Quantum information technologies offer significantly more computational power for certain tasks and secure communication lines compared to the available classical machines. In recent years there have been numerous proposals for the implementation of quantum computers in several different systems that each come with their own advantages and challenges. This dissertation primarily focuses on challenges, specifically interactions with the environment, and applications of two of such systems: Semiconductor quantum dots and topological insulators. The first part of the dissertation is devoted to the study of semiconductor quantum dots as candidates for quantum information storage and sources of single-photon emission. The spin of the electron trapped in a self-assembled quantum dot can be used as a quantum bit of information for quantum technology applications. This system possesses desirable photon emission properties, including efficiency and tunability, which make it one of the most advanced single-photon emitters. This interface is also actively explored for the generation of complex entangled photonic states with applications in quantum computing, networks, and sensing. First, an overview of the relevant developments in the field will be discussed and our recent contributions, including protocols for the control of the spin and a scheme for the generation of entangled photon states from coupled quantum dots, will be presented. We then look at the interaction between the electron and the surrounding nuclear spins and describe how its interplay with optical driving can lead to dynamic nuclear polarization. The second part of the dissertation follows a similar study in topological insulators: The role of time-reversal breaking magnetic impurities in topological materials and how spinful impurities enable backscattering mechanisms by lifting the topological protection of edge modes. I will present a model that allows for an analytical study of the effects of magnetic impurities within an experimental framework. It will be discussed how the same platform also enables a novel approach for applications of spintronics and quantum information, such as studying the entanglement entropy between the impurities and chiral modes of the system. / Doctor of Philosophy / Quantum information science has received special attention in recent years due to its promising advantages compared to classical machines. Building a functional quantum processor is an ongoing effort that has enjoyed enormous advancements over the past few years. Several different condensed matter platforms have been considered as potential candidates for this purpose. This dissertation addresses some of the major challenges in two of the candidate platforms: Quantum dots and topological insulators. We look at methods for achieving high-performance optical control of quantum dots. We further utilize quantum dots special ability to emit photons for specific quantum technology applications. We also address the nuclear spin problem in these systems which is the main source of destruction of quantum information and one of the main obstacles in building a quantum computer. This is followed by the study of a similar problem in topological insulators: Addressing the interaction with magnetic impurities of topological insulators. Included with each of these topics is a description of relevant experimental setups. As such, the studies presented in this dissertation pave the way for a better understanding of the two major obstacles of hyperfine interactions and the optical controllability of these platforms.

Στην παρούσα διπλωματική εργασία εξετάζονται οι παράγοντες που επιδρούν στην απόδοση και τη διάρκεια ζωής των συνθετικών μονωτήρων. Ειδικότερα εξετάζεται η γήρανση των συνθετικών μονωτήρων από σιλικονούχα πολυμερή με κύριο παράγοντα επιβάρυνσης την απώλεια της υδροφοβικότητας σε συνθήκες ηλεκτρικής καταπόνησης λόγω ηλεκτρικών εκκενώσεων, εκκενώσεων corona γύρω από σταγόνες ύδατος που επικάθονται στην επιφάνεια των μη κεραμικών μονωτήρων χρήσεως εξωτερικού χώρου καθώς επίσης και άλλων περιβαλλοντικών παραγόντων που επηρεάζουν τους συνθετικούς μονωτήρες όσο αφορά τις φυσικές και μηχανικές τους ιδιότητες. Τα πολυμερή υλικά τα οποία χρησιμοποιούνται στην κατασκευή των συνθετικών μονωτήρων θα υποστούν γήρανση με το χρόνο. Στη διάρκεια της χρησιμοποίησης τους είναι σημαντικό να μην αστοχήσουν ή να μην επιτρέψουν ηλεκτρική υπερπήδηση που είναι το αποτέλεσμα της εμφάνισης υψηλών τιμών της έντασης ηλεκτρικού πεδίου σε ορισμένα τμήματα της επιφάνειας του μονωτήρα. Για την αξιολόγηση της γήρανσης των συνθετικών μονωτήρων χρησιμοποιήσαμε μεθόδους φασματοσκοπίας σε πολυμερή υλικά και συγκεκριμένα σε EPSB και SiR, τα οποία υπέστησαν τεστ πολλαπλών εντάσεων σε συνθήκες εργαστηρίου, εξομοιώνοντας τις πραγματικές συνθήκες λειτουργίας. Η αξιολόγηση πραγματοποιήθηκε για μια περίοδο 15 χρόνων εξομοιώνοντας πραγματικά χρόνια λειτουργίας(15 εργαστηριακά χρόνια) σε θάλαμο πολλαπλών καταπονήσεων επιταχυνόμενης γήρανσης. Σε σύγκριση με το παρθενικό δείγμα. Οι μονωτήρες από EPSB εμφάνισαν κρητιδική μορφή, μειωμένη υδροφοβικότητα και μείωση μορίων υδρογονάνθρακα (CH) κατά την ανάλυση με την φασματοσκοπική μέθοδο ATR-FTIR (Attenuated Total Reflection Fourier Transform Infrared Spectroscopy). Οι μονωτήρες από SiR δεν εμφάνισαν αξιοσημείωτες αλλαγές κατά τις μετρήσεις FTIR. Παρόλα αυτά όμως με την μέθοδο SEM (Scanning Electron Microscopy) σημείωσαν αυξημένη επιφανειακή σκληρότητα καθώς και αποσύνθεση του υλικού για τις επιφάνειες και των δύο υλικών. Τα αποτελέσματα των ερευνών μας δείχνουν πως τα τεστ πολλαπλών καταπονήσεων είναι αξιόπιστα από τη στιγμή που ανταποκρίνονται στην πραγματικότητα. Λέξεις Κλειδιά: Γήρανση, συνθετικός μονωτήρας, σιλικονούχο καουτσούκ, τάξη υδροφοβικότητας, εκκενώσεις corona σταγόνων ύδατος, υπερπήδηση, ηλεκτρικό πεδίο συνθετικών μονωτήρων, προσομοίωση, αστοχία υλικού, σιλικονούχα ελαστομερή, ρύπανση, αντοχή στη διάβρωση, αντοχή στην υπεριώδη ακτινοβολία, πρόσθετα, πληρωτικά υλικά, διάρκεια ζωής, τεστ πολλαπλών καταπονήσεων, φασματοσκοπία FTIR, μέθοδος SEM. / This diplopma thesis investigates the factors which can play an important role in long term performance and life expectancy of composite insulators. The aging of non ceramic silicone rubber and EPDM insulators is especially studied. Especially is examined the loss of their hydrophobicity due to electrical stresses, corona effect due to water drops that lay on outdoor polymeric insulators surface and other environmental factors which affect the polymeric insulators as far as their physical and mechanical properties concerns. All polymers used in the manufacturing of composite insulators will age over time but what is important is that they do not fail and not permit an electrical flashover which is a result of high values of electric field localized on some parts of the insulator surface. To evaluate the aging of polymeric insulators we used spectroscopy methods in polymeric materials and specifically at EPSB and SiR, with multistress tests in laboratory conditions, simulating real time conditions in the field. The evaluation was conducted for a period of 15 simulated service years of aging (15 lab-years) in the multistress accelerated aging chamber. Compared to the unaged (virgin) sample, the EPSB insulators showed chalking, reduced hydrophobicity, and reduction in hydrocarbon (CH) group molecules in the Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) measurements. The SiR arrester surfaces showed no noticeable changes in the FTIR measurements. However, Scanning Electron Microscopy (SEM) micrographs indicated increased surface roughness and disintegration of the material for both the SiR and EPSB surfaces. Research results indicated that multistress accelerated tests are reliable since they reflect the real world. Key words: Aging, composite insulator, silicone rubber, hydrophobicity classification, water drops corona discharges, flashover, electric field of composite insulator, simulation, material failure, silicone elastomer, pollution, erosion resistance, UV stability, additives, fillers, life expectancy, multistress accelerating tests, FTIR spectroscopy, SEM micrographs.

Thesis (MScEng)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: The main objective of this research project was to develop a methodology which can be used to assist in developing an insulator pollution severity application map (INSMAP), more particularly for South Africa. The techniques must be able to assist in determining pollution severity levels. At the inception of this project a phase one insulator pollution severity application map was available; this map was based on an existing corrosion map, climatic maps and dust gauge measurements. The phase one map was conservative and did not highlight localised pollution. To confirm and possibly improve on this map, a phase two map was developed, which was based on feedback from Eskom field personnel via an electronic insulator pollution questionnaire survey (based on IEC 60815 guidelines).Jhe questionnaire was incorrectly completed in most cases due to a lack of understanding by field personnel of environmental pollution and the impact on insulator performance. This resulted in an unrealistic map with exaggerated contamination levels. To confirm the findings of the phase one map, a phase three approach was followed, which was focused on an investigation into pollution measurement techniques and methodologies. Based on the phase one map, a need was identified to confirm by measurements the impact of distance-to-coast. Also, the validity of installing a dust gauge at 3 metres above ground level, while insulators are at a height of 8 to 40 meters, had to be investigated. Two pollution monitoring programmes were established: a distance-to-coast and height-above-ground environmental pollution study. Various test methods are described with a complete testing procedure, descriptive equations and related pollution severity classifications for insulator pollution (DDG and ESDD) and corrosion (CLIMAT and metal specimens). The distance-to-coast test results confirm the phase one map, while the height-aboveground measurements justify the installation of dust gauges at 3 metres above ground level. Good correlations were found for the distance-to-coast study between insulator pollution severity and corrosion rates. From the height-above-ground study, a slight (insignificant) decrease in pollution levels was found for increase in height for insulator pollution and corrosion tests. Recommendations are made on setting up a basic pollution monitoring station. Key words: Insulator pollution, Corrosion, Distance, Height. / AFRIKAANSE OPSOMMING: Die hoofoogmerk van hierdie navorsingprojek was die ontwikkeling van 'n metodiek om 'n isolator besoedelingskaart saam te stel, meer spesifiek vir Suid Afrika. Die tegnieke moet die besoedelingsvlakke en impak-afstand van elke besoedelingvlak aandui. 'n Fase een isolator besoedelingskaart het reeds bestaan by die aanvang van hierdie projek, hierdie kaart was gebaseer op 'n bestaande korrosiekaart. Die fase een kaart was baie konserwatief en het nie lokale besoedeling aangedui nie. Dit was besluit om die fase een kaart te verbeter en 'n fase twee kaart was ontwikkel wat gebaseer was op 'n elektroniese isolator-besoedeling vraelys wat deur Eskom se tegniese veldpersoneel voltooi is. Die vraelys is in baie gevalle verkeerdelik ingevul, as gevolg van 'n gebrek aan die kennis oor besoedeling en die impak daarvan op isolatorprestasie. Dit het tot 'n onrealistise kaart gelei met onakurate besoedelingsvlakke. 'n Fase drie ontwikkeling is begin, waar 'n ondersoek gekyk het na metingstegnieke en metodiek vir besoedelingsmetings. 'n Volledige toetsprosedure, beskrywende vergelykings en besoedelingklassifikasies is ontwikkel vir die isolator-besoedeling (DDG en ESDD) en korrosietoetse (CLIMAT en metaal monsters). Toetsstasie seleksiekriterea vir die geografiese plasing van 'n toetsstasie is vasgestel. Twee toetsprogamme is vasgestel: 'n afstand-van-see en 'n hoogte-bo-grondvlak besoedelingsstudie. Die afstand-van-see studie het 'n impak-afstand vir elke besoedelingsvlak vir beide isolatorbesoedeling en korrosietoetse vasgestel. 'n Goeie korrelasie is vasgestel tussen isolator-besoedeling en korrosievlakke. Vir die hoogte-bo-grondvlak studie was gevind dat daar 'n klein (weglaatbare) vermindering in besoedelingsvlakke met toename in hoogte bo grondvlak vir isolatorbesoedeling- en korrosievlakke onstaan. 'n Goeie korrelasie bestaan tussen isolatorbesoedeling- en korrosievlakke. Aanbevelings word gemaak vir die opstel van 'n basiese besoedelingsmetingstasie. Sleutelwoorde: Isolator besoedeling, Korrosie, Afstand, Hoogte.

abstract: This paper presents a theoretical model for evaluating flashover performance of insulators under contaminated conditions. The model introduces several new features when compared with existing models such as, the formation of dry bands, variations in insulator geometry and surface wettability. The electric field distribution obtained from software for 3-Dimensional models along with form factor are used to determine the dimensions of the dry bands and the onset of arcing. The model draws heavily from experimental measurements of flashover voltage and surface resistance under wet conditions of porcelain and composite insulators. The model illustrates the dominant role played by the insulator shape and housing material on the flashover performance. / Dissertation/Thesis / M.S. Electrical Engineering 2012

A rich variety of phases can exist in quantum systems. For example, the fractional quantum Hall states have persistent topological characteristics that derive from strong interaction. This thesis uses the exact diagonalization method to investigate quantum lattice models with strong interaction. Our research topics revolve around quantum phase transitions between novel phases. The goal is to find the best schemes for realizing these novel phases in experiments. We studied the fractional Chern insulator and its transition to uni-directional stripes of particles. In addition, we studied topological Mott insulators with spontaneous time-reversal symmetry breaking induced by interaction. We also studied emergent kinetics in one-dimensional lattices with spin-orbital coupling. The exact diagonalization method and its implementation for studying these systems can easily be applied to study other strongly correlated systems. / PHD / Topological quantum states are a new type of quantum state that have properties that cannot be described by local order parameters. These types of states were first discovered in the 1980s with the integer quantum Hall effect and the fractional quantum Hall effect. In the 2000s, the predicted and experimentally discovered topological insulators triggered studies of new topological quantum states. Studies of strongly correlated systems have been a parallel research topic in condensed matter physics. When combining topological systems with strong correlation, the resulting systems can have novel properties that emerge, such as fractional charge. This thesis summarizes our work that uses the exact diagonalization method to study topological states with strong interaction.

Outdoor insulators are one of the most important parts of a power system. The reliability of a power system depends also on the reliability of the insulators. The main functions of an insulator used for outdoor applications are to give the necessary insulation, provide the necessary mechanical support to the transmission line conductor and also to resist the various environmental stresses like pollution, ultra violet rays etc. Traditionally porcelain and glass insulators have been used for outdoor insulator applications. They are good insulators under normal conditions and the cap and pin arrangement allows them to take up the mechanical load of the line. But owing to their large weight and brittle nature they are susceptible to vandalism and also they have increased cost of installation and commissioning. But the main problem of porcelain and glass insulators is its performance under polluted environmental condition. Under wet and polluted conditions, the porcelain insulators allow the formation of a conducting layer on the surface which results in setting up of leakage current, dry band arcing and power loss. This problem is further augmented under dc voltages where the stress is unidirectional and the contaminant deposition is higher as compared to ac. Polymeric insulators are a good alternative for porcelain and ceramic insulators for use especially under dc voltages because of their good pollution performance. The property of surface hydrophobicity resists the setting up of leakage currents and hence polymeric insulators help in reducing power loss. They are also light in weight and vandalism resistant and hence easier to install. But being polymeric, they form conductive tracks and erode when exposed to high temperatures which occur at the surface during dry band arcs and when exposed to corona discharges. The surface hydrophobicity is also temporarily lost when exposed to different electrical stresses. Silicone rubber is the most popular among the various choices of polymers for outdoor insulator applications. They have good surface hydrophobicity and tracking performance. But polymers in their pure form cannot be used as insulators because of their poor mechanical strength. Adding inorganic fillers into the polymer matrix not only improves its mechanical properties but also its erosion resistance. Micron sized Alumina Trihydrate (ATH) is used traditionally to improve the tracking and erosion resistance of polymeric insulators. A very high loading (up to 60%) is used. Adding such a high filler loading to the base polymer hampers its flexibility and the material processing. With the advent of nanotechnology, nano fillers have come into vogue. Studies conducted on nano filled polymers showed exciting results. A small amount of nano fillers in the polymer matrix showed significant improvement in the mechanical strength without hampering its flexibility. The electrical properties like tracking and erosion also improved with filler loading. Hence the use of nano filled silicone rubber is a good alternative for use as a high voltage insulator especially under dc voltages. Reports suggest that adding nano fillers into the silicone rubber matrix improves the tracking and erosion resistance and the corona degradation as compared to the unfilled samples under ac voltages. The literature on the dc performance of silicone rubber nano composites is scarce. So the present study aims to evaluate the performance of silicone rubber nano composites for tracking and erosion resistance and corona degradation under dc voltages. The tracking and erosion resistance under dc voltages was measured using the Inclined Plane Tracking and Erosion Resistance set up as per ASTM D2303 which was modified for dc voltage studies. The performance of nano Alumina and nano Silica fillers were evaluated under negative dc and the performance was compared with micron sized Alumina Trihydrate filled samples. The effect of filler loading was also studied. It was seen that the performance of the silicone rubber improved with filler loading. A small loading percentage of nano fillers were enough to give performance similar to silicone rubber filled with micron sized ATH filler. The silicone rubber performed better under negative dc as compared to ac and positive dc. The positive dc tests showed a migration of ions from the electrodes onto the sample surface. The increased surface conductivity resulted in very heavy erosion in the case of positive dc tested samples. The corona aging studies were also conducted on silicone rubber nano composites. Nano silica was used as filler in this case. Different filler loadings were employed to understand the effect of filler loading. The corona was generated using a needle plane electrode and samples were exposed to both positive and negative dc corona. The samples were exposed to corona for different time intervals – 25 and 50 hours to study the effect of exposure time. The hydrophobicity, crack width and surface roughness were measured after the tests. Adding nano fillers into the polymer matrix improved the corona performance. With filler loading, the performance improved. The samples exposed to positive dc corona performed better than those under negative dc corona. The loss of hydrophobicity, surface cracks and the surface roughness was less in the case of positive dc corona tested samples. With exposure time, the performance of silicone rubber became poorer for positive dc corona tested samples. For the negative dc corona tested samples, the performance seemed to improve with exposure time. The tracking and erosion resistance and the corona aging studies conducted showed that the performance of silicone rubber is improved by adding nano fillers into the polymer matrix. A small amount of nano filler loading was enough to perform similar to a heavily loaded micron filled sample. Hence nano fillers can be used as a good functional material to improve the performance of silicone rubber insulators especially under wet and polluted conditions.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The worldwide increase in the popularity of high voltage direct current (HVDC) power transmission application has led to questions regarding the performance of high voltage alternating current (HVAC) insulators when energized under HVDC excitation. These questions have led researchers conduct numerous research projects worldwide. A particular question NamPower (the power utility in Namibia) has faced is: how do these insulators perform and age when energized under HVDC excitation in heavily polluted environments? This question was only partly answered by some insulator ageing tests conducted under laboratory conditions for HVDC excitation. However, the natural ageing tests on insulators, which are preferred, have to date been confined predominantly to HVAC excitation voltages. Thus, this research was initiated to investigate the natural ageing performance of insulators under both HVAC and HVDC excitations, when subjected to harsh marine pollution environments. This research project involved performance and ageing tests on three identical sets of line insulators made from different insulator materials, energised under HVAC and both polarities HVDC excitations respectively. The tests were conducted at Koeberg insulator pollution test station (KIPTS), which is a natural marine pollution insulator test station located near Cape Town, along the west coast of South Africa, approximately 50 m from the sea. The set of insulators consisted of EPDM silicone alloy rubber, HTV silicone rubber, RTV silicone rubber coated porcelain, Porcelain and Glass insulators. The HVAC excitation voltage was chosen as 12.7 kV r.m.s. phase-to-ground and it was decided to use a HVDC excitation voltage equal to this value. The research results showed that the insulators made from HTV silicone rubber performed better than the insulators made from EPDM silicone alloy rubber under all excitation voltages. It is also showed that RTV silicone rubber coated porcelain insulators performed better than Glass and Porcelain insulators under all excitation voltages. / AFRIKAANSE OPSOMMING: Die wêreld wye toename in gewildheid van hoë spanning gelyk stroom (HSGS) krag transmisie aplikasie het gelei na vrae oor die effektieweteit van hoë spanning wissel stroom (HSWS) insolators in HSGS aplikasies. Hierdie vrae het gelei na baie navorsings projekte. Een vraag waarmee NamPower (die krag voorsienings maatskapy in Namibia) gekonfronteer was, was hoe gaan die insolators wat onderworpe is aan ’n HSGS toevoer reageer in ’n baie besoedelde omgewing. Hierdie vraag was slegs gedeeltelik beantwoord deur verouderings toetse op insolators wat gedoen is in ’n labaratorium met ’n HSGS toevoer. Inteendeel, die meeste natuurlike verouderings toetse op insolators, soos verkies, is meestal gedoen met ’n HSWS toevoer. Om hierdie rede was hierdie navorsing begin om natuurlike verouderings toetse te doen op insolators onderworpe beide aan HSWS en HSGS toevoere binne ’n marien besoedelde omgewing. Hierdie navorsings projek gaan oor prestering en verouderings toetse op drie, identiese, transmissie lyn insolator stelle, wat onderskeidelik onderworpe was aan HSWS en beide polariteite HSGS toevoere. Die toetse was gedoen by Koeberg insolator besoedeling toets stasie (KIBTS) wat naby Kaapstad geleë is langs die weskus van Suid Africa omtrent 50 m van die see. ’n Stel toets insolators bestaan uit EPDM silikon allooi rubber, HTV silikon rubber, RTV silikon rubber bedekte porselein, Porselein en Glas insolators. Die HSWS waarde waarmee die insolators getoets was, was 12.7 kV w.g.k., fase-na-grond, en dit was besluit om ’n HSGS waarde gelyk aan hierdie spannings waarde te gebruik. Die navorsings resultate wys dat insolators wat gemaak is met HTV silikon rubber presteer beter as insolators wat met EPDM silikon allooi rubber gemaak is onder al die verskillende toevoere. Dit wys ook dat RTV silikon rubber bedekte porselein presteer beter as Porselein en Glas insolators onder al die verskillende toevoere.

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Recently, Bi₂Se₃ was added to a new class of materials known as topological insulators. While several studies have provided tantalizing hints towards novel physical properties, such as backscatter suppression and spin-polarized transport, several concerns remain in actual materials. In particular, high defect densities, strong surface band bending, and potential fluctuations have been observed. Here, scanning tunneling microscopy and spectroscopy are used to reveal surface effects in Bi₂Se₃ and CuxBi₂Se₃. First, a detailed examination of defects in bulk-grown samples is described. Then, I provide an analysis of molecular beam epitaxy results, done in collaboration with colleague Yuxuan Chen. Following this, I provide a detailed study of individual point defects in Cu-doped Bi₂Se₃ and examine how Cu is incorporated into the Bi₂Se₃ lattice. Finally, through spectroscopic analysis, a novel depth-sensitive measurement of the local band bending field is developed. Furthermore, for the first time, fluctuations of the Dirac point can be correlated to specific near-surface defects, namely Se vacancies. These analyses provide valuable insights into the preparation of future samples for the investigation of topological insulators. / text

No description available.