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We have studied the electron quantum phase by fabricating low dimensional (d ≤ 2) mesoscopic interferometers in high-quality narrow-gap semiconductor (NGS) heterostructures. The low effective-mass electrons in NGS heterostructures enable observation of delicate quantum phases; and the strong spin-orbit interaction (SOI) in the systems gives us means by which we can manipulate the quantum-mechanical spin of these electrons through the orbital properties of the electrons. This enables the observation of spin-dependent phenomena otherwise inaccessible in non-magnetic systems. We have performed low temperature (0.4 K ≤ T ≤ 8 K), low noise (â V ~ 1μV ) transport measurements, and observed evidence of Aharonov-Bohm (AB) and Alâ tshuler-Aronov-Spivak (AAS) quantum oscillations in meso- scopic devices that we fabricated on these NGSs. Our measurements are unique in that we observe both AB and AAS in comparable magnitude in ballistic networks with strong SOI. We show that, with appropriate considerations, diffusive formalisms can be used to describe ballistic transport through rings, even in the presence of SOI. This work also contains an introduction to the physics of geometric phases in mesoscopic systems, and the experimental and analytic processes through which these phases are probed. A discussion of the results of our measurements presents the case that quantum interferometric measurements of geometric phases can be understood quite thoroughly, and that these measurements may have deeper utility in discovery than has yet been recognized. / Ph. D.

This thesis experimentally studies quantum interference phenomena and quantum coherence in mesoscopic systems, and quantum transport as well as magnetotransport in various materials system. One overarching aim is exploring the different mechanisms that give rise to quantum phase decoherence in lithographically patterned mesoscopic structures, of importance in the field of quantum technologies and spintronics. Various mesoscopic structures, namely quantum stadia, quantum wires, and side-gated rings, were fabricated to function as quantum interference devices and platforms to study quantum coherence on two-dimensional electron systems in InGaAs/InAlAs heterostructures. The mesoscopic structures were fabricated by photolithography and electron-beam lithography. The dependence of quantum coherence on geometry or temperature is investigated for each of the quantum interference devices. In the case of quantum stadia, phase coherence lengths were extracted by universal conductance fluctuations, and the extracted phase coherence lengths show a dependence on both temperature and geometry. Phase coherence lengths decreased with increasing temperature, as expected. Moreover, phase coherence lengths also varied with the width-to-length ratio and length of the side wires connected to the stadia, where competition between Nyquist decoherence and environmental coupling decoherence mechanisms coexists. For the quantum wires studied, the phase coherence lengths were extracted from antilocalization signals. Antilocalization measurements provide a sensitive mean of probing the quantum mechanical correction to electronic transport. The phase coherence lengths increased as the wire length increased, due to reduction of the environmental coupling that induces decoherence at the ends of a wire; longer wires tend to have longer phase coherence lengths. In related work, the thesis shows that the spin coherence length, as limited by spin-orbit interaction, increases as the wire width decreases. Decoherence in side-gated rings was measured from the amplitudes of the quantum-mechanical Aharonov-Bohm oscillations. The side gates allow for an in-plane controllable electric field. Asymmetrically biased side-gate voltages allow for the breaking of the two-dimensional parity symmetry of the ring device, effectively resulting in reduced amplitude of the Aharonov-Bohm oscillations. The mechanism that contributes to decoherence in these rings appears to be related to the breaking of the spatial symmetry. Measurements of antilocalization and weak-localization as well as magnetotransport were used to probe interesting or unique quantum mechanical phenomena in the following two, quite different, materials system: bismuth iridate thin films, and Ge/AlAs heterostructures on GaAs or Si substrates. Both materials are of interest for future quantum technologies and devices. Measurements in bismuth iridate thin films reveal interesting transport characteristics such as logarithmic temperature dependence of the resistivity, multiple charge carriers, and antilocalization due to spin-orbit interaction in the system. Weak localization measurements in the Ge/AlAs heterostructure on GaAs or Si substrates show that single carrier transport is essentially located in the Ge layer only. Further, the weak localization results indicate the near-absence of spin-orbit interaction for carriers in the electronically active Ge layer, suggesting the potential use of this materials system as a promising candidate for future electronic device applications. In short, quantum transport and interference measurements probe the quantum-mechanical behavior of materials system for future quantum, spin and electronic technologies. Mesoscopic patterned geometries in InGaAs/InAlAs heterostructures offer a wide range of interesting and unique platforms to study quantum-mechanical phenomena, specifically quantum decoherence, in the solid state. The decoherence phenomena observed and the investigations to the underlying mechanisms studied and modeled in this thesis may be transferred to similar materials system, enriching the knowledge in the field of quantum technologies. Magnetotransport and quantum transport were also applied to Ge/AlAs heterostructures and bismuth iridate thin films, to study the properties of their carrier systems. / Ph. D.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2005. / A process was developed to design and manufacture a dc SQUID magnetometer. Superconductor theory is given as a foundation to explain the Josephson junction. This knowledge is applied to explain the ideal and practical dc SQUID. The design of the dc SQUID is done with inductance calculation formulas. Each step of the manufacturing process is discussed in detail. Many improvements have been made to the process to make each step reproducible. The steps in the manufacturing process that were done in-house are: manufacturing an YBa2Cu3O7−d pellet, depositing the thin film with the pulsed laser deposition process, creating a mask with the UV lithography process, wet etching the circuit and depositing silver contact pads with thermal evaporation. The device is packaged on a printed circuit board device holder and shielded with a mu-metal shield. A test setup is developed to test the final device. Each step in the manufacturing process was successful. The final device did not work, due to oxidisation of the YBa2Cu3O7−d thin film.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: This thesis presents an attempt at creating a fully functional SQUID for the purposes of a predefined application. The goal of the work is to provide a method of development, primarily utilising the facilities of the Engineering Faculty of the University of Stellenbosch, with minimal assistance from other departments. This method is then analysed and the successes and failures discussed in order to provide guidelines and recommendations for future research. The device consists of a gradiometric SQUID, as well as electronics which provide the SQUID bias current, output linearisation and output signal filtering. YBCO is used for the superconducting material, allowing the use of liquid nitrogen as coolant which eases the operation and portability of the device. Various methods of creating Josephson junctions are tested, such as constriction bridges using AFM lithography and step-edge junctions, with and without different buffer layers. Proof of concept simulations demonstrate the viability of the device. Unfortunately, a functional device could not be constructed, mostly due to the design requiring higher levels of resolution and process control than some of the available facilities can provide. Recommendations are provided for future researchers. / AFRIKAANSE OPSOMMING: Hierdie tesis beskryf ’n poging om ’n volledige funksionele SQUID te vervaardig vir die doeleindes van ’n vasgestelde toepassing. Die doel van die werk is om ’n metode te voorsien wat hoofsaaklik gebruik maak van die fasiliteite van die Ingenieurswese Fakulteit van die Universiteit van Stellenbosch, met minimale hulp van ander departemente. Hierdie metode word dan ondersoek en die suksesse en mislukkings bespreek om riglyne en voorstellings te voorsien vir toekomstige navorsing. Die toestel bestaan uit ’n gradiometriese SQUID, sowel as elektronika wat voorsiening maak vir voorspanning, uittree-linearisering en ’n uittree-filter. YBCO is as supergeleidende materiaal gebruik, wat die gebruik van vloeibare stikstof as verkoeler moontlik maak en dus die hantering en draagbaarheid van die toestel vergemaklik. Verskillende metodes om Josephson-vlakke te vervaardig is getoets, onder andere vernouingsbrugvlakke deur middel van AFM litografie en stapvlakke met en sonder bufferlae. Bewys van konsep simulasies demonstreer die lewensvatbaarheid van die toestel. Ongelukkig kon ’n funksionele weergawe van die toestel nie vervaardig word nie, hoofsaaklik as gevolg van die ontwerp wat hoër resolusievlakke en beter prosesbeheer benodig as wat sekere van die beskikbare fasiliteite kan voorsien. Aanbevelings word voorsien vir toekomstige navorsers.

Building complex quantum systems has the potential to reveal phenomena that cannot be studied using classical simulation. Photonics has proven to be an effective test-bed for the investigation of such quantum-enhanced technologies, however, the proliferation of bulk optical components is unlikely to be a scalable route towards building more complex devices. Instead, the miniaturisation, inherent phase stability and trivial alignment afforded by integrated photonic systems has been shown to be a promising alternative. In the first half of this thesis, we describe experiments exploiting the quantum interference of three single photons on a reconfigurable integrated photonic chip. We develop a low-loss source of single photons and introduce a low-loss silica-on-silicon waveguide architecture which enables us to show the first genuine quantum interference of three single photons on an integrated platform. A loss-tolerant, element-wise characterisation scheme is developed along with a statistical test to verify that this multi-photon circuit behaves as expected. We then make use of this three-photon interference to detail the first proof-of-principle demonstration of a new intermediate model of quantum computation called boson sampling. Finally, we perform an on-chip demonstration of the quantum teleportation protocol where all key parts --- entanglement preparation, Bell-state analysis and quantum state tomography --- are performed on a reconfigurable photonic chip. The element-wise characterisation scheme developed earlier is shown to be critical to mitigate fabricated component errors. We develop a theoretical model to account for all sources of possible error in the circuit and find good agreement with the measured teleported state fidelities, which exceed the average teleportation fidelity possible with a classical device. We identify the elements of this error budget relevant to scaling and propose improvements to chip characterisation and fabrication in order to achieve high fidelity operation. In the second half, we discuss the use of high efficiency superconducting transition edge sensors in enabling quantum experiments using more photons. We detail the installation and characterisation of these detectors in a new lab in Oxford. We achieve good photon number-resolution and high-efficiency operation. Work to integrate these detectors on the silica-on-silicon waveguide architecture is discussed and we detail the optical and thermal device modelling performed to optimise the on-chip detection efficiency. New, on-chip detectors, fabricated according to this design are shown to operate as expected and achieve high-efficiency and good energy resolution.

abstract: Seeking an upper limit of the Neutron Electric Dipole Moment (nEDM) is a test of charge-parity (CP) violation beyond the Standard Model. The present experimentally tested nEDM upper limit is 3x10^(26) e cm. An experiment to be performed at the Oak Ridge National Lab Spallation Neutron Source (SNS) facility seeks to reach the 3x10^(28) e cm limit. The experiment is designed to probe for a dependence of the neutron's Larmor precession frequency on an applied electric eld. The experiment will use polarized helium-3 (3He) as a comagnetometer, polarization analyzer, and detector. Systematic influences on the nEDM measurement investigated in this thesis include (a) room temperature measurements on polarized 3He in a measurement cell made from the same materials as the nEDM experiment, (b) research and development of the Superconducting QUantum Interference Devices (SQUID) which will be used in the nEDM experiment, (c) design contributions for an experiment with nearly all the same conditions as will be present in the nEDM experiment, and (d) scintillation studies in superfluid helium II generated from alpha particles which are fundamentally similar to the nEDM scintillation process. The result of this work are steps toward achievement of a new upper limit for the nEDM experiment at the SNS facility. / Dissertation/Thesis / Doctoral Dissertation Physics 2019

This thesis reports results from the investigation of optically-induced carrier dynamics in graphite and graphitic carbon nanostructures. In this first set of experiments, the dynamics of photo-excited carriers in exfoliated graphene and thin graphitic films are studied by optical pump-probe spectroscopy. Samples ranging in thickness from 1 to 260 carbon layers are deposited onto an oxidized silicon substrate. Time-resolved reflectivity and transmissivity are measured at 1300 nm, following excitation by 150 fs, 800 nm pump pulses at room temperature. Two time scales are identified over which the extracted transient dielectric function returns to its quiescent value. A fast decay time of ~200 fs in graphene is associated with hot phonon emission and increases to ~300 fs for thicknesses greater than only a few carbon layers. The slow decay time, associated with hot phonon interaction and/or carrier recombination, increases more gradually, from ~2.5 to 5 ps over ~30 layers. A simple model suggests the thickness dependence of the slow decay time is likely a result of thermal diffusion into the substrate. In the second set of experiments, coherently-controlled two-colour injection photocurrents are generated via quantum interference of single- and two-photon absorption in bulk graphite and a variety of single-walled carbon nanotube samples, such as a CVD-grown aligned forest of nanotubes (tube diameter dt = 2.5 ± 1.5 nm), and both arc discharge (dt = 1.44 ± 0.15 nm) and HiPco (dt = 0.96 ± 0.14 nm) nanotube films separated by electronic type (metallic vs. semiconducting). At pump wavelengths of 1500 and 750 nm, the emitted terahertz radiation is used to estimate a peak current density of ~12 kA/cm² in graphite and a peak current of ~8 nA per nanotube. From the dependence of the injected current on pump polarization, the relative values of the current injection tensor elements are measured, and information is gained on the alignment and birefringence of the nanotube samples. The dependence of the injected current on pump wavelength implies that the currents are likely based on band-band electronic transitions and not on excitonic effects, which govern most linear optical processes.

This thesis reports results from the investigation of optically-induced carrier dynamics in graphite and graphitic carbon nanostructures. In this first set of experiments, the dynamics of photo-excited carriers in exfoliated graphene and thin graphitic films are studied by optical pump-probe spectroscopy. Samples ranging in thickness from 1 to 260 carbon layers are deposited onto an oxidized silicon substrate. Time-resolved reflectivity and transmissivity are measured at 1300 nm, following excitation by 150 fs, 800 nm pump pulses at room temperature. Two time scales are identified over which the extracted transient dielectric function returns to its quiescent value. A fast decay time of ~200 fs in graphene is associated with hot phonon emission and increases to ~300 fs for thicknesses greater than only a few carbon layers. The slow decay time, associated with hot phonon interaction and/or carrier recombination, increases more gradually, from ~2.5 to 5 ps over ~30 layers. A simple model suggests the thickness dependence of the slow decay time is likely a result of thermal diffusion into the substrate. In the second set of experiments, coherently-controlled two-colour injection photocurrents are generated via quantum interference of single- and two-photon absorption in bulk graphite and a variety of single-walled carbon nanotube samples, such as a CVD-grown aligned forest of nanotubes (tube diameter dt = 2.5 ± 1.5 nm), and both arc discharge (dt = 1.44 ± 0.15 nm) and HiPco (dt = 0.96 ± 0.14 nm) nanotube films separated by electronic type (metallic vs. semiconducting). At pump wavelengths of 1500 and 750 nm, the emitted terahertz radiation is used to estimate a peak current density of ~12 kA/cm² in graphite and a peak current of ~8 nA per nanotube. From the dependence of the injected current on pump polarization, the relative values of the current injection tensor elements are measured, and information is gained on the alignment and birefringence of the nanotube samples. The dependence of the injected current on pump wavelength implies that the currents are likely based on band-band electronic transitions and not on excitonic effects, which govern most linear optical processes.

Thesis (M.S.)--West Virginia University, 2004 / Title from document title page. Document formatted into pages; contains vi, 30 p. : ill. (some col.) Includes abstract. Includes bibliographical references (p. 29-30).