Testing the mass-proportional CSL model, that describes quantum-mechanical wave function collapse, by measuring a small offset in energy due to that same collapse, requires ultra-low temperatures....Show moreTesting the mass-proportional CSL model, that describes quantum-mechanical wave function collapse, by measuring a small offset in energy due to that same collapse, requires ultra-low temperatures. These temperatures can be reached, using adiabatic nuclear demagnetization as a refrigeration method. To obtain the lowest temperature possible and to do so for a long time, dissipation has to be minimized. Theoretical work in this thesis provides a way to decrease dissipation through an optimized demagnetization ramp, resulting in a final magnetic field of 5 mT and a field ramp rate of 0.5 mT/s. Experimentally, a decrease in dissipation is found by comparing demagnetization ramps with and without an LCR circuit. The ramp with such a circuit has approximately 2.5 times less dissipation. Also discussed in this thesis is SQUID thermometry, a reliable way of measuring the temperature at ultra-low temperatures. An analysis method is presented to reduce the influence of mechanical interference when determining the temperature.Show less
Well isolated mechanical systems have the potential to be developed into systems for magnetic, accelerometric and gravitational sensing, as well as to investigate the limits of quantum theory. This...Show moreWell isolated mechanical systems have the potential to be developed into systems for magnetic, accelerometric and gravitational sensing, as well as to investigate the limits of quantum theory. This holds especially for mechanical resonators which consist of levitated nano- and microparticles, since an advantage of this type of system is the lack of clamping losses, potentially resulting in an extremely low energy dissipation. Here, a mechanical resonator is presented, where a magnetic microparticle is levitated in a cylindrical trap of a type I superconductor. SQUID detection has been used to measure the vibrational modes of the particle. The damping factors of the resonator have been analytically calculated, resulting in an expected quality factor Q of 10^11. The coupling and energy of the six translational and rotational rigid body modes of the particle have been simulated, based on analytical approximations. Experimentally, a resonance is detected with a damping time of 47 seconds and a Q of 2.2*10^4. These are promising first results, since this difference in damping and Q factor can be explained as the Earth's magnetic field was trapped inside the experiment. With these complications resolved, an extremely sensitive micromechanical resonator can be developed. This opens a new road in the investigation of the boundary between the quantum and classical regime and gravitational research.Show less
Magnetic resonance force microscopy (MRFM) is a scanning probe technique capable of producing three-dimensional images with nanometer-scale spatial resolution. MRFM relies on the mechanical...Show moreMagnetic resonance force microscopy (MRFM) is a scanning probe technique capable of producing three-dimensional images with nanometer-scale spatial resolution. MRFM relies on the mechanical detection of a weak and oscillating magnetic force between a tip magnet attached to a high compliance cantilever and magnetic moments. Measuring a single electron spin (abbreviated as single-spin) would open the way towards a macroscopic spin-cantilever superposition and three-dimensional images of molecular complexes, e.g. protein structure, with angstrom precision. Although single-spin detection has already been accomplished at 1.6 K, we aim to repeat this feat at millikelvin temperatures to achieve an improved force sensitivity and reduced thermal noise. In this thesis we report on the requirements a setup has to satisfy to enable the detection of an individual spin at millikelvin temperatures. These conditions are drastically more stringent compared to the prerequisites of single-spin detection at a temperature of several kelvin. Moreover, it turned out that our setup does not meet the criteria so we studied several technical enhancements that bring single-spin detection within reach, such as a sample with a lower spin density, nanometer-scale probe magnets and nanometer-sized cantilevers. Provided that these improvements are implemented successfully, detection of an individual spin at millikelvin temperatures appears to be feasible. Furthermore, we present several test experiments with a novel piezoelectric based vibration isolation device. This damping apparatus was designed to actively reduce the level of environmental vibrations near the sample stage, which is required to be ultra-low to achieve a sufficiently large superposition to measure a visible interference.Show less
The study of stepped surfaces is fundamental to the fields of catalysis, nanostructure and chemical surface bonding. In this thesis, we characterize the range of surface structures present in a...Show moreThe study of stepped surfaces is fundamental to the fields of catalysis, nanostructure and chemical surface bonding. In this thesis, we characterize the range of surface structures present in a curved platinum crystal, miscut such that the curvature is perpendicular to the [112] direction. Surface preparations include argon sputtering and annealing cycles. Auger electron spectroscopy has been used to confirm the chemical composition at the surface and low energy electron diffraction has been used to determine the surface structure. Scanning tunneling microscopy was used to image the surface and study its terrace width distribution and step density. Vacancy islands were studied to determine the chirality of the surface. A short to long edge ratio of 0.65 +/- 0.06 is found in vacancy islands. Quantifying defect sites shows a surplus of reactive sites on the surface where step density is lower than 0.005 /nm.Show less
In this thesis, we address our progress to send high currents and generating high magnetic fields at milliKelvin temperatures for the use in MRFM measurements. Multiple ways for sending a current...Show moreIn this thesis, we address our progress to send high currents and generating high magnetic fields at milliKelvin temperatures for the use in MRFM measurements. Multiple ways for sending a current while at 20mK inside a dilution refrigerator are described. The use of a heatsink and an option for splitting the current over multifillament connections are analyzed and tested. We find starting resistances in our spotwelds contradicting with earlier measurements of 3pΩ conducted in our group. Next, the design of a transformer in the form of a cone complement is showed and preliminary tests are presented. Furthermore, the inductance is calculated from a sweep over a frequency range from 500Hz to 20kHz. Our measurements show high potential for an experiment to generate 500mT at 20mK. This experiment is described and in addition, a possible use for B1-fields of this cone complement coil is briefly discussed.Show less
We performed numerical simulations to verify a model of spontaneous collapse of the wavefunction by an infinitesimally small, non-unitary noise field. This noise field breaks time reversal and...Show moreWe performed numerical simulations to verify a model of spontaneous collapse of the wavefunction by an infinitesimally small, non-unitary noise field. This noise field breaks time reversal and translation symmetry of the Hamiltonian and depends only linearly on the wavefunction itself. We found that the probabilty of collapse from a uniform wavefunction goes to infinity in the limit of continuous space and depends on B^{2}m. Here B is the strength of the noise and m the mass of the system. Furthermore, we investigated the stability of a collapsed state and found that this depends on the shape of the noise distribution: an asymmetrical, positive distribution seems to enhance the stabilty. We finally performed analytical calculations to further understand this dependency and the role of normalisation of the wavefunction in our model.Show less
Curved crystals provide the possibility to study different vicinal surfaces in a single crystal. In this thesis, we analyzed a curved Pt (111) single crystal with kinked steps under UHV conditions...Show moreCurved crystals provide the possibility to study different vicinal surfaces in a single crystal. In this thesis, we analyzed a curved Pt (111) single crystal with kinked steps under UHV conditions using STM images to characterize the sample. We present and compare terrace width histograms of different images taken on the crystal.Show less
Majorana quasiparticles can arise at the ends of one dimensional superconducting quantum wires. As a true one-dimensional experimental realization of such a quantum wire, a carbon nanotube can be...Show moreMajorana quasiparticles can arise at the ends of one dimensional superconducting quantum wires. As a true one-dimensional experimental realization of such a quantum wire, a carbon nanotube can be employed. The one-dimensional carbon nanotube can inherit the superconducting properties by bringing it in close contact to a superconductor, such as van der Waals superconductor niobium diselenide. In this work, the design and fabrication of devices are discussed that contain a carbon nanotube connected to a few atomic layer thick niobium diselenide. Bi- or trilayers of niobium diselenide crystal flakes are obtained using mechanical exfoliation techniques in inert conditions. A stamping resolution of about 1 mm is obtained in positioning the flakes with respect to each other which is shown by Atomic Force Microscopy. Initial characterizations have shown an improvement of the conductance of more than two orders of magnitude. The stability diagrams, however, exhibit a complex behaviour which we were unable to explain in terms of a proximitized carbon nanotube. We have shown that it is possible to fabricate the complete hybrid device that is predicted to host Majorana Fermions in its non-trivial phase. This work was conducted at Regensburg University in the group of Professor Christoph Strunk.Show less
While Magnetic Resonance Force Microscopy is capable of imaging three dimensional structures on nanoscopic scales, the number of practical applications so far has been limited, partly due to the...Show moreWhile Magnetic Resonance Force Microscopy is capable of imaging three dimensional structures on nanoscopic scales, the number of practical applications so far has been limited, partly due to the complexity of the device. In this thesis we introduce the Easy MRFM, a way to increase the usability of MRFM. By seperating all MRFM components from the sample, we hope to remove some of the drawbacks of the previous Oosterkamp MRFM, allowing for easier data analysis and sample exchange. This thesis provides the theoretical calculations for the optimal set-up of the Easy MRFM and a preliminary proof of concept. Furthermore it describes a new way to analyse the measurements of the cantilever properties which would be better suited for the Easy MRFM. It also It includes the characterization of a new cantilever which could possibly be used inside the Easy MRFM to increase its sensitivity.Show less
For a phenomenological explanation of the absence of macroscopic superpositions, mass-dependent nonlinear and stochastic terms are added to the Schrödinger equation in some models of wave function...Show moreFor a phenomenological explanation of the absence of macroscopic superpositions, mass-dependent nonlinear and stochastic terms are added to the Schrödinger equation in some models of wave function collapse. This thesis describes a feasibility study of a microwave interferometer experiment with the purpose of falsifying the existence of spontaneous wave function collapse. Traveling wave parametric amplifiers in the interferometer arms effectively increase the mass of these superpositions, resulting in an interference visibility and an expected vanishing visibility in case of wave function collapse. The main question arises if the visibilities for these cases can be distinguished from each other in case of a gain of 20 dB and a (typical) insertion loss of 4 dB of the parametric amplifiers. Calculations of the interference visibilities were extended and performed by a Python program with the QuTiP module by using a Master Equation solver and Monte Carlo simulations on different computer infrastructures with up to 1.5 TB of RAM available. The limitations of the solvers were the QuTiP internals and computational time respectively. Visibilities are calculated using the Master Equation solver for gains up to 13.4 dB when neglecting losses, and for gains up to 5.3 dB and insertion losses up to 6.5 dB when including the effect of dissipation using both solvers. The differences between both visibilities appear to converge to constant values in the limit of large gain. Further calculations can be carried out on the Lisa Cluster to explore this promising observation. In addition, a cryogenic calorimeter for determining the insertion loss of an amplifier is characterized by its thermal time constants. Efforts were made to reduce the time constant attributed to a 3 dB attenuator, allowing for a precise determination of its power dissipation. On the basis of the results of this study, it can be concluded that significant steps in determining the feasibility of the proposed interferometer experiment are taken and future research is encouraged.Show less
In this thesis we describe the potential application of Si3N4 cantilevers in a Magnetic Resonance Force Microscopy (MRFM) setup. In a characterization of these cantilevers we find quality factors...Show moreIn this thesis we describe the potential application of Si3N4 cantilevers in a Magnetic Resonance Force Microscopy (MRFM) setup. In a characterization of these cantilevers we find quality factors up to 26000 at 100 mK and determine the thermal force noise SF to be 0.66 aN/√(Hz), which is competitive with currently used single crystal silicon cantilevers. With this we show that Si3N4 cantilevers are suitable replacements for the currently used MRFM cantilevers. We perform a study of the higher order resonance modes of this cantilever and compare this to a simulation of the eigenfrequencies of the cantilever. Lastly we describe a method of applying feedback with a specific phase or gain to the cantilever. We use this feedback to cool the effective temperature of the fundamental resonance mode of the cantilever from a saturation temperature of 100 mK to 28 mK. We show that this result is limited by the high detection noise in the setup and make suggestions for further improvements. This new, more convenient, feedback scheme should allow for easier implementation of feedback cooling in future MRFM experiments.Show less
A hinge specifically designed for continuous friction measurements during ice skating was tested and used. The hinge can handle large vertical normal forces to simulate the weight of a real person...Show moreA hinge specifically designed for continuous friction measurements during ice skating was tested and used. The hinge can handle large vertical normal forces to simulate the weight of a real person on a skate, and is very flexible in the horizontal direction, so it deforms under a friction force. Two sensors on the hinge measure the deformation. Friction measurements were done with a part of a real skate, with varying temperatures, skating speeds and normal forces on the skate. A clear dependence of friction on temperature was found. Friction coefficients for an ice temperature of -20 C and air temperature of -10 C varied between 0.04 and 0.1, and coefficients for an ice temperature of -10 C and air temperature of -6 C varied from 0.006 to 0.016. The temperature of the skate was held at -10 C for both cases. The results also suggest friction dependence on skating speed and normal force, but this has to be verified. During the calibration of the setup it was found that the vertical force, controlled by air pressure, could be determined up to a factor of 2. Furthermore there was a large variation (up to a factor 2) in friction coefficients from measurements under the same circumstances, on the same ice layer. These could have been caused by changing humidity in the setup, as this was not monitored during the measurements. The setup works, but needs to be improved for more precise friction measurements. A humidity sensor in the setup is recommended.Show less
An indispensable ingredient for nanoscale imaging within the field of Magnetic Resonance Force Microscopy (MRFM) is a radiofrequency (RF) source. Conventional RF-sources constitute a significant...Show moreAn indispensable ingredient for nanoscale imaging within the field of Magnetic Resonance Force Microscopy (MRFM) is a radiofrequency (RF) source. Conventional RF-sources constitute a significant impediment for MRFM experiments at extreme low temperatures and consequently form a major obstacle towards the single-spin measurement. In this thesis we have introduced a non-trivial method where an intrinsic property of an MRFM force sensor is exploited for the generation of an ultra-low dissipative RF-field. Using MRFM as a probe for local nuclear magnetic resonance (NMR) experiments on copper nuclei at millikelvin temperatures, we have demonstrated that the correct implementation of this feature resulted in an amplification of the NMR signal by more than a factor 2. Based on these findings, we propose an adjusted design of the force sensor that could contribute towards a significantly improved imaging sensitivity as established in MRFM experiments to date.Show less
The working temperature is a limiting factor for improving the sensitivity of Magnetic Resonance Force Microscopy towards imaging of a single nuclear spin. In this report we take a step in lowering...Show moreThe working temperature is a limiting factor for improving the sensitivity of Magnetic Resonance Force Microscopy towards imaging of a single nuclear spin. In this report we take a step in lowering the working temperature by reducing the energy loss of the radio frequent source, using a superconducting NbTiN microwire. A cryogenic calorimeter with 100 nW resolution at 4 Kelvin is developed to investigate the power dissipation of the detection chip, giving new insights on the current design. The use of a NbTiN microwire enables to keep the mixing chamber of a dilution refrigerator at 10mK. The presence of flux vortices penetrating the NbTiN material seems to play an active role in the energy losses. Improvements are proposed on the design of the detection chip to reduce the power dissipation further for future experiments.Show less
