherence time considerably. Thus, the protection of individual quantum systems from environmental perturbations constitutes a fundamentally important but also a challenging task for the further development of quantum appliances. In this thesis, we address this challenge by extending the widely used approach of dynamical decoupling to protect a quantum system from decoherence. Specifically, we study three-level dressed states that emerge under continuous, `closed-contour' interaction driving. To implement and investigate these dressed states, we exploit well-established methods for coherent microwave and strain manipulation of the NV center spin in a hybrid spin-mechanical system. Our results reveal that this novel continuous decoupling mechanism can overcome external magnetic fluctuations in a robust way. We demonstrate experimentally that the dressed states we created are long-lived and find coherence times nearly two orders of magnitude longer than the inhomogeneous dephasing time of the NV spin, even for moderate driving strengths. To realize direct and efficient access to the coherence-protected dressed states under closed-contour driving, we further demonstrate the use of state transfer protocols for their initialization and readout. In addition to an adiabatic approach, we apply recently developed protocols based on `shortcuts to adiabaticity' to accomplish the initialization process, which ultimately accelerates the transfer speed by a factor of $2.6$ compared to the fastest adiabatic protocol with similar fidelity. Moreover, we show bidirectionality of the accelerated state transfer, which allows us to directly read out the dressed state population and to quantify the transfer fidelity of $\approx$$\,99\,\%$. By employing the methods to prepare and read out the dressed states, we lay the foundation to meet the remaining key requirement for quantum systems -- coherent quantum control. We present direct, coherent manipulation of the dressed states in their own manifold and exploit this for extensive characterization of the dressed states' properties. Thus, our results constitute an elementary step to establish the dressed states as a powerful resource in prospective quantum sensing applications. Harnessing quantum systems like the dressed states as nanoscale sensors of external fields requires the detailed characterization of the local internal environment. In the final part of this thesis, we report on the determination of intrinsic effective fields of individual NV center spins. We study single NVs in high purity diamond and find that local strain dominates over local electric fields. In addition, we experimentally demonstrate and theoretically describe a new technique for performing single spin-based polarization analysis of microwave fields in a tunable, linear basis