To reach the error threshold required to successfully perform error-correcting algorithms in quantum computers, geometric quantum gates have been considered because of their natural resilience against noise. Non-cyclic geometric gates have been proposed to reduce the run time of conventional geometric gates, to further guard against decoherence. However, while these proposed gates remove the dynamical phase from the computational basis, they do not in general remove it from the eigenstates of the time evolution operator. For a non-cyclic gate to genuinely be considered geometric the dynamical phase should be removed from both the computational basis and the eigenstates. Here, a scheme for finding genuine non-cyclic geometric gates is proposed. The gates are designed to evolve the computational basis along non-cyclic paths, consisting of two geodesic segments, chosen such that the dynamical phase is removed from the eigenstates. The gates found with this scheme did not have shorter runtimes than cyclic gates, but it was possible to implement any gate with this scheme. The findings are important for the understanding of how general quantum computations can be implemented with geometric gates.

High entropy alloys (HEAs) are made of at least five principal elements in near-equimolar proportions. The vast number of possible alloys and unconventional combinations of properties are the main benefits of HEAs. Ti, Nb, Cr, Al and N were chosen in order to create a hard, corrosion resistant coating with good thermal stability. TiNbCrAl multicomponent nitride thin films with Ti content between 0 to 14.4 at.% were deposited using multi-magnetron reactive high power impulse magnetron sputtering (R-HiPIMS) to investigate the feasibility of this method and to study how the Ti content affects the properties of the film. The samples deposited using reactive direct current magnetron sputtering (R-DCMS) were used as benchmarks. The settings required for near-equimolar composition were fixed, with Ti magnetron power as the only variable. Substrate was grounded and not intentionally heated. The composition of HiPIMS samples was more stable while the DCMS samples had significant fluctuations in Al and N content when varying the Ti target power, and were understoichiometric in nitrogen, (T iCrN bAl)1N1−δ, due to low degree of ionization of N. All crystalline samples had NaCl-type fcc structure. Crystalline DCMS samples were (111) textured, while the higher ionization characteristic for HiPIMS resulted in samples with competitive growth between two growth directions. The energetic particle bombardment caused the columnar structure of the film to be denser and less jagged, while DCMS samples containing Ti were significantly more porous. Denser, harder and stiffer films with significantly higher compressive stress were produced with HiPIMS. The hardness and stiffness were almost linearly dependent on Ti content, with density slightly decreasing as the Ti content increased. Higher Ti content increased the rate of corrosion of the films.

Due to recent advances in computational hardware and code accessibility, state-of-the-art calculations are currently employed to investigate materials at the nanoscale with varying levels of accuracy. As such, this dissertation highlights a series of materials ranging from one-dimensional wires, to reactive surfaces, to bulk crystals. Initial characterizations for all considered materials are carried out using density functional theory where additional approximations are utilized to obtain more complex quantities. For Millon's salt, first-principles calculations confirm a quasi-one-dimensional description where the metallic backbone influences electronic properties while hydrogen-bonding between ligands results in structural stability. We show that valence band dispersion can be controlled via strain or ligand substitution, pointing to tunable hole-carrier possibilities. Optical properties are also addressed with respect to experimental and theoretical findings. Our focus then shifts to titanium dioxide, a popular and promising photocatalyst. Specific nitrogen doping on the anatase (001) surface introduces intra gap states accessible for photoactivation in the visible. The additional presence of a fluorine dopant or oxygen vacancy enhances the density of these particular states available for transitions. Titanium dioxide also has experimentally displayed involvement in carbon dioxide reduction mechanisms. From first-principles calculations, anatase (001) surfaces containing an oxygen vacancy exhibit an increased potential for carbon dioxide to undergo reduction due to an exposed titanium atom in comparison to the pristine case. Other binding configurations on both types of surfaces suggest the existence of alternative conversion pathways. As a recently realized plasmonic material, titanium nitride proves advantageous in relation to more traditional materials, e.g., gold or silver; one of the main factors stems from its tunable permittivity. We investigate this aspect by theoretically incorporating defects into titanium nitride, which introduces a systematic approach to control plasmonic activity over a broad frequency range. Finally, lifetimes of hot-electrons, originating from plasmonic decay, for instance, possess finite lifetimes in titanium nitride, as well as in other similar materials, that are described by electron-electron interactions through the electron self-energy. Average lifetimes resemble those obtained with a free electron gas model while details of the band structure influence lifetime behavior. Calculations exploring factors affecting these lifetimes are presented.

Kohn-Sham density functional theory is a rigorous formulation of many-electron quantum mechanics which, for practical purposes, requires approximation of one term in its total energy expression: the exchange-correlation energy. This work elucidates systematic methods for constructing approximations to the exchange-correlation energy solely from first-principles physics. We review the constraints that can be built into approximate density functionals, and use thermochemical data to argue that satisfaction of these constraints permits a more general description of electronic matter. Contact with semiclassical physics is made by studying the turning surfaces of Kohn-Sham potentials in solids. Perfect metals and covalently-bound, narrow-gap insulators do not have turning surfaces at equilibrium, but do under expansive strain. Wide-gap insulators, ionic crystals, and layered solids tend to have turning surfaces at equilibrium. Chemical bonds in solids are classified using the turning surface radii of its constituent atoms. Depletion of the charge density, such as near a monovacancy in platinum, is shown to produce a turning surface. Further, this work demonstrates why generalized gradient approximations (GGAs) are often able to describe some properties of sp-bonded narrow-gap insulators well. A Laplacian-level pure-density functional is developed with the goal of describing metallic condensed matter. This functional is derived from the r2SCAN orbital-dependent meta-GGA, and reduces its tendency to over-magnetize ferromagnets; improves its description of the equation of state properties of alkali metals; and improves its description of intermetallic thermodynamics. It is constructed to enforce the fourth-order exchange gradient expansion constraint (not satisfied by r2SCAN), and a few free parameters are fitted to paradigmatic metallic systems: jellium surfaces and closed-shell jellium clusters. Last, we modify an exchange-correlation kernel that describes the density-density response of jellium to better satisfy known frequency sum rules. We also constrain the kernel to reproduce the correlation energies of jellium, and compare it to a wide variety of common kernels in use for linear response, time-dependent density functional theory calculations. / Physics

Fiber optic sensors offer the ability to measure different types of physical quantities in more harsh environments, such as temperature, pressure and deformations. Some of these demanding environments include chemicals that affect the sensitivity of the sensor, and therefore its resili-ence deteriorates. This work focuses on using experimental techniques to find a method that protects the optical fiber in these chemically demand-ing environments, by coating the fiber with a polymer layer which has the task of protecting it in such environments. A challenge that comes with coating the fiber optic sensor with a polymer layer is that the ability to obtain information becomes more difficult as, its sensitivity deterio-rates. In this project, a type of polymer called polyimide will be tested, using different concentrations and number of layers coated on the optical sensor to investigate the extent that these factors affect the sensor´s ability to cope in chemically demanding environments and also how the sensi-tivity is affected. Thus, the coating method used was soap film coating (SFC). A spectrometer was used to examine the sensitivity of the sensor (using total internal reflection (TIR) and surface plasmon resonance (SPR)). The examination of the resistance of the optical fiber was meas-ured by immersing the polymer-coated sensor in a corrosive liquid for various time intervals and then examining its protective ability. The re-sults obtained through this work demonstrate that polyimide as a coating material provides a protective effect by improving the resistance. The sen-sitivity was most affected when the concentration of the polyimide layer increased from 1-layer to 2-layer polyimide at high concentrations. Re-sistance also increased as the concentration increased, however, 1-layer and 2-layer protection did not have a major impact. The results of this project can be used to further test different types of polymers, for example PVDF. Even more tests with the same attitude and conditions should be carried out to ensure the conclusions and results, and to estimate the measurement uncertainties in the work.

The experimental quest for a quantum spin-liquid state (QSL) in frustrated magnetic systems addresses fundamental scientific interests, as this intriguing quantum phase provides excellent grounds for discovering exotic collective phenomena. ZnCu3(OH)6Cl2 (herbertsmithite), an S=1/2 kagomé-lattice Heisenberg antiferromagnet, is the most promising candidate for experimentally realizing a QSL. However, despite years of intense research, the nature of its paramagnetic ground state remains highly debated. The root cause of the controversy lies in the difficulty in distinguishing the effects of defects from the intrinsic properties of the kagomé lattice. In this thesis, we present 17-O nuclear magnetic resonance (NMR) measurements of an isotope-enriched ZnCu3(OH)6Cl2 single crystal. We succeeded in distinguishing the intrinsic magnetic behavior of the kagomé lattice from the defect-induced phenomena down to T~0.01J, where J~200K is the Cu-Cu super-exchange interaction. We identify NMR signals arising from the nearest-neighbor 17-O sites of Cu2+ defects occupying the Zn2+ interlayer sites. From the 17-O Knight shift measurements, we show that these Cu2+ defects induce a large Curie-Weiss contribution to the bulk-averaged susceptibility at low temperatures. Moreover, our 17-O single-crystal lineshapes show no signature of nonmagnetic Zn2+ defects within the kagomé lattice, and therefore, we rule out “anti-site disorder” as a cause of the paramagnetic ground state in ZnCu3(OH)6Cl2. Most importantly, we demonstrate that the intrinsic spin susceptibility of the kagome lattice asymptotically tends to zero below T~0.03J, indicating the presence of a finite gap Δ = 0.03~ 0.07J in the spin excitation spectrum; this gap is completely suppressed under the application of a high magnetic field of ~ 9T. The behavior of low-energy spin fluctuations probed by the 17-O nuclear spin-lattice relaxation rate is consistent with the gap signature observed for the 17-O Knight shift. In short, our 17-O NMR results provide the first experimental evidence for a gapped QSL realized in ZnCu3(OH)6Cl2. / Thesis / Doctor of Philosophy (PhD)

No description available.

Quantum conductance is a phenomenon associated with nanowires / quantum point contacts where the current through a wire is quantized. Experiments have shown that this phenomenon can be manifested at room temperature using macroscopic wires. This project is aimed to recreate these experiments with emphasis on simplicity. By briefly contacting gold wires and measuring the current using an oscilloscope, current quantization can occasionally be seen as the contact breaks.

The intention with this thesis is to investigate a dimer in a spin chain. Inorder to do that, a model from Z2-gauge theory is taken as the theoretical motivation to construct a discrete lattice with Ising spin properties. A dimer is then allowed to exist indirectly in the empty space between sites. We choose to tackle the problem through a quantum mechanical approach in 1+1-dimensions, distancing ourselves from the original description in quantum field theory. The exposition begins by reviewing the spatial construction of the entire chain as well as its components, and ends with a discussion of time development where the main concern is dispersion in addition to reflection against a static charge.

No description available.