Quantum computing promises to deliver exponential speed-up over classical machines in solving specific problems. However, quantum information is susceptible to decoherence and errors, and fault...Show moreQuantum computing promises to deliver exponential speed-up over classical machines in solving specific problems. However, quantum information is susceptible to decoherence and errors, and fault-tolerant quantum computing (FTQC) is the only realistic approach. In FTQC, operations are performed on logical qubits which are encoded in physical qubits such that errors are correctable or trackable. Specifically, in the repetition code for quantum error correction (QEC), qubits are encoded in Greenberger-Horne-Zeilinger (GHZ)-type states to be protected from bit-flip or phase-flip errors. It is important not to leave the protected subspace at any time to meet the basic requirement for FTQC. Previous demonstrations of the repetition code in various physical systems have circumvented this requirement, detecting errors at the cost of decoding the logical qubit. Using five superconducting qubits in circuit quantum electrodynamics, we demonstrate quantum bit-flip error detection at the logical-qubit level by stabiliser measurements for the first time. These stabilisers are assessed by their ability to generate GHZ-type entanglement: projecting a maximal superposition state into the subspaces being stabilised, while maintaining the coherence within each. To further characterise the error detection, we intentionally apply errors on all qubits and assess the fidelities both in the encoded subspace and at the logical-qubit level. Although current fidelities of the stabiliser measurements preclude improvements by error detection over idling, this demonstration is a critical step towards larger codes based on stabiliser measurements in the paradigm of FTQC.Show less
In this thesis, the necessary elements to build up a quantum switch, the central element in a quantum random access memory, are proposed and analyzed. A network with quantum switches at its nodes...Show moreIn this thesis, the necessary elements to build up a quantum switch, the central element in a quantum random access memory, are proposed and analyzed. A network with quantum switches at its nodes forms the bifurcation path that leads an address register from a root node to an array of memory cells, activating, quantum coherently, only the quantum switches that the register encounters in its path to the memory cells. Transmon qubits and SQUIDs are used to design a superconducting device capable of routing a register of microwave photons through a bifurcation network, allowing for superposition of paths. In order to give rise to all the required interactions between the device and the address register, a non-linear capacitor, composed of two plates with carbon nanotubes in between, is introduced into the transmon. The dynamic operation of the quantum switch is analyzed using Langevin equations and a scattering approach, and probabilities of reflection and transmission of photons by (or through) the switch are computed, both for single- and two-photon processes. Computations show that, with parameters taken from up-to-date similar devices, probabilities of success are above 94%. Applications of quantum random access memories are discussed, as well as other applications of quantum switches. Also, solutions are proposed to the challenges that emerge during the study of the dynamics of the quantum switch.Show less
Big Bang Nucleosynthesis theory together with Hubble expansion law, Cosmic Microwave Background and Large-Scale structure formation constitute the basis of experimental confirmations of the...Show moreBig Bang Nucleosynthesis theory together with Hubble expansion law, Cosmic Microwave Background and Large-Scale structure formation constitute the basis of experimental confirmations of the Standard Cosmology. Primordial nucleosynthesis observables are very sensitive to the details of particle physics at the time of formation of the first nuclei. We analyze the influence of the heavy sterile neutrinos on the primordial plasma and abundances of light chemical elements (H, He, Li incl. isotopes) using the numerical simulations of non-equilibrium particle dynamics in the expanding Universe. Next, demanding the computed abundances to be consistent with modern measurements, we can limit the mass-lifetime parameter space of sterile neutrinos. The obtained constraints will complement the experimental constraints from direct accelerator searches, including currently planned experiments. We apply this approach to the Neutrino Minimal Standard Model that aims to explain simultaneously 3 Beyond the Standard Model problems: dark matter, neutrino oscillations and baryon asymmetry of the Universe.Show less
Motional control of mechanical resonators is crucial for their applications. In particular, cooling the mechanical mode to overcome the thermal noise has been greatly explored, and has recently...Show moreMotional control of mechanical resonators is crucial for their applications. In particular, cooling the mechanical mode to overcome the thermal noise has been greatly explored, and has recently been pushed into the quantum regime. In this thesis, we study an almost-forgotten cooling technique: resistive cooling with an artificial cold resistor (ACR) which is physically at room temperature. We perform a proof-of-principle demonstration to cool a mechanical mode of a quartz crystal with a “cold” resistor. The “cold” resistor is realised either by a normal resistor cooled by liquid nitrogen or by an ACR made of a special circuitry. We show that the ACR can cool the mode in the same way as a real cold resistor, and the cooling mechanism can be qualitatively understood in the basic thermodynamic picture. We also discuss the feasibility of applying such resistive cooling to an optomechanical system, with a nested trampoline resonator.Show less
In this thesis, we will present some exact Finslerian solutions to a Finsler Einstein Equation. The first chapter contains an introduction to the main concepts of Finsler geometry. In particular,...Show moreIn this thesis, we will present some exact Finslerian solutions to a Finsler Einstein Equation. The first chapter contains an introduction to the main concepts of Finsler geometry. In particular, the mathematical concepts necessary to develop Finsler General Relativity will be introduced. We will briefly elaborate on properties of pp-waves in General Relativity, followed by a presentation of the main ideas of Very Special Relativity (VSR). VSR assumes a reduced symmetry of nature that is a subgroup of the full Lorentz group. A deformation of VSR symmetries gives rise to a Finsler treatment of spacetime, General Very Special Relativity (GVSR). We will extend GVSR in order to incorporate curved spacetimes. Then, we investigate whether the GVSR versions of VSI metrics (including pp-wave metrics) and AdS metrics in four and five dimensions are solutions to the Finsler Einstein equation proposed by Pfeifer. For the former, this is indeed the case (under certain conditions), but the latter raise the problem of a Finsler generalization of a cosmological constant, which we cannot resolve yet. Finally, we give an overview on experiments that have been proposed to detect (G)VSR deviations from Riemannianity and violations of Lorentz invariance. We conclude with a summary of our results and an outlook on further research.Show less
Despite the success of the Standard Model in the last few decades, we know it is not complete. There is strong motivation for assuming the existence of aditional heavy neutral leptons, which can...Show moreDespite the success of the Standard Model in the last few decades, we know it is not complete. There is strong motivation for assuming the existence of aditional heavy neutral leptons, which can account for active neutrino masses and possibly also have cosmological implications. In this work I consider the Standard Model with two neutral lepton singlets (sterile neutrinos) with degenerated masses in the range $20MeV - 2GeV$. The constraints on the active-sterile neutrino mixing angles are evaluated based on recent neutrino oscillations data. Using these constraints the bounds from accelerator experiments are reanalyzed for the case of the considered model. Finally, the results are compared with cosmological constraints coming from Big Bang nucleosynthesis and the $\nu MSM$ resonant leptogenesis.Show less
This report concerns the creation of a machine that creates graphene using chemical vapor deposition. This machine is a ultra high vacuum device, where high quality graphene is created on top of a...Show moreThis report concerns the creation of a machine that creates graphene using chemical vapor deposition. This machine is a ultra high vacuum device, where high quality graphene is created on top of a single chrystal copper substrate. Additionally an analysis of substrate motility and graphene stretching is presented.Show less
We report the design and construction of a mechanism, consisting of rigid plates and freely hinging connectors, that is used to describe and study the motions of a mechanical metamaterial called...Show moreWe report the design and construction of a mechanism, consisting of rigid plates and freely hinging connectors, that is used to describe and study the motions of a mechanical metamaterial called the ‘metacube’. For this design, a novel method is used, where the mechanism is 3D printed and the accuracy is mechanically tested. We present an accurate mechanism that describes the motions of the metacube. This mechanism can be used to not only gain further understanding of the properties of the metacube, but also shows previously undiscovered properties of this mechanical metamaterial.Show less
In recent years Electronic Double Layer (EDL) gating using ionic liquids or organic electrolytes has been successful in tuning carrier densities in materials such as ZnO [1], SrTiO3 [2–4], La2...Show moreIn recent years Electronic Double Layer (EDL) gating using ionic liquids or organic electrolytes has been successful in tuning carrier densities in materials such as ZnO [1], SrTiO3 [2–4], La2-xSrxCuO4 [5] and YBa2Cu3O7- [6]. The potential for studying High-Tc superconductors and Metal-Insulator transition with great flexibility has been shown. The carriers are pulled into a 1nm layer by the accumulation of ions on the surface (EDL). Here the formation the EDL in time is studied on atomically flat SrTiO3 (100) single crystals and electron doped High-Tc superconductor Nd2-xCexCuO4 thin films. At temperatures close to the melting point of the ionic liquid (DEMETFSI) the ability to separate the EDL formation on the contacts from the formation on STO is demonstrated by measuring resistance and gate current as a function of time. This despite the presence of a significant Faradaic current most likely due to oxidation/reduction at the gold surface. It is argued that the EDL formation on the STO originates from the contacts and its growth largely depends on the mobility of the ions. Measurements on (undoped) Nd2CuO4 show a resistance decrease up to 90% at Vg = 3V and thereby showing the potential of EDL gating on electron doped cuprates.Show less
In this paper a new method of distinguishing shower-like from track-like neutrino interaction events using event reconstruction data is introduced. This method is then used to minimise the error...Show moreIn this paper a new method of distinguishing shower-like from track-like neutrino interaction events using event reconstruction data is introduced. This method is then used to minimise the error that a shower-like event is misidenti ed as a track-like event and vice versa, which is necessary for measuring the neutrino mass hierarchy in ORCA. In its current format, the method leads to errors of around 0.40 for noisy simulation data and approximately 0.15 for noiseless data. However, the method can still be extended to take into account more information of the reconstructions, thereby possibly improving the results.Show less
This report presents a first attempt to introduce noise into the protocol of reference-frame- independent quantum key distribution. It is found that a frequently accepted manner to introduce noise,...Show moreThis report presents a first attempt to introduce noise into the protocol of reference-frame- independent quantum key distribution. It is found that a frequently accepted manner to introduce noise, according to the model of Eckert et al. proposed in ref. [1] leads to non-physical state matrices and therefore another model is proposed: the $\beta_{\pi}$-noise model. In this model the basis states composing the state matrix are perturbed by a complex quantity. For pure states this approach is applied to all state matrix elements, whereas for mixed states it is applied only to the diagonal elements. The off-diagonal elements in the mixed state are perturbed by a complex quantity that is independent of the perturbations on the basis states that the matrix element consists of. Using a Monte Carlo simulation, statistics on the quantum bit error rate as well as the transverse correlation factor are obtained for this model. However, although the $\beta_{\pi}$-noise model solves the main issues that lead to the conclusion that the model of Eckert et al. might infer non-physical state matrices, it does not yet guarantee the state matrix is always physical: a mixed state may still violate positive semi-definiteness. Therefore the original model is improved by perturbing all basis states as before and using this approach for all state matrix elements. In this improved version of the $\beta_{\pi}$-noise model Eve is present as a (complex) scaling of the off-diagonal state matrix elements. Thus, positive semi-definiteness is guaranteed for this noise model. Also for this improved version of the model statistics on the quantum bit error rate and the transverse correlation factor are presented, thereby describing the implications on an experiment.Show less
In this proposal an experiment is envisioned that is principally able to determine whether there exists a fundamental transition between quantum mechanics (QM) and classical mechanics (CM). In the...Show moreIn this proposal an experiment is envisioned that is principally able to determine whether there exists a fundamental transition between quantum mechanics (QM) and classical mechanics (CM). In the quantum realm spatial superpositions of a particle may exist, whereas these superpositions are not observed in the classical world. Many theories have been put forward to explain this difference. Of these, decoherence theories state that there is no explicit difference between QM and CM, but that quantum phenomena are extinguished upon increasing the coupling of the studied object to its environment. Other theories, however, predict that there is a fundamental scale - apart from the environmental coupling - at which spontaneous collapse of a superposition occurs. To test whether such a scale exists, it is proposed to build an interferometer that has a tuneable, coherent amplifier in each of its two arms. The interferometer is fed by a microwave single photon source that yields an entangled superposition of zero and one photon in the arms. The advantage of using microwave photons in a transmission line is that these are carried by electrons, which have a rest mass. Therefore, spontaneous collapse may be expected upon amplifying the superposition of the single photon to such an extent, that the QM-CM transition scale is exceeded. This set-up circumvents the limitation of contemporary experiments that study superpositions of objects fixed in size. By tuning the amplifiers one is now able to smoothly vary the size of the superposition, so that studying the transition scale and its nature becomes experimentally feasible. After introducing the theoretical ideas and the experimental status of the topic, this proposal elaborates on the research goal of the proposed experiment. Then, the method is presented including the experiment's preliminary design options and considerations as well as research steps and risks. Furthermore, the proposal includes a four-year plan of work, discusses the facilities and resources of the host group and stresses collaborations with both theorists and experimentalists.Show less
During this study, a device was developed which is capable of measuring the friction coefficient between two macroscopic solid samples for different rotation angles. It is based on an Anton Paar...Show moreDuring this study, a device was developed which is capable of measuring the friction coefficient between two macroscopic solid samples for different rotation angles. It is based on an Anton Paar rheometer. It can measure torques ranging from 10 nNm up to 200 mNm, while controlling the rotation angle with a minimum resolution of 10 nrad. The normal load exerted on the samples can be controlled with mN accuracy. Measurements on HOPG and silicon dioxide have verified that the instrument works and is capable of measuring very low friction coefficients as a function of the rotation angle between the two samples. The obtained friction coefficients are in agreement with values found in literature.Show less
The Low energy effective Lagrangian of a supersymmetric variation of a Yang--Mills--Higgs Lagrangian, will be derived, where both Lagrangians are still N=2 supersymmetric massless Lagrangians. The...Show moreThe Low energy effective Lagrangian of a supersymmetric variation of a Yang--Mills--Higgs Lagrangian, will be derived, where both Lagrangians are still N=2 supersymmetric massless Lagrangians. The effective Lagrangian is the fixed in a couple of steps in an indirect way. First quantum field theoretic perturbation theory is used for asymptotic behaviour near some identified singularities. Then, with the use of a family of cubic curves as manifolds, and some complex analysis on holomorphic functions, the effective Lagrangian is fixed.Show less
