Tesis sobre el tema "Quantum devices"
Siga este enlace para ver otros tipos de publicaciones sobre el tema: Quantum devices.
Crea una cita precisa en los estilos APA, MLA, Chicago, Harvard y otros
Consulte los 50 mejores tesis para su investigación sobre el tema "Quantum devices".
Junto a cada fuente en la lista de referencias hay un botón "Agregar a la bibliografía". Pulsa este botón, y generaremos automáticamente la referencia bibliográfica para la obra elegida en el estilo de cita que necesites: APA, MLA, Harvard, Vancouver, Chicago, etc.
También puede descargar el texto completo de la publicación académica en formato pdf y leer en línea su resumen siempre que esté disponible en los metadatos.
Explore tesis sobre una amplia variedad de disciplinas y organice su bibliografía correctamente.
1
Felle, Martin Connor Patrick. "Telecom wavelength quantum devices". Thesis, University of Cambridge, 2017. https://www.repository.cam.ac.uk/handle/1810/270019.
Texto completoResumen
Semiconductor quantum dots (QDs) are well established as sub-Poissonian sources of entangled photon pairs. To improve the utility of a QD light source, it would be advantageous to extend their emission further into the near infrared, into the low absorption wavelength windows utilised in long-haul optical telecommunication. Initial experiments succeeded in interfering O-band (1260—1360 nm) photons from an InAs/GaAs QD with dissimilar photons from a laser, an important mechanism for quantum teleportation. Interference visibilities as high as 60 ± 6 % were recorded, surpassing the 50 % threshold imposed by classical electrodynamics. Later, polarisation-entanglement of a similar QD was observed, with pairs of telecom-wavelength photons from the radiative cascade of the biexciton state exhibiting fidelities of 92.0 ± 0.2 % to the Bell state. Subsequently, an O-band telecom-wavelength quantum relay was realised. Again using an InAs/GaAs QD device, this represents the first implementation of a sub-Poissonian telecom-wavelength quantum relay, to the best knowledge of the author. The relay proved capable of implementing the famous four-state BB84 protocol, with a mean teleportation fidelity as high as 94.5 ± 2.2 %, which would contribute 0.385 secure bits per teleported qubit. After characterisation by way of quantum process tomography, the performance of the relay was also evaluated to be capable of implementing a six-state QKD protocol. In an effort to further extend the emitted light from a QD into the telecom C-band (1530—1565 nm), alternative material systems were investigated. InAs QDs on a substrate of InP were shown to emit much more readily in the fibre-telecom O- and C-bands than their InAs/GaAs counterparts, largely due to the reduced lattice mismatch between the QD and substrate for InAs/InP (~3 %) compared to InAs/GaAs (~7 %). Additionally, to minimize the fine structure splitting (FSS) of the exciton level, which deteriorates the observed polarisation-entanglement, a new mode of dot growth was investigated. Known as droplet epitaxy (D-E), QDs grown in this mode showed a fourfold reduction in the FSS compared to dots grown in the Stranski-Krastanow mode. This improvement would allow observation of polarisation-entanglement in the telecom C-band. In subsequent work performed by colleagues at the Toshiba Cambridge Research Labs, these D-E QDs were embedded in a p-i-n doped optical cavity, processed with electrical contacts, and found to emit entangled pairs of photons under electrical excitation. The work of this thesis provides considerable technological advances to the field of entangled-light sources, that in the near future may allow for deterministic quantum repeaters operating at megahertz rates, and in the further future could facilitate the distribution of coherent multipartite states across a distributed quantum network.
2
Wettstein, Andreas. "Quantum effects in MOS devices /". Zürich, 2000. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=13649.
Texto completo
3
Forsberg, Erik. "Electronic and Photonic Quantum Devices". Doctoral thesis, KTH, Microelectronics and Information Technology, IMIT, 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3476.
Texto completoResumen
In this thesis various subjects at the crossroads of quantummechanics and device physics are treated, spanning from afundamental study on quantum measurements to fabricationtechniques of controlling gates for nanoelectroniccomponents.
Electron waveguide components, i.e. electronic componentswith a size such that the wave nature of the electron dominatesthe device characteristics, are treated both experimentally andtheoretically. On the experimental side, evidence of partialballistic transport at room-temperature has been found anddevices controlled by in-plane Pt/GaAs gates have beenfabricated exhibiting an order of magnitude improvedgate-efficiency as compared to an earlier gate-technology. Onthe theoretical side, a novel numerical method forself-consistent simulations of electron waveguide devices hasbeen developed. The method is unique as it incorporates anenergy resolved charge density calculation allowing for e.g.calculations of electron waveguide devices to which a finitebias is applied. The method has then been used in discussionson the influence of space-charge on gate-control of electronwaveguide Y-branch switches.
Electron waveguides were also used in a proposal for a novelscheme of carrierinjection in low-dimensional semiconductorlasers, a scheme which altogether by- passes the problem ofslow carrier relaxation in suchstructures. By studying aquantum mechanical two-level system serving as a model forelectroabsorption modulators, the ultimate limits of possiblemodulation rates of such modulators have been assessed andfound to largely be determined by the adiabatic response of thesystem. The possibility of using a microwave field to controlRabi oscillations in two-level systems such that a large numberof states can be engineered has also been explored.
A more fundamental study on quantum mechanical measurementshas been done, in which the transition from a classical to aquantum "interaction free" measurement was studied, making aconnection with quantum non-demolition measurements.
4
Holder, Jenna Ka Ling. "Quantum structures in photovoltaic devices". Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:d23c2660-bdba-4a4f-9d43-9860b9aabdb8.
Texto completoResumen
A study of three novel solar cells is presented, all of which incorporate a low-dimensional quantum confined component in a bid to enhance device performance. Firstly, intermediate band solar cells (IBSCs) based on InAs quantum dots (QDs) in a GaAs p-i-n structure are studied. The aim is to isolate the InAs QDs from the GaAs conduction band by surrounding them with wider band gap aluminium arsenide. An increase in open circuit voltage (VOC) and decrease in short circuit current (Jsc) is observed, causing no overall change in power conversion efficiency. Dark current - voltage measurements show that the increase in VOC is due to reduced recombination. Electroreflectance and external quantum efficiency measurements attribute the decrease in Jsc primarily to a reduction in InGaAs states between the InAs QD and GaAs which act as an extraction pathway for charges in the control device. A colloidal quantum dot (CQD) bulk heterojunction (BHJ) solar cell composed of a blend of PbS CQDs and ZnO nanoparticles is examined next. The aim of the BHJ is to increase charge separation by increasing the heterojunction interface. Different concentration ratios of each phase are tested and show no change in Jsc, due primarily to poor overall charge transport in the blend. VOC increases for a 30 wt% ZnO blend, and this is attributed largely to a reduction in shunt resistance in the BHJ devices. Finally, graphene is compared to indium tin oxide (ITO) as an alternative transparent electrode in squaraine/ C70 solar cells. Due to graphene’s high transparency, graphene devices have enhanced Jsc, however, its poor sheet resistance increases the series resistance through the device, leading to a poorer fill factor. VOC is raised by using MoO3 as a hole blocking layer. Absorption in the squaraine layer is found to be more conducive to current extraction than in the C70 layer. This is due to better matching of exciton diffusion length and layer thickness in the squaraine and to the minority carrier blocking layer adjacent to the squaraine being more effective than the one adjacent to the C70.
5
Dikme, Altay. "A Quantum Neural Network for Noisy Intermediate Scale Quantum Devices". Thesis, KTH, Tillämpad fysik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-300394.
Texto completoResumen
Neural networks have helped the field of machine learning grow tremendously in the past decade, and can be used to solve a variety of real world problems such as classification problems. On another front, the field of quantum computing has advanced, with quantum devices publicly available via the cloud. The availability of such systems has led to the creation of a new field of study, Quantum Machine Learning, which attempts to create quantum analogues of classical machine learning techniques. One such method is the Quantum Neural Network (QNN) inspired by classical neural networks. In this thesis we design a QNN compatible with Noisy Intermediate Scale Quantum (NISQ) devices, which are characterised by a limited number of qubits and small decoherence times. Furthermore we provide an implementation of the QNN classifier using the open source quantum computing software development kit, Qiskit provided by IBM. We perform a binary classification experiment on a subset of the MNIST data set, and our results showed a classification accuracy of 80.6% for a QNN with circuit depth 20.Neurala nätverk har varit en stor del av utvecklingen av maskininlärning som ett forskningsområde i det senaste årtiondet, och dessa nätverk har flera appliceringsområden, som till exempel klassificieringsproblemet. Parallelt med denna utveckling, har forskning kring kvantdatorer vuxit fram, med flera kvantsystem allmänt tillgängliga via molnet. Denna tillgänglighet har lett till skapandet av ett nytt forskningsområde; kvantmaskininlärning, som försöker skapa motsvarigheter till klassiska maskininlärningsmetoder på kvantdatorer. En sån metod är kvantneurala nätverk som inspireras av klassiska neurala nätverk. I denna avhandling designar vi ett kvantneuralt närverk som är kompatibel med nuvarande kvantsystem, som kännetecknas av ett begränsat antal qubits och korta dekoherenstider. Dessutom tillhandahåller vi en implementering av en klassificerare med ett kvantneuralt nätverk, med hjälp av IBMs programvaruutvecklingsmiljö Qiskit. Vi utför ett binärt klassificeringsexperiment på en delmängd av MNIST-datamängden, och våra resultatvisar en klassificeringsnoggrannhet på 80,6% för ett kvantneuralt nätverk med kretsdjup 20.
6
Autebert, Claire. "AlGaAs photonic devices : from quantum state generation to quantum communications". Thesis, Sorbonne Paris Cité, 2016. http://www.theses.fr/2016USPCC166/document.
Texto completoResumen
Un des plus grands challenges dans le domaine de l’information quantique est la génération, manipulation et détection de plusieurs qubits sur des micro-puces. On assiste ainsi à un véritable essor des technologies pour l’information quantique et pour transmettre l’information, les photons ont un grand avantage sur les autres systèmes, grâce à leur grande vitesse et leur immunité contre la décohérence.Mon travail de thèse porte sur la conception, fabrication et caractérisation d’une source de photons intriqués en matériaux semiconducteurs d’une très grande compacité. Ce dispositif fonctionne à température ambiante, et émet dans la bande de longueurs d’onde télécom. Après une présentation des concepts fondamentaux (chap. 1), le chap. 2 explique la conception et la fabrication des dispositifs.Le chap. 3 présente les caractérisations opto-électroniques des échantillons pompés électriquement, et le chap. 4 les résultats des mesures de pertes et des caractérisations non-linéaires optiques (génération de seconde harmonique, conversion paramétrique spontanée et reconstruction de l’intensité spectrale jointe). Les chap. 5 et 6 se concentrent sur la caractérisation des états quantiques générés par un dispositif passif (démonstration de l’indiscernabilité et de l’intrication en énergie-temps) et leur utilisation dans un protocole de distribution de clés quantiques multi-utilisateurs (intrication en polarisation). Finalement le travail sur le premier dispositif produisant des pairs de photons dansles longueurs d’onde télécoms, injecté électriquement et fonctionnant à température ambiante est présenté (chap. 7)One of the main issues in the domain of quantum information and communication is the generation,manipulation and detection of several qubits on a single chip. Several approaches are currentlyinvestigated for the implementation of qubits on different types of physical supports and a varietyof quantum information technologies are under development: for quantum memories, spectacularadvances have been done on trapped atoms and ions, while to transmit information, photons arethe ideal support thanks to their high speed of propagation and their almost immunity againstdecoherence. My thesis work has been focused on the conception, fabrication and characterization ofa miniaturized semiconductor source of entangled photons, working at room temperature and telecomwavelengths. First the theoretical concepts relevant to understand the work are described (chapter1). Then the conception and fabrication procedures are given (chapter 2). Chapter 3 presents theoptoelectronics characterization of the device under electrical pumping, and chapter 4 the resultson the optical losses measurements and the nonlinear optical characterization (second harmonicgeneration, spontaneous parametric down conversion and joint spectral intensity reconstruction).Chapters 5 and 6 focus on the characterization of the quantum state generated by a passive sample(demonstration of indistinguishability and energy-time entanglement) and its utilization in a multiuserquantum key distribution protocol (polarization entanglement). Finally the work on the firstelectrically driven photon pairs source emitting in the telecom range and working at room temperatureis presented (chapter 7)
7
Jones, Gregory Millington. "Quantum transport in nanoscale semiconductor devices". College Park, Md. : University of Maryland, 2006. http://hdl.handle.net/1903/3831.
Texto completoResumen
Thesis (Ph. D.) -- University of Maryland, College Park, 2006.Thesis research directed by: Electrical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
8
Koch, Jens. "Quantum transport through single molecule devices". [S.l.] : [s.n.], 2006. http://www.diss.fu-berlin.de/2006/380/index.html.
Texto completo
9
Earnshaw, Mark Peter. "Quantum well electrorefraction materials and devices". Thesis, University of York, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.298387.
Texto completo
10
McNeil, Robert Peter Gordon. "Surface acoustic wave quantum electronic devices". Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.610718.
Texto completo
11
Herrera-Marti, David A. "Implementations of fault-tolerant quantum devices". Thesis, Imperial College London, 2012. http://hdl.handle.net/10044/1/10209.
Texto completoResumen
Accurate control and addressability of quantum devices will come with the promise of improvement in a wide variety of theoretical and applied fields, such as chemistry, condensed matter physics, theoretical computer science, foundational physics, communications, metrology and others. Decoherence of quantum states and the loss of quantum systems have adverse effects and deter a satisfactory usage of quantum devices. This is the main problem to be overcome, which is the goal of quantum fault tolerance. In this thesis we present a series of works that contribute to some of the fields mentioned above, in the direction of fighting decoherence and loss. These works fall in two categories: on one hand, we looked at computer architectures which can be used to combat errors, using techniques of quantum error correcting codes. In a first project we found decoherence and loss probability thresholds below which quantum computing is provably possible. We assumed a very particular error model tailored specially to quantum dots as single photon sources and linear optics. Subsequently we looked at the problem of loss, both of heralded and unheralded, and devised some ways to fight it. The framework under which this work was done was used to develop theory which is currently being tested in a quantum optics experimental group and will be reported in an article later this year. On the other hand, we studied how the error probability can be reduced at the physical level, thanks exclusively to the properties of the system in which information is stored, as opposed to making use of quantum codes. We looked at a particular superconducting circuit, which is potentially very well protected against some types of decoherence. In particular, we observed that the interaction with the environment become weaker for certain values of the circuit external parameters.
12
Turner, Keith. "Hot carrier relaxation in GaAs quantum wells and V-groove quantum wires". Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.308680.
Texto completo
13
Bhat, Jerome C. "Electroluminescent hybrid organic/inorganic quantum dot devices". Thesis, University of Oxford, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.298766.
Texto completo
14
Wu, Junling y 吳峻嶺. "Frequency-dependent quantum transport through nano-devices". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2004. http://hub.hku.hk/bib/B3124614X.
Texto completo
15
Sullivan, Robert James. "High field quantum transport in submicron devices". Thesis, University of Exeter, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.317641.
Texto completo
16
Nash, C. R. "Circuit elements for quantum phase-slip devices". Thesis, University College London (University of London), 2017. http://discovery.ucl.ac.uk/1558026/.
Texto completoResumen
If proved to exist, the phenomenon of quantum phase slips (QPS) allows us to provide a new standard for current. In order to investigate this effect a QPS circuit needs to be constructed with a superconducting nanowire connected in series with an inductor and two thin film resistors. It was found that it was possible to control the low temperature resistance of chromium oxide films by increasing the oxygen pressure at the time of deposition, meaning chromium oxide films could be fabricated for a large resistance range of thin film resistors, from values in the hundreds of ohms to megaohms. These films were found to be amorphous when measured with XRD and displayed extremely low magnetoresistance (with a peak magnetic moment of 5x10 3 μB per atom, compared to 2 μB per atom in bulk ferro- magnetic Cr2O3). The contact resistance of joining these chromium oxide resistors to gold or niobium-silicon was measured using a transmission-line model and this revealed that a gold interlayer provided a much lower contact resistance than direct contact of the circuit components (a contact resistivity of 0.15 m Ohm cm2 for chromium oxide to gold compared to 65 m Ohm cm2 for chromium oxide to niobium silicon). Several methods were used to restrict nanowire dimensions. It was found that combining a HSQ fabrication method with a neon mill resulted in the creation of wires below the coherence length of niobium nitride (30 nm), which made these wires suitable for QPS measurements.
17
Herbschleb, Ernst David. "Charge-writing induced quantum devices in graphene". Thesis, University of Cambridge, 2014. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.708368.
Texto completo
18
Bagwell, Philip F. (Philip Frederick). "Quantum mechanical transport in submicron electronic devices". Thesis, Massachusetts Institute of Technology, 1990. http://hdl.handle.net/1721.1/44264.
Texto completo
19
Stace, Thomas Michael. "Quantum information transfer in solid state devices". Thesis, University of Cambridge, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.615715.
Texto completo
20
Fry, Daniel J. "Multimode quantum interference in integrated photonic devices". Thesis, University of Bristol, 2015. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.685350.
Texto completoResumen
This thesis contains experimental and theoretical work based on pair-photon interference in integrated photonic devices. Multimode interference (MMI) devices fabricated with silicon oxynitride technology are used to experimentally demonstrate high visibility pair-photon Hong-Ou-Mandel (HOM) interference. A theoretical model of pair-photon interference is derived for the self-imaging multimode interference that occurs between guided modes in the MMI device , and the scalability of MMI devices for use in quantum photonic networks is considered by modelling serially concatenated devices for decoherent intermodal dispersion effects.
21
Boonkoom, Thitikorn. "InP quantum dots for hybrid photovoltaic devices". Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/17778.
Texto completoResumen
Significant research efforts have been directed towards the development of solar cells comprising blends of conjugated polymers and II-VI inorganic semiconductors (e.g. CdSe and CdS). Despite recent advances in the power conversion efficiency of such devices, the toxicity of Cd-based materials remains a concern with regard to widespread implementation. This thesis focuses on alternative (lower toxicity) InP nanocrystals for use as electron acceptors and light-harvesting materials in solution-processed polymer solar cells. In this thesis a combination of novel materials design/processing, transient absorption spectroscopy (TAS) and time-resolved photoluminescence spectroscopy (TRPL) is used to study the charge generation in InP:polymer photoactive layers. These studies are complimented by morphological characterisation of the photoactive layers as well as device studies. One aim of this thesis is the elucidation of quantitative structure function relationships that can be used to guide the design of new hybrid nanocomposite materials for photovoltaic devices. As such the data presented in this thesis helps to advance the present day understanding how hybrid solar cells work. The first chapter focuses on the synthesis of InP quantum dots (QDs) using an organometallic reaction. The aim of the work in this chapter was to prepare InP QDs with a size that provides an appropriate energy offset relative to the selected the electron donating polymer, poly(3-hexylthiophene) (P3HT). Detailed studies on the growth of InP QDs and how the reaction conditions affect the particle size are provided. The process of ligand exchange from hexadecylamine (HDA) to pyridine prior to blending with P3HT is also described. The second chapter focuses on charge transfer between the P3HT and the InP QDs which is a key process for achieving efficient photovoltaic device operation. Steady state and time-resolved photoluminescence and absorption spectroscopy were used to better understand the parameters influencing charge separation. After the blending and annealing conditions had been optimised to maximise the yield of photogenerated charges, the P3HT:InP blend was found to provide approximately twice yield of standard P3HT:PCBM blends. In addition, the decay lifetime of the polaron in P3HT:InP was found to be longer than that of P3HT:PCBM, suggesting the P3HT:InP blend is a promising active layer material for hybrid solar cells. The third chapter focuses on the fabrication and characterisation of hybrid solar cells. The fabrication conditions were optimised before carrying out detailed studies on the effect of thermal annealing. Although the device performance improved significantly with increasing annealing temperature, the net photocurrent was found to be low, compared to standard P3HT:PCBM devices, suggesting poor charge transport within the device. Nevertheless, if the charge transport can be improved, P3HT:InP still has potential to provide efficient hybrid solar cells. The last result chapter focuses on preliminary studies of quantum dot based light emitting diodes (QDLEDs) using InP QDs as light emitters. ZnO was used as electron transporting and hole blocking layer and poly(9,9-dioctylfluorene) (PFO) as a host medium and a hole transporting layer. The device structure and the PFO:InP blend composition were investigated to obtain QDLEDs with electroluminescence from the InP quantum dots. The findings suggest that ZnO plays a key role in suppressing the electroluminescence of PFO, most likely due to the hole blocking effect of the ZnO layer. Despite the low efficiencies of the InP-based QDLEDs, the results suggest that InP QDs are potential candidates for emitters in QDLEDs.
22
Allen, Monica Theresa. "Quantum Electronic Transport in Mesoscopic Graphene Devices". Thesis, Harvard University, 2016. http://nrs.harvard.edu/urn-3:HUL.InstRepos:33493258.
Texto completoResumen
Graphene provides a rich platform for the study of interaction-induced broken symmetry states due to the presence of spin and sublattice symmetries that can be controllably broken with external electric and magnetic fields. At high magnetic fields and low temperatures, where quantum effects dominate, we map out the phase diagram of broken symmetry quantum Hall states in suspended bilayer graphene. Application of a perpendicular electric field breaks the sublattice (or layer) symmetry, allowing identification of distinct layer-polarized and canted antiferromagnetic v=0 states. At low fields, a new spontaneous broken-symmetry state emerges, which we explore using transport measurements.
The large energy gaps associated with the v=0 state and electric field induced insulating states in bilayer graphene offer an opportunity for tunable bandgap engineering. We use local electrostatic gating to create quantum confined devices in graphene, including quantum point contacts and gate-defined quantum dots.
The final part of this thesis focuses on proximity induced superconductivity in graphene Josephson junctions. We directly visualize current flow in a graphene Josephson junction using superconducting interferometry. The key to our approach involves reconstruction of the real-space current density from magnetic interference using Fourier methods. We observe that current is confined to the crystal boundaries near the Dirac point and that edge and bulk currents coexist at higher Fermi energies. These results are consistent with the existence of "fiber-optic" edge modes at the Dirac point, which we model theoretically. Our techniques also open the door to fast spatial imaging of current distributions along more complicated networks of domains in larger crystals.Physics
23
Marquardt, Ronald R. McGill T. C. McGill T. C. "Quantum magnetotransport studies of semiconductor heterostructure devices /". Diss., Pasadena, Calif. : California Institute of Technology, 1995. http://resolver.caltech.edu/CaltechETD:etd-10172007-111750.
Texto completo
24
Vukmirovic, Nenad. "Physics of intraband quantum dot optoelectronic devices". Thesis, University of Leeds, 2007. http://etheses.whiterose.ac.uk/1590/.
Texto completoResumen
In last two decades, semiconductor nanostructures, such as quantum wells, wires and dots, have been recognised as sources and detectors of radiation in the mid- and far-infrared region of the spectrum. Much of a success has been obtained with quantum well based intraband devices, such as quantum cascade lasers and quantum well infrared photodetectors. However due to longer carrier lifetimes in quantum dots, it is expected that optoelectronic devices based on intraband transitions in self-assembled quantum dots would have superior performance to their quantum well counterparts. In order to fully exploit this prospect, appropriate theoretical models describing electronic, optical and transport properties of the active region of these devices need to be developed, which was the subject of this thesis. It was shown how symmetry of the dot shape can be exploited to efficiently calculate the energy levels within the framework of the multiband envelope function method. The implementation of the method in the plane wave representation of the Hamiltonian eigenvalue problem and the results of its application to square based pyramidal InAs/GaAs quantum dots and hexagonal III-nitride quantum dots were given. A semiclassical model of intraband carrier dynamics in quantum dots was then developed and applied to design an optically pumped long wavelength mid-infrared laser based on intersublevel transitions in InAs/GaAs quantum dots. Two orders of magnitude lower pumping flux was predicted than in similar quantum well based devices. Next, simulations of the optical absorption spectrum in the existing quantum dot infrared photodetector structures were performed. A special emphasis was put into quantum dots-in-a-well structures and explanation of the effect of well width on the detection wavelength. A theory of transport in quantum dot infrared photodetectors starting from the energy levels and wavefunctions obtained by solving the envelope Hamiltonian, yielding as output the device characteristics such as dark current and responsivity, was then developed. The comparison with experimental data available in the literature was made, yielding a good agreement. Finally, the theory of electron transport through arrays of closely stacked quantum dots, where coherent and polaronic effects become important, therefore requiring the treatment within the formalism of the nonequilibrium Green's functions, rather than the semiclassical approach, was presented. A design of a structure promising to act as a terahertz quantum dot cascade laser was given.
25
Alecce, Antonio. "Selected problems in quantum mechanics: towards topological quantum devices and heat engine". Doctoral thesis, Università degli studi di Padova, 2017. http://hdl.handle.net/11577/3421931.
Texto completoResumen
The work presented in this thesis meanly addresses two topics in theoretical physics which are quantum thermodynamics and topological order. In the first case, physicists are trying to build up a theory able to describe quite in general phenomena involving heat and energy exchanges in quantum systems. The second topic, instead, is related to exotic phenomena and states of matter like the quantum Hall effect (QHE) or topological insulators and topological superconductors.
In the first part od the thesis we define the quantum dynamics for closed and open systems. This is a key ingredient to address the field of quantum thermodynamics. Then, after an introductory part about the quantum thermodynamic transformations, we move toward the field of nonequilibrium fluctuation relations. We address the problem of irreversibility in classical as well as quantum mechanics. Here we present one of our main result. We characterize the "thermodynamic" irreversible adiabatic evolution of a quantum system starting such branch in a thermal equilibrium state at inverse temperature ßi. We give the amount of thermodynamic entropy growth for the process. As direct application of the preceding result we then address a quantum Otto cycle (QOC) working at finite power. We saw that the increasing of irreversible character of the evolution affects the main figures of merit of the cycle.
The second part of the thesis addresses the field of topological order. At first we introduce the concept of topological orders, classes and invariants. Then we introduce the well known Kitaev model for 1 D superconductors. This model predicts Majorana zero mode at the ends of the wire (the 1 D system). MZM are topological states showing great resistance against disorder, local perturbations and any dissipative element. Then we consider a generalized Kitaev model where long range interactions are accounted. We get rich topological phase diagrams showing the presence of several MZM per edge. We study the appearing/disappearing dynamics of the modes according to the time reversal symmetry, that is fundamental in the study of topological phase. The phase diagrams we obtained also show the presence of massive edge modes. In this last case the topological invariants do not well describe any transition. At last we focused on a very limit cases where MZM are obtained at finite length of the wire. Such cases are really interesting since the great advance we can get from the finiteness of the wire in an experimental setup.
The last part is about single electron tunneling devices. Here we got a different ability to work as "heat-to-current harvester" for a device using quantum dots respect to an analogue one using metallic dots.
These different arguments find their unity by considering recent scientific works in which heat transport is addressed in single electron transistor devices where some element of the circuit shows a topological behaviour. This is a perfect system from hich we can get new transport phenomena.Il lavoro presentato in questa tesi tratta principalmente due argomenti quali le termodinamica quantistica e l'ordine topologico. Nel primo caso fisici stanno provando a costruire una teoria capace to descrivere abbastanza in generale gli scambi di calore ed energia in sistemi quantistici. Il secondo argomento, invece, si relaziona a fenomini e stati della meteria esotici come l'effetto "fractional quantum hall" o gli isolanti e superconduttori topologici. Nella prima parte della tesi definiamo la dinamica quantistica per un sistema chiuso ed aperto. Questo é fondamentale per trattare il campo della termodinamica quantistica. Poi, dopo una parte introduttiva sulle trasformazioni termodinamiche quantistiche, ci si sposta verso il campo delle relazioni di fluttuazione non all'equilibrio. Viene trattato il problema dell'irreversibilità tanto nella meccanica classica quanto in quella quantistica. Qui presentiamo uno dei nostri maggiori risultati. Caratterizziamo un'evoluzione adiabatica "termodinamica" irreversibile di un sistema quantistico il cui stato iniziale é uno di equilibrio alla temperatura inversa iniziale ßi. Viene ricavato l'incremento di entropia termodinamica del processo. Come applicazione diretta del risultato precedente si é considerato un ciclo Otto quantistico (QOC). Abbiamo notato che l'aumentare del carattere irreversibile dell'evoluzione inficia le principali figure di merito del ciclo. La seconda parte della tisi, invece, guarda al campo dell'ordine topologico. All'inizio introduciamo i concetti di ordine, classi ed invarianti topologici. Poi introduciamo il ben noto modello di Kitaev per superconduttori 1 D. Questo modello prevede "Majorana zero mode" (MZM) ai capi del filo (il sistema 1 D). I Majorana zero modes sono stati topologici che mostrano una grande resistenza contro il disordine, perurbazioni locali e ogni genere di elemento dissipativo. In viene considerata una generalizzazione del modello di Kitaev con interazioni a molti vicini. Vengono ricavati diagrammi di fase topologica molto "ricchi" che mostrano la presenza di molti MZM per lato. Inoltre si studia l'apparire e scomparire di tali modi a seconda della simmetria di inversione temporale, che é fondamentale per lo studio della fase topologica. I diagrammi di fase mostrano anche la presenza di massive edge modes. In questo ultimo caso gli invarianti topologici non descrivono bene tutte le transizioni. In fine ci siamo focalizzati sul caso limite dove gli MZM sono ottenuti quando il sistema ha una lunghezza finita. Tali casi sono molto interressanti visto il grande vantaggio che possiamo ricavarne in un setup sperimentale dato che il sistema può grandezza ridotta. L'ultima parte é sui dispositivi single electron tunneling. Qui abbiamo descritto la differente capacità a lavorare come "heat-to-current harvester" per un dispositivo che usa quantum dots rispetto ad uno analogo che usa metallic dots. Questi argomenti differenti trovano un punto di unione considerando lavori scientifici recenti in cui si considera trasporto di calore su dispositivi "single electron tuunneling" in cui alcune delle componenti circuitali dei dispositivi mostrano una natura topologica. Sono sistemi perfetti dai quali possiamo ottenere nuovi fenomeni di trasporto.
26
Sudirgo, Stephen. "Quantum and spin-based tunneling devices for memory systems /". Link to online version, 2006. https://ritdml.rit.edu/dspace/handle/1850/2066.
Texto completo
27
Peyvast, Negin. "Quantum well and quantum dot broadband optical devices for optical coherence tomography applications". Thesis, University of Sheffield, 2016. http://etheses.whiterose.ac.uk/15906/.
Texto completoResumen
In this thesis quantum well (QW) and quantum dot (QD) based devices are investigated with the aim of obtaining broad bandwidth light sources for optical coherence tomography (OCT) applications. QD based structures have many possible advantages for broadband applications due to their inhomogeneous broadening. However, more investigation is required in order to fulfill this potential. Firstly, in chapter one, an introduction to the fundamental principles of semiconductor heterostructures is provided followed by basic concepts of OCT. The experimental techniques used in this thesis are outlined and briefly discussed. Brief reviews of the gain measurement techniques which have been used throughout this thesis are presented. Free carrier effects have been highlighted as a source of line-width broadening in QD structures. However, to date the effects of free carriers have mostly been experimentally determined at comparatively high carrier densities. In chapter 2 I develop a model for the gain and spontaneous emission spectra of QD active elements and show that not only are free carrier effects important at high QD occupancies, but also at much lower carrier densities where QD lasers would normally operate. Furthermore, it is shown that the choice of carrier distribution function is far less important than was previously thought in describing the experimentally observed gain and spontaneous emission spectra. The literature has suggested that incorporating QW layers in hybrid QW/QD structures changes the behaviour of the QDs. Optical pumping of the QD active element by emission from the QW active element is investigated experimentally in chapter 3. Analysis of a QD laser, a hybrid QW/QD super luminescence diode (SLD) and mesa diodes with different active element designs show that emission from the quantum well layer does indeed modify the QD spontaneous emission, suggesting optical pumping of the QD states and the prospect for enhanced gain from the QD ground-state. Finally in chapter 4, different configurations of swept light sources (SLSs) are implemented with the aim of obtaining broader spectral bandwidth. It is demonstrated that increasing the gain of the QD-SOA is important in enhancing the sweep range. The use of complimentary SOAs is then explored. InP QW-SOAs and GaAs based QD-SOAs have overlapping gain and SE spectra which is utilised in a swept source laser (SSL) and filtered ASE configuration SLS. The results suggest that such sources may be able to achieve ~220nm sweep bandwidths. Chapter 5 summarizes the whole thesis and provides an overview of future work.
28
Griffiths, Andrew. "Exploring MBE growth of quantum dots : low density growth for quantum information devices". Thesis, University of Sheffield, 2014. http://etheses.whiterose.ac.uk/9413/.
Texto completoResumen
This thesis concerns the growth and subsequent application of InAs/GaAs quantum dots for novel research applications including entangled photon sources, and coupled quantum well/quantum dot devices. This thesis gives an overview of quantum dot growth and tunable parameters of quantum dots (QDs), showing a review of prior work detailing their composition and properties. Special attention is given to wavelength tuning, and optimisation of the linewidth for a target application, including analysis of post-growth annealing. Subsequent discussion focuses on low-density quantum dot samples, both in terms of growth and application. In conjunction with Cambridge University, micro-photoluminescence results of a short-wavelength QD sample are demonstrated, showing successful use of the rotation stop growth method, with further analysis and comparisons made with photoluminescence maps and atomic force microscopy. Analysis of long- wavelength (>1300nm at room temperature) QDs is also shown, with single dot work being performed on rotation-stop growth samples in analysis of Fine Structure Splitting (FSS) of individual QDs by Heriot Watt University. Results show an unexpectedly low FSS value for samples grown at the University of Sheffield, with potential for long wavelength entangled photon emitters. Growth optimisation of both the long- and short-wavelength structures is described, with optimisation required for the longer-wavelength samples, due to a comparative lack of cross-wafer QD density variation. A novel adjustment to photoluminescence excitation is also discussed, with polarisation analysis being performed on QD samples designed for optical emission. Results indicate that QD properties have little to no effect on the spin retention in GaAs-capped QDs: instead surrounding material has a larger effect, indicating that spin loss happens primarily between excitation and capture.
29
Zhang, Zhigang. "Modeling, analysis and control of quantum electronic devices". [College Station, Tex. : Texas A&M University, 2006. http://hdl.handle.net/1969.1/ETD-TAMU-1811.
Texto completo
30
Wetzler, Reinhard. "Spatial Coulomb effects in semiconductor quantum dot devices". [S.l.] : [s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=971082073.
Texto completo
31
Kutluer, Kutlu. "Quantum memory protocols for photonic solid-state devices". Doctoral thesis, Universitat Politècnica de Catalunya, 2017. http://hdl.handle.net/10803/461494.
Texto completoResumen
A photonic quantum memory (QM) is a device that has the capability of storing a quantum state of light and retrieving back after a controlled time. It is an important element in quantum information science and is, among other applications, a crucial device for quantum repeater architectures which have been proposed to overcome the loss and the decoherence issues in long distance transmission of photons. Rare earth ion doped solid state systems are promising candidates for QMs which combine the advantages of solid state systems, such as scalability and reduced experimental complexity, with the long coherence time typically found in atomic systems. In this thesis, I investigated three different QM protocols in a Pr3+:Y2SiO5 crystal.
The first part describes here the first demonstration of the spectral hole memory (SHoMe) protocol which was proposed theoretically in 2009. This protocol relies on slowing down the light in a long-lived spectral hole and transferring the excitations to the spin state. We first prepare a spectral hole, then send an input pulse whose bandwidth is comparable with the hole and stop the compressed light in the crystal by transferring the off-resonant coherence to the spin state with an optical p pulse. Later a second p pulse transfers the coherence back and leads to the emission of the stored light. We reached a storage and retrieval efficiency of around 40% in the classical regime, and of 31% in the single photon level, with a signal-to-noise ratio of 33 ± 4 for a mean input photon number of 1. These results demonstrate the most efficient and noiseless spin-wave solid-state optical memory at the single photon level to date.
The second part of the thesis describes new experiments using the well-known atomic frequency comb (AFC) protocol. It is based on tailoring the inhomogeneously broadened absorption profile of the rare earth with periodic absorptive peaks, which induces the re-emission of the absorbed light field after a certain time determined by the separation between the peaks. In this chapter I describe several AFC experiments. First I present the storage of frequency converted telecom photons into our crystal where we obtained a total efficiency of 1.9 ± 0.2 % for a storage time of 1.6 µs storage time and signal-to-noise ratio of more than 200 for a mean input photon number of 1. Then I discuss the results of improved excited state storage efficiency values for long storage times where we achieved 30% at short storage times and up to 17% at 10 µs storage time. And finally I present a spin-wave AFC experiment where we obtained a signal-to-noise ratio value of 28 ± 8 for a mean input photon number of 1, the highest value achieved so far for this kind of experiment.
Finally, in the last part, I describe the first demonstration of a solid-state photon pair source with embedded multimode quantum memory. The aim of the protocol is to combine a single photon source and a QM in one ensemble as in the well-known Duan-Lukin-Zoller-Cirac (DLCZ) scheme however this time not in a cold atomic ensemble but in a solid-state crystal.
The protocol takes advantage of the AFC protocol for rephasing the ions and obtaining efficient read-out. The use of AFC also makes the protocol temporally multi-mode. In the experiment, after the AFC preparation we send an on-resonant write pulse and detect the decayed Stokes photons which herald single spin excitations. At a later time a read pulse transfers the spin excitation back to the excited state and we detect the anti-Stokes photons. We show strong non-classical second order cross-correlations between the Stokes and anti-Stokes photons and demonstrate storage of 11 temporal modes.
The results presented in this thesis represent a significant contribution to the field of solid-state quantum memories and an important steps towards the realization of scalable quantum network architectures with solid state systems.Una memòria quàntica (MQ) és un dispositiu que té la capacitat d'emmagatzemar l'estat quàntic de la llum i retornar-lo després d'un temps controlat.És un element important en la ciència de la informació quàntica i és un dispositiu crucial per a arquitectures de repetidors quàntica.Els sistemes d'estat sòlid basats en ions de terres rares són candidats prometedors per implementar MQs, ja que combinen els avantatges dels sistemes sòlids (escalabilitat i poca complexitat experimental) amb els llargs temps de coherència dels sistemes atòmics.En aquesta tesis he investigat tres protocols diferents de MQ en un cristall de Pr3+:Y2SiO5. La primera part descriu la primera demostració del protocol de memòria basat en forats espectrals (MFE), que va ser proposat teòricament el 2009. Aquest protocol es basa en disminuir la velocitat de la llum en un forat espectral de vida llarga i transferir les excitacions a un estat d'espín. Comencem preparant un forat espectral, després enviem un pols de llum amb una amplada espectral comparable a la del forat i aturem la llum comprimida en el cristall transferint la coherència fora de ressonància a l'estat d'espín amb un pols òptic.Seguidament un segon pols retorna la coherència i porta a l'emissió de la llum emmagatzemada. Aconseguim una eficiència d'emmagatzematge i recuperació de 40% en el règim clàssic i de 31% al nivell de fotons individuals, amb una relació senyal-soroll de 33 ±4 per un nombre mitjà de fotons incidents igual a 1. Aquests resultats demostren la memòria òtica operant al nivell de fotons individuals amb més eficiència i més lliure de soroll. La segona part de la tesis descriu nous experiments que utilitzen el protocol de pintes de freqüència atòmiques (PFA). Aquest està basat en modificar el perfil d'absorció eixamplat inhomogèniament dels ions de terres rares, creant pics d'absorció periòdics que indueixen la reemissió del camp de llum absorbit, després d'un cert temps que ve determinat per la separació dels pics. En aquest capítol descric varis experiments de PFA. Primer presento l'emmagatzematge en el nostre cristall de fotons amb freqüència convertida des de telecom, obtenint una eficiència total de 1.9 ± 0.2% per un temps d'emmagatzematge de 1.6us i una relació senyal-soroll de més de 200 per un nombre mitjà de fotons incidents igual a 1. Seguidament discuteixo els resultats obtinguts amb una millorada eficiència d'emmagatzematge en l'estat excitat per temps d'emmagatzematge llargs, on vam obtenir 30% per temps curts i 17% a 10us. I finalment presento un experiment de PFA amb ona d'espín on vam obtenir una relació senyal-soroll de 28 ± 8 per un nombre mitjà de fotons incidents igual a 1, el valor més alt assolit mai en un experiment d'aquest tipus. Finalment, en la última part, descric la primera demostració d'una font de parelles de fotons d'estat sòlid integrada amb una memòria quàntica multimodal. L'objectiu del protocol és combinar en un sol sistema una font de fotons individuals i una MQ, com té lloc en el conegut esquema de Duan-Lukin-Cirac-Zoller (DLCZ), però en aquest cas amb un cristall en lloc d'un sistema d'àtoms freds.El protocol agafa els avantatges del protocol PFA per refasar els ions i obtenir una recuperació eficient. Utilitzant PFA fa que el protocol sigui temporalment multimodal.En l'experiment, després de la preparació de la PFA, enviem un pols d'escriptura en ressonància i detectem un fotó Stokes que anuncia excitacions d'espín individuals. Un temps més tard, un pols de lectura transfereix l'excitació d'espín de tornada cap a l'estat excitat i detectem fotons anti-Stokes. Mostrem fortes correlacions de segon ordre no-clàssiques entre els fotons de Stokes i anti-Stokes i demostrem l'emmagatzematge de 11 modes temporals. Els resultats presentats en aquesta tesis representen una contribució significativa en el camp de les memòries quàntiques d'estat sòlid i un pas important cap a la realització d'arquitectures de xarxes quàntiques amb sistemes d'estat sòlid
32
Barrios, Andres J. "Nonlinear quantum transport in low-dimensional electronic devices". FIU Digital Commons, 2002. http://digitalcommons.fiu.edu/etd/1414.
Texto completoResumen
The study of transport processes in low-dimensional semiconductors requires a rigorous quantum mechanical treatment. However, a full-fledged quantum transport theory of electrons (or holes) in semiconductors of small scale, applicable in the presence of external fields of arbitrary strength, is still not available. In the literature, different approaches have been proposed, including: (a) the semiclassical Boltzmann equation, (b) perturbation theory based on Keldysh's Green functions, and (c) the Quantum Boltzmann Equation (QBE), previously derived by Van Vliet and coworkers, applicable in the realm of Kubo's Linear Response Theory (LRT).
In the present work, we follow the method originally proposed by Van Vliet in LRT. The Hamiltonian in this approach is of the form: H = H°(E, B) + λV, where H0 contains the externally applied fields, and λV includes many-body interactions. This Hamiltonian differs from the LRT Hamiltonian, H = H° - AF(t) + λV, which contains the external field in the field-response part, -AF(t). For the nonlinear problem, the eigenfunctions of the system Hamiltonian, H°(E, B) , include the external fields without any limitation on strength.
In Part A of this dissertation, both the diagonal and nondiagonal Master equations are obtained after applying projection operators to the von Neumann equation for the density operator in the interaction picture, and taking the Van Hove limit, (λ → 0 , t → ∞ , so that (λ2 t)n remains finite). Similarly, the many-body current operator J is obtained from the Heisenberg equation of motion.
In Part B, the Quantum Boltzmann Equation is obtained in the occupation-number representation for an electron gas, interacting with phonons or impurities. On the one-body level, the current operator obtained in Part A leads to the Generalized Calecki current for electric and magnetic fields of arbitrary strength. Furthermore, in this part, the LRT results for the current and conductance are recovered in the limit of small electric fields.
In Part C, we apply the above results to the study of both linear and nonlinear longitudinal magneto-conductance in quasi one-dimensional quantum wires (1D QW). We have thus been able to quantitatively explain the experimental results, recently published by C. Brick, et al., on these novel frontier-type devices.
33
Clarke, Warrick Robin Physics Faculty of Science UNSW. "Quantum interaction phenomena in p-GaAs microelectronic devices". Awarded by:University of New South Wales. School of Physics, 2006. http://handle.unsw.edu.au/1959.4/32259.
Texto completoResumen
In this dissertation, we study properties of quantum interaction phenomena in two-dimensional (2D) and one-dimensional (1D) electronic systems in p-GaAs micro- and nano-scale devices. We present low-temperature magneto-transport data from three forms of low-dimensional systems 1) 2D hole systems: in order to study interaction contributions to the metallic behavior of 2D systems 2) Bilayer hole systems: in order to study the many body, bilayer quantum Hall state at nu = 1 3) 1D hole systems: for the study of the anomalous conductance plateau G = 0.7 ???? 2e2/h The work is divided into five experimental studies aimed at either directly exploring the properties of the above three interaction phenomena or the development of novel device structures that exploit the strong particle-particle interactions found in p-GaAs for the study of many body phenomena. Firstly, we demonstrate a novel semiconductor-insulator-semiconductor field effect transistor (SISFET), designed specifically to induced 2D hole systems at a ????normal???? AlGaAs-on-GaAs heterojunction. The novel SISFETs feature in our studies of the metallic behavior in 2D systems in which we examine temperature corrections to ????xx(T) and ????xy(T) in short- and long-range disorder potentials. Next, we shift focus to bilayer hole systems and the many body quantum Hall states that form a nu = 1 in the presence of strong interlayer interactions. We explore the evolution of this quantum Hall state as the relative densities in the layers is imbalanced while the total density is kept constant. Finally, we demonstrate a novel p-type quantum point contact device that produce the most stable and robust current quantization in a p-type 1D systems to date, allowing us to observed for the first time the 0.7 structure in a p-type device.
34
Zibold, Tobias. "Semiconductor based quantum information devices theory and simulations /". kostenfrei, 2007. http://mediatum2.ub.tum.de/doc/617147/document.pdf.
Texto completo
35
Carmmona, Humberto de Andrade. "Quantum magneto-transport in AlGaAs/GaAs nano-devices". Thesis, University of Nottingham, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.319672.
Texto completo
36
Wallace, Chik-Ho Choy. "Modelling and electro-optic quantum-wells modulation devices". Thesis, University of Surrey, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.267967.
Texto completo
37
Sohn, Choong S. (Choong Sik) 1977. "Commercialization potential of quantum dot light emitting devices". Thesis, Massachusetts Institute of Technology, 2003. http://hdl.handle.net/1721.1/7973.
Texto completoResumen
Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2003.Includes bibliographical references (leaves 45-48).
The use of quantum dots as discrete emitters in hybrid organic/inorganic light emitting devices is an attractive approach for producing novel display products. These structures exhibit narrow-band emission tunable across the visible spectrum - characteristics allowing for display devices not possible with current OLEO materials. In this work, quantum dot light emitting devices (QD-LEDs) using small molecule host materials are evaluated as a potential platform for the growing OLEO industry. Specific applications are suggested and the primary technology hurdles identified. A search of relevant patents pertaining to quantum dot synthesis and device structure was conducted to reveal a significant opportunity for the commercialization of QD-LED devices. A business model has been devised based upon several developing companies in the OLEO industry with a focus on licensing of technology as the primary source of revenue.
by Choong S. Sohn.
M.Eng.
38
Lee, Alan Wei Min. "Terahertz imaging and quantum cascade laser based devices". Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/58175.
Texto completoResumen
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2010.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 108-114).
The terahertz (THz) frequency range (f=0.3-10 THz, [lambda]=30-1000 lam) is much less technologically developed that the adjacent microwave and infrared frequency ranges, but offers several advantages for imaging applications: THz wavelengths offer better spatial resolutions than microwave frequencies, and THz radiation is able to penetrate materials that are opaque at infrared frequencies (e.g. packaging, plastics, paints and semiconductors). These features, combined with the unique THz spectral signatures of chemicals have lead to the development of terahertz imaging systems for non-destructive test. However, the weak radiation sources in these existing systems result in single pixel scanning architectures requiring minutes to acquire images or enhanced speed at the expense of signal to noise ratio (SNR). In this thesis, a system for real-time imaging is demonstrated using recently developed terahertz quantum-cascade laser (QCL) sources, along with commercial, focal plane array thermal detectors. The system uses a high power (48 mW) 4.3-THz QCL, which is also used to characterize the focal plane array, resulting in a noise equivalent power (NEP) of 320 pW/Hz. The source and detector are used in a synchronous detection scheme, resulting in an SNR of ~25 dB/pixel at a 20-Hz frame rate. This represents a two order of magnitude improvement in speed over previous systems at comparable SNRs. Real-time imaging over a 25-m distance is described, using a QCL adjusted for emission in the narrow 4.9 THz atmospheric transmission window.
(cont.) The challenges posed by the long THz wavelengths in QCL waveguide design leads to a tradeoff between high temperature operation (<186K) and high power/good beam patterns (248 mW peak, l2deg FWHM). To mitigate these tradeoffs, a technique for buttcoupling a metal-metal waveguide QCL to an index matched lens is developed. The resulting device achieves the highest reported power for a MM waveguide (145 mW peak) and while retaining a high operating temperature (160 K) and achieving a narrow beam pattern (<5deg). The lens coupling technique is also used to add spectroscopic capability to the system, through the development of an external cavity QCL. The butt-coupling of an antireflection coated lens to a semi-insulating surface plasmon waveguide QCL results in increased optical losses and suppression of lasing. Lasing is recovered using an external optical system with a reflective grating for frequency selective feedback. A device is characterized showing 4% tuning range at ~4.4 THz, and is among the first demonstrations of tunable THz QLCs.
by Alan Wei Min Lee.
Ph.D.
39
Washington, Zoe. "Quantum-bit devices inspired by classical stochastic analogies". Thesis, Loughborough University, 2013. https://dspace.lboro.ac.uk/2134/13022.
Texto completoResumen
As systems/structures get smaller we need to take into account noise and quantum effects and so, we need to develop some quantum devices. Quantum devices work using quantum principles like qubits that have already been developed, i.e., superconducting qubits that are going to be discussed in chapter 1. Initially, scientists wanted to use qubits to do quantum computations, this is not easy so scientists developed methods to do something different, e.g. quantum metamaterials. Here in this thesis we describe two examples of quantum devices. Our first device is the parametric quantum amplifier. Used when we need to amplify very weak signals. Amplifying a weak signal on the nanoscale is a very big challenge, this is due to classical and quantum noise, and so, we need to employ quantum physics to resolve this issue. The proposed two-qubit system amplifies weak signals at very small scales. We have shown that we can construct a multitude of novel devices on the nano-scale with the use of qubits Our second device uses harmonic mixing. It can be used where rectification is needed, for example, when we need to rectify some fluctuations and in principle some quantum fluctuations in order to pump either an excited or ground state of the two qubit device. In this thesis we propose how to do this. Firstly, we propose that if we apply harmonic mixing of two signals for two qubits, using the structure of the equation and basically the structure of quantum mechanics we can pump a desirable quantum state. We can pump either the upper or ground state by changing the signal.
40
Al-Owaedi, Oday. "Electronic properties of nano and molecular quantum devices". Thesis, Lancaster University, 2016. http://eprints.lancs.ac.uk/84113/.
Texto completoResumen
The exploring and understanding the electronic properties of molecules connected to metallic leads is a vital part of nanoscience if molecule is to have a future. This thesis documents a study for various families of organic and organometallic molecules, which offer unique concepts and new insights into the electronic properties of molecular junctions. Different families of molecules were studied using a combination of density functional theory (DFT) and non-equilibrium Green’s function formalism of transport theory. The main results of this thesis are as follows: A quantum circuit rule for combining quantum interference effects in the conductive properties of oligo(phenyleneethynylene) (OPE)-type molecules possessing three aromatic rings was investigated both theoretically and experimentally. Molecules were of the type X-Y-X, where X represents pyridyl anchors with para (p), meta (m) or ortho (o) connectivities and Y represents a phenyl ring with p and m connectivities. The conductances Gxmx (Gxpx) of molecules of the form X-m-X (X-p-X), with meta (para) connections in the central ring, were predominantly lower (higher), irrespective of the meta, para or ortho nature of the anchor groups X, demonstrating that conductance is controlled by the nature of quantum interference in the central ring Y. The singlemolecule conductances were found to satisfy the quantum circuit rule Gppp/Gpmp=Gmpm/Gmmm. This demonstrates that the contribution to the conductance from the central ring is independent of the para versus meta nature of the anchor groups. The conductance and the decay of conductance as a function of molecular length within a homologous series of oligoynes, Me3Si― (C≡C)n―SiMe3 (n = 2, 3, 4, or 5), is shown to depend strongly on the solvent medium. Single molecule junction conductance measurements have been made with the I(s) method for each member of the series Me3Si―(C≡C)n―SiMe3 (n = 2, 3, 4, and 5) in mesitylene (MES), 1,2,4- trichlorobenzene (TCB), and propylene carbonate (PC). In mesitylene, a lower conductance is obtained across the whole series with a higher length decay (β ≈ 1 nm−1). In contrast, measurements in 1,2,4-trichlorobenzene and propylene carbonate give higher conductance values with lower length decay (β ≈ 0.1 and 0.5 nm−1 respectively). This behaviour is rationalized through theoretical investigations, where β values are found to be higher when the contact Fermi energies are close to the middle of the HOMO−LUMO gap but decrease as the Fermi energies approach resonance with either the occupied or unoccupied frontier orbitals. The different conductance and β values between MES, PC, and TCB have been further explored using DFT-based models of the molecular junction, which include solvent molecules interacting with the oligoyne backbone. Good agreement between the experimental results and these “solvated” junction models is achieved, giving new insights into how solvent can influence charge transport in oligoyne-based single molecule junctions. The single molecule conductances of a series of bis-2,2′:6′,2″-terpyridine complexes featuring Ru(II), Fe(II), and Co(II) metal ions and trimethylsilylethynyl (Me3SiC≡C−) or thiomethyl (MeS-) surface contact groups have been determined theoretically and experimentally. The single molecule conductance of metal complexes of general form transRu(C≡CArC≡CY)2(dppe)2 and trans-Pt(C≡CArC≡CY)2(PPh3)2 (Ar = 1,4-C6H2-2,5- (OC6H13)2; Y = 4-C5H4N, 4-C6H4SMe) have been determined theoretically and experimentally. The complexes display high conductance (Y = 4-C5H4N, M = Ru (0.4±0.18 nS), Pt (0.8±0.5 nS); Y = 4-C6H5SMe, M = Ru (1.4±0.4 nS), Pt (1.8±0.6 nS)) for molecular structures of ca. 3 nm in length, which has been attributed to transport processes arising from tunneling through the tails of LUMO states.
41
Walsh, Michael P. Ph D. Massachusetts Institute of Technology. "Statistical metrology and process control of quantum devices". Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/126998.
Texto completoResumen
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, May, 2020Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 169-187).
Quantum emitters, such as color centers (e.g., nitrogen-vacancy color centers in diamond), have a wide range of applications in quantum information processing, bioimaging, and quantum sensing. Such quantum emitters are typically addressed optically and store their quantum state as an electron spin that can subsequently be read out optically. For this process to work effectively, an efficient light-matter interaction must be achieved, which is difficult given the small interaction cross section of an atomic memory with the optical field. In this thesis, I address three problems that relate to the engineering of a quantum device. The first problem centers on the fact that most quantum emitters are randomly positioned throughout their host lattice making it difficult to lithographically pattern structures intended to increase the light-matter interaction. While there is a non-zero chance that a small number of randomly aligned structures will coincide with randomly positioned emitters, when efforts to scale such a system are made the yield drops exponentially. The second problem has to do with scaling. As systems scale up to larger sets of interacting qubits, it becomes increasingly necessary to produce quantum emitters with narrow optical transitions and long spin coherence times. The third problem is related to the development of tools to manage experiments and data in a more robust, team-centric, and structured manner. The automation of systems to measure qubits and devices that enables improvement of each step in the design process will be crucial if efforts to scale devices beyond a handful of qubits are to be successful. Here, I will review the progress that I made in each of these areas.
by Michael P. Walsh.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science
42
Zhou, Kejia. "Development of advanced GaAs based quantum dot devices". Thesis, University of Sheffield, 2014. http://etheses.whiterose.ac.uk/6177/.
Texto completoResumen
This thesis details research on the development of ~1.3μm quantum dot (QD) devices. QD devices which are theoretically ideal for the realisation of temperature insensitive lasers. A method to measure the recombination coefficients in a semiconductor laser is developed, and the role of Auger recombination in the realisation of temperature insensitive lasers is discussed. Moreover, due to a broad spectral linewidth and strong state-filling effects, QD structures are promising for application as broadband light sources. It is reported that the Auger recombination coefficient decreases with increasing device temperature, as measured by several complicated experimental techniques. In chapter 2, a simple analysis method (small signal modulation) to measure all of the recombination coefficients is introduced and discussed. In chapter 3, experimental data based on the small signal modulation technique is analysed. Which shows that all of the recombination coefficients, including the Auger coefficient, are a function of temperature and modulation doping in QD lasers. Following on from chapter 3, in chapter 4 the dynamic characteristic (differential carrier lifetime) of a 3μm-ridge QD laser device fabricated from commercial QD material is investigated. The modelled GS peak gain as a function of current density is determined based on the recombination coefficients, the random population model and the measured gain (via the iv Hakki-Paoli method). Then, by comparing the modelled GS gain to the experimental results, the carrier thermal escape parameter is determined. Finally in chapter 4, the variation of the Auger coefficient is explored to investigate the possibility of a temperature independent current density. The selective intermixing technique can be used in order to achieve broadband light source devices. In chapter 5, the intermixing method is introduced based on both quantum well and quantum dot structures. Then, a number of different capping materials on samples with different active region structures are discussed based on photoluminescence measurements from intermixed structures. The potential for selective area intermixing of an integrated device with a TiO2 and SiO2 cap annealed on a p-doped sample is demonstrated at the end of the chapter. Finally, in chapter 6, two integrated devices are fabricated based on this TiO2 and SiO2 cap. These devices demonstrate a broad emission bandwidth, and by applying a fast Fourier transform to the spectra in order to determine the point spread function of the instrument, and application of the Rayleigh criterion for resolution, an estimation of the resolution in an OCT system is made.
43
Doychinov, Viktor. "Quantum barrier devices for sub-millimetre wave detection". Thesis, University of Leeds, 2015. http://etheses.whiterose.ac.uk/11906/.
Texto completoResumen
Resonant-Tunnelling Diodes (RTDs) are a specific class of Quantum Barrier Devices, which offer a lot of potential for customisation through careful engineering of their semiconductor layer structure. They exhibit characteristics that make them good candidates for use in both subharmonic mixers and signal amplifiers, that operate at millimetre and sub-millimetre frequencies. In this thesis RTDs fabricated at the University of Leeds from three different layer structures are investigated. Initially, device measurements are presented along with a device model for use in circuit simulation software. Planar transmission media circuits were designed for subharmonic mixers and two types of amplifiers, all using these devices. Additional circuits, implemented in waveguide technology, were also studied in preparation for realising the RTD based amplifiers at sub-millimetre and terahertz frequencies. The sub-harmonic mixer circuits were simulated at microwave, millimetre, and sub-millimetre frequencies. Best predicted conversion loss performance is on the order of 20 dB. It was found that amongst the devices used an optimum size exists, offering best trade-off between junction capacitance and current density. The amplifier circuits are divided into two groups, reflection based amplifiers and active transmission line. Their purpose would be to complement the mixers towards eventually building a receiver with low power requirements and low overall conversion loss. The former were found to either exhibit high narrow-band gain, while the latter had low wide-band gain, with an additional, resonant peak at frequencies in the sub-millimetre wave region. The project was primarily a parametric design study, rather than a build and test project. Therefore, the simulation results are used to determine what characteristics of the devices studied would make them suited for use in circuits at high frequencies; and to come up with recommendations for future optimum RTD layer design.
44
Lo, Piparo Nicolo. "Long-distance quantum key distribution with imperfect devices". Thesis, University of Leeds, 2015. http://etheses.whiterose.ac.uk/8582/.
Texto completoResumen
Quantum key distribution (QKD) is one of the most promising techniques for the secure exchange of cryptographic keys between two users. Its unique property of relying on the laws of physics makes it an appealing tool for secure communications. While QKD has been implemented over distances on the order of a few hundreds of kilometers, the transmission rate of the key severely drops, when we go to further distances. An easy solution to this could rely on a network of trusted nodes. This solution, however, is far from ideal. In this thesis, we focus on obtaining long-distance secure communications by using trust-free intermediate nodes between two users. Quantum repeaters will then be at the core of our work and we analytically study different systems under realistic scenarios. We cover a range of repeater setups incorporating quantum memories (QMs), in terms of their short-term and long-term feasibility and in terms of ease of access for end users. We consider the main imperfections of the employed devices. In particular, we consider ensemble-based QMs, which offer a feasible route toward the implementation of probabilistic quantum repeaters. We study the effects of multiple excitations in such QMs and its effects on the key rate in a memory-assisted measurement device- independent QKD (MDI-QKD) system. We then analytically compare the performance of two probabilistic quantum repeater protocols by calculating their secure key rates. We identify under which regimes of operation one system outperforms the other. Source and memory imperfections are considered in our analysis. Finally, we combine a quantum repeater scheme with the MDI-QKD protocol and we derive the largest distances that is possible to reach under practical assumptions. Overall we obtain a realistic account of what can be done with existing technologies in order to improve the reach and the rate of QKD systems within a larger quantum network.
45
Baker, Luke James. "Superconducting nanowire devices for optical quantum information processing". Thesis, University of Glasgow, 2018. http://theses.gla.ac.uk/8440/.
Texto completoResumen
Near infrared photons are a promising choice for quantum information processing; their low transmission loss is necessary for applications such as long distance Quantum Key Distribution (QKD) in optical fibre and integrated quantum optics. An ideal proof-of-concept test of such applications would be to create, manipulate and detect single photons on a monolithic chip. Superconducting nanowire single photon detectors promise high system detection efficiencies, low dark count and low jitter under near-infrared photon illumination. Superconducting nanowire devices using NbTiN films show improved coupling efficiencies with the aid of oxidized silicon cavities. NbTiN devices were characterised in a fibre-coupled package, achieving high SDE (43%) coherent key generation rates over 200km in a T12 QKD protocol simulation. Hairpin superconducting nanowires offer excellent integration with silicon waveguide optics and can achieve near unity absorption efficiencies. Hairpin devices fabricated from MoSi films were characterised using a custom pulse tube He-3 cryostat engineered for low vibration operation at 350mK and capable of near-infrared optical maps of superconducting nanowires. The devices exhibited high critical currents 40uA), low jitter (51ps) and a dark count rate <10cps. Tests of perpendicular coupling efficiencies yield low system detection efficiencies due to high coupling losses. Using an alternative coupling method via grating couplers or cleave mounting, it is expected a much higher system detection efficiency can be achieved.
46
Giannopoulos, Mihail. "Tunable bandwidth quantum well infrared photo detector (TB-QWIP)". Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2003. http://library.nps.navy.mil/uhtbin/hyperion-image/03Dec%5FGiannopoulos.pdf.
Texto completoResumen
Thesis (M.S. in Applied Physics)--Naval Postgraduate School, December 2003.Thesis advisor(s): Gamani Karunasiri, James Luscombe. Includes bibliographical references (p. 59-61). Also available online.
47
Jiang, Lin. "Investigation of a novel multicolor quantum well infrared photodetector and advanced quantum dot infrared photodetectors". [Gainesville, Fla.] : University of Florida, 2003. http://purl.fcla.edu/fcla/etd/UFE0001249.
Texto completo
48
Scannell, William Christian 1970. "Chaotic electron transport in semiconductor devices". Thesis, University of Oregon, 2010. http://hdl.handle.net/1794/10933.
Texto completoResumen
xix, 171 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number.The field of quantum chaos investigates the quantum mechanical behavior of classically chaotic systems. This dissertation begins by describing an experiment conducted on an apparatus constructed to represent a three dimensional analog of a classically chaotic system. Patterns of reflected light are shown to produce fractals, and the behavior of the fractal dimension D F is shown to depend on the light's ability to escape the apparatus. The classically chaotic system is then used to investigate the conductance properties of semiconductor heterostructures engineered to produce a conducting plane relatively free of impurities and defects. Introducing walls that inhibit conduction to partition off sections considerably smaller than the mean distance between impurities defines devices called 'billiards'. Cooling to low temperatures enables the electrons traveling through the billiard to maintain quantum mechanical phase. Exposure to a changing electric or magnetic field alters the electron's phase, leading to fluctuations in the conductance through the billiard. Magnetoconductance fluctuations in billiards have previously been shown to be fractal. This behavior has been charted using an empirical parameter, Q , that is a measure of the resolution of the energy levels within the billiard. The relationship with Q is shown to extend beyond the ballistic regime into the 'quasi-ballistic' and 'diffusive' regimes, characterized by having defects within the conduction plane. A model analogous to the classically chaotic system is proposed as the origin of the fractal conductance fluctuations. This model is shown to be consistent with experiment and to account for changes of fine scale features in MCF known to occur when a billiard is brought to room temperature between low temperature measurements. An experiment is conducted in which fractal conductance fluctuations (FCF) are produced by exposing a billiard to a changing electric field. Comparison of D F values of FCF produced by electric fields is made to FCF produced by magnetic fields. FCF with high D F values are shown to de-correlate at smaller increments of field than the FCF with lower D F values. This indicates that FCF may be used as a novel sensor of external fields, so the response of FCF to high bias voltages is investigated.
Adviser: Stephen Kevan, Chairperson, Physics; Richard Taylor, Advisor, Physics; Robert Zimmerman, Member, Physics; Stephen Gregory, Member, Physics; David Johnson, Outside Member, Chemistry
49
Baniahmad, Ata. "QUANTUM MECHANICAL Study and Modelling of MOLECULAR ELECTRONIC DEVICES". Master's thesis, Alma Mater Studiorum - Università di Bologna, 2017. http://amslaurea.unibo.it/13193/.
Texto completoResumen
Molecular electronics pursues the use of molecules as fundamental electronic components. The inherent properties of molecules such as nano-size, low cost, scalability, and self-assembly are seen by many as a perfect complement to conventional silicon electronics. Molecule based electronics has captured the attention of a broad cross section of the scientific community.
In molecular electronic devices, the possibility of having channels that are just one atomic layer thick, is perhaps the most attractive feature that takes the attention to graphene.The conductivity, stability, uniformity, composition, and 2D nature of graphene make it an excellent material for electronic devices.
In this thesis we focused on Zigzag Graphene NanoRibbon(ZGNR) as a transmission channel. Due to the importance of an accurate description of the quantum effects in the operation of graphene devices, a full-quantum transport model has been adopted: the electron dynamics has been described by Density Functional Theory(DFT) and transport has been solved within the formalism of Non-Equilibrium Green’s Functions (NEGF). Using DFT and NEGF methods, the transport properties of ZGNR and ZGNR doped with Si are studied by systematically computing the transmission spectrum. It is observed that Si barrier destroyed the electronic transport properties of ZGNR, an energy gap appeared for ZGNR, and variations from conductor to semiconductor are displayed. Its followed by a ZGNR grown on a SiO2 crystal substrate, while substituting the Graphene electrodes with the Gold ones, and its effect on transmission properties have been studied. Improvement in transmission properties observed due to the formation of C-O bonds between ZGNR and substrate that make the ZGNR corrugated. Finally, we modeled a nano-scale Field Effect Transistor by implementing a gate under SiO2 substrate. A very good I-ON/I-OFF ratio has been observed although the device thickness.
50
Colomés, Capón Enrique. "Quantum transport with Bohmian mechanics: application to graphene devices". Doctoral thesis, Universitat Autònoma de Barcelona, 2018. http://hdl.handle.net/10803/663823.
Texto completoResumen
La ley de Moore ha sido una piedra fundamental en la mejora de la electónica
y la causa del aumento de nuestra capacidad computacional y de la existencia de la
electónica. En estas dimensiones, herramientas de simulación clásica deben ser susti-
tuidas por cuánticas. Dicha substitución implica enfrentarse a nuevos problemas fun-
damentales. Primero, el problema de muchos cuerpos imposibilita la exacta simulación
de sistemas con muchas partículas. Por otra parte, el problema de la medida, que es
especialmente relevante en escenarios de alta frequencia donde se debe medir muchas
veces.
Alternativamente a la teoría ortodoxa, la mecánica cuántica Bohmiana emerge como una
teoría cuántica especialmente bien equipada para simular parámetros de alta frecuencia
en dispositivos electrónicos cuánticos, gracias a la función de onda condicional que guía
a las partículas con posiciones bien definidas.
En esta tesis, el transporte cuántico fue explorado bajo la mecánica cuántica Bohmiana,
poniendo especial atención en grafeno, un novedoso material con estructura de bandas
lineal, del que se espera que ocupe una posición central en el futuro próximo de la
electrónica. Difiere de otros materiales en que obedece la ecuación biespinor de Dirac y
el electrón se comporta entonces como una partícula sin masa, exhibiendo efectos como
el Klein tunneling.
Durante la tesis, el simulador cuántico BITLLES fue mejorado y ahora es capaz de sim-
ular correctamente nanodispositivos con materials de banda parabólica o linear (gracias
a la inclusión de la ecuación de Dirac) en sistemas balísticos o con disipación (gracias
a la inclusión del método completamente positivo de scattering Bohmiano). Por tanto,
el BITLLES se ha convertido en un excelente candidato a ocupar el lugar del versátil
método Monte Carlo en el régimen cuántico, manteniendo su versatibilidad para calcular
parámetros de DC, AC y ruido.
Como aplicaciones prácticas, durante la tesis se predijeron probabilidades (no esperadas)
de encontrar dos electrones en el mismo lugar. También se obtuvieron curvas IV en
diferentes escenarios. Finalmente, se encontró un nuevo límite (debido a la naturaleza
discreta de los electrones) para nanodispositivos para aplicaciones lógicas trabajando a
altas frecuencias.Moore’s law has been the milestone for electronics improvement, and the cause for the exponential growth in our computational abilities and for reaching nanoscale electron devices. At such dimensions, classical simulation tools must be substituted by quan- tum ones. Such substitution implies to tackle new fundamental problems. First, the many-body problem makes (almost) impossible the exact simulation of many particles scenarios. Second, the measurement problem is especially relevant in high-frequency scenarios where multi-time measurements are mandatory. Alternatively to the orthodox theory, Bohmian mechanics emerges as a quantum theory which is specially well-equipped for the simulation of high-frequency characteristic of quantum electron devices. The Bohmian theory provides the conditional wave function that guides particles with well-defined positions. In this thesis, I explored quantum transport using Bohmian mechanics, putting special emphasis to graphene, a 2D material with linear bandstructure, which is expected to play an important role in the next future electronics. Differently from other materi- als, graphene obeys the bispinor Dirac equation, electrons behave as massless particles, exhibiting exotic behaviors, such as the Klein Tunneling effect. During the thesis, the quantum BITLLES simulator has been improved to correctly model electron nanodevices with either linear (by the inclusion of the complex bispinor Dirac equation) or parabolic bandstructures in either ballistic or dissipative (by the inclusion of the complete positive Bohmian scattering approach) systems. Thus, the BITLLES has become the candidate for substituting the versatile semiclassical Monte Carlo approach in the quantum regime, while keeping the versatility to predict DC, AC and noise performances. As practical applications, during the thesis, unexpected scattering probabilities in a Hong-Ou-Mandel experiment were predicted. In addition, different current-voltage char- acteristics were analyzed. Finally, a new limit (due to the discrete nature of charge) for ultra-small logic applications working at THz frequencies was predicted.