Dissertations / Theses: 'Quantum electronics'

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Li, Elise Yu-Tzu. "Electronic structure and quantum conductance of molecular and nano electronics." Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/65270.

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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 129-137).
This thesis is dedicated to the application of a large-scale first-principles approach to study the electronic structure and quantum conductance of realistic nanomaterials. Three systems are studied using Landauer formalism, Green's function technique and maximally localized Wannier functions. The main focus of this thesis lies on clarifying the effect of chemical modifications on electron transport at the nanoscale, as well as on predicting and designing new type of molecular and nanoelectronic devices. In the first study, we suggest and investigate a quantum interference effect in the porphyrin family molecules. We show that the transmission through a porphyrin molecule at or near the Fermi level varies by orders of magnitude following hydrogen tautomerization. The switching behavior identified in porphyrins implies new application directions in single molecular devices and molecular-size memory elements. Moving on from single molecules to a larger scale, we study the effect of chemical functionalizations to the transport properties of carbon nanotubes. We propose several covalent functionalization schemes for carbon nanotubes which display switchable on/off conductance in metallic tubes. The switching action is achieved by reversible control of bond-cleavage chemistry in [1+2] cycloadditions, via the 8p 3 8s p 2 rehybridization it induces; this leads to remarkable changes of conductance even at very low degrees of functionalization. Several strategies for real-time control on the conductance of carbon nanotubes are then proposed. Such designer functional groups would allow for the first time direct control of the electrical properties of metallic carbon nanotubes, with extensive applications in nanoscale devices. In the last part of the thesis we address the issue of low electrical conductivity observed in carbon nanotube networks. We characterize intertube tunneling between carbon nanotube junctions with or without a covalent linker, and explore the possibility of improving intertube coupling and enhance electrical tunneling by transition metal adsorptions on CNT surfaces. The strong hybridization between transition metal d orbitals with the CNT [pi] orbitals serves as an excellent electrical bridge for a broken carbon nanotube junction. The binding and coupling between a transition metal atom and sandwiching nanotubes can be even stronger in case of nitrogendoped carbon nanotubes. Our studies suggest a more effective strategy than the current cross-linking methods used in carbon nanotube networks.
by Elise Yu-Tzu Li.
Ph.D.

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Midgley, Stuart. "Quantum waveguide theory." University of Western Australia. School of Physics, 2003. http://theses.library.uwa.edu.au/adt-WU2004.0036.

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The study of nano-electronic devices is fundamental to the advancement of the semiconductor industry. As electronic devices become increasingly smaller, they will eventually move into a regime where the classical nature of the electrons no longer applies. As the quantum nature of the electrons becomes increasingly important, classical or semiclassical theories and methods will no longer serve their purpose. For example, the simplest non-classical effect that will occur is the tunnelling of electrons through the potential barriers that form wires and transistors. This results in an increase in noise and a reduction in the device?s ability to function correctly. Other quantum effects include coulomb blockade, resonant tunnelling, interference and diffraction, coulomb drag, resonant blockade and the list goes on. This thesis develops both a theoretical model and computational method to allow nanoelectronic devices to be studied in detail. Through the use of computer code and an appropriate model description, potential problems and new novel devices may be identified and studied. The model is as accurate to the physical realisation of the devices as possible to allow direct comparison with experimental outcomes. Using simple geometric shapes of varying potential heights, simple devices are readily accessible: quantum wires; quantum transistors; resonant cavities; and coupled quantum wires. Such devices will form the building blocks of future complex devices and thus need to be fully understood. Results obtained studying the connection of a quantum wire with its surroundings demonstrate non-intuitive behaviour and the importance of device geometry to electrical characteristics. The application of magnetic fields to various nano-devices produced a range of interesting phenomenon with promising novel applications. The magnetic field can be used to alter the phase of the electron, modifying the interaction between the electronic potential and the transport electrons. This thesis studies in detail the Aharonov-Bohm oscillation and impurity characterisation in quantum wires. By studying various devices considerable information can be added to the knowledge base of nano-electronic devices and provide a basis to further research. The computational algorithms developed in this thesis are highly accurate, numerically efficient and unconditionally stable, which can also be used to study many other physical phenomena in the quantum world. As an example, the computational algorithms were applied to positron-hydrogen scattering with the results indicating positronium formation.

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Lynch, Alastair M. "Low Cost and Flexible Electronics for Quantum Key Distribution and Quantum Information." Thesis, University of Bristol, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.520592.

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Hinzer, Karin. "Semiconductor quantum dot lasers." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape15/PQDD_0003/MQ36702.pdf.

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El, Kass Abdallah. "Milli-Kelvin Electronics at the Quantum-Classical Interface." Thesis, The University of Sydney, 2021. https://hdl.handle.net/2123/26889.

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The primary research topic is the design of readout circuits for quantum systems at cryogenic temperatures. The work is divided into 3 parts. The first part addresses the modelling of the I-V characteristics of the SiGe HBT over a wide range of temperatures. I empirically prove that the logarithmic slope of the collector current as a function of base-emitter bias is linearly dependent on the y-intercept over the temperature range from 300 K to 6 K. The forward active characteristics at different temperatures can be extrapolated to intersect at a single point. This point is labelled by its temperature-invariant voltage that is predicted to be very close to the bandgap potential at the junction. The second part focuses on the scalability of on-chip readout of semiconductor qubits. I analyze the performance characteristics of a low-power common-emitter transimpedance amplifier. I simulate the electrical behaviour of the amplifier with 70 mK SiGe HBT literature data to understand the achievable fidelity and bandwidth of the readout. The analysis shows that sharper scaling of the transistor characteristics down to the mK range is required to lower the noise temperature of the amplifier below 1 K. I also explore the thermal ramifications of heat generation on the temperature of qubits. The results show a relation between readout circuit integration density and the qubit temperature. Lastly, I present my work on designing, fabricating, and testing the QCPA for the purposes of amplifying qubit readout signals. The amplifier uses the capacitance between a metallic gate and the 2DEG in a GaAs/AlGaAs heterostructure as a medium of frequency mixing resulting in parametric amplification. The paramp, fabricated with the same semiconductor material and processing steps as qubits in GaAs, provides an on-chip, low-noise, wide dynamic range, and magnetically robust method for amplification at mK temperatures.

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Little, Reginald Bernard. "The synthesis and characterization of some II-VI semiconductor quantum dots, quantum shells and quantum wells." Diss., Georgia Institute of Technology, 1999. http://hdl.handle.net/1853/30573.

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Nakanishi, Toshihiro. "Coupled-resonator-based metamaterials emulating quantum systems." 京都大学 (Kyoto University), 2016. http://hdl.handle.net/2433/204563.

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Khalid, Ahmed Usman. "FPGA emulation of quantum circuits." Thesis, McGill University, 2005. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=98979.

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In recent years, new and novel forms of computation employing different natural phenomena such as the spin of atoms or the orientation of protein molecules have been proposed and are in the very initial stages of development. One of the most promising of these new computation techniques is quantum computing that employs various physical effects observed at the quantum level to provide significant improvement in certain computation tasks such as data search and factorization. An assortment of software-based simulators of quantum computers have been developed recently to assist in the development of this new computation process. However, efficiently simulating quantum algorithms at the software level is quite challenging since the algorithms have exponential run-times and memory requirements. Furthermore, the sequential nature of software-based computation makes simulating the parallel nature of quantum computation exceedingly difficult. In this thesis, the first hardware-based quantum algorithm emulation technique is presented. The emulator uses FPGA technology to model quantum circuits. Parallel computation available at the hardware level allows considerable speed-up as compared to the state-of-the-art software simulators as well as provides a greater insight into precision requirements for simulating quantum circuits.

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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.

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Jiang, Jun. "A Quantum Chemical View of Molecular and Nano-Electronics." Doctoral thesis, Stockholm : Biotechnology, Kungliga tekniska högskolan, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4335.

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Buchler, Benjamin Caird. "Electro-optic control of quantum measurements." View thesis entry in Australian Digital Theses Program, 2001. http://thesis.anu.edu.au/public/adt-ANU20020527.131758/index.html.

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Bennett, Steven. "Charge and momentum in quantum electromechanical systems." Thesis, McGill University, 2010. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=95081.

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We address theoretical questions in quantum nanoelectromechanical systems. These are systems where a mechanical oscillator is coupled to a conductor in which single electrons or the quantum coherence of electrons plays an important role. The interplay of quantum electronics with the motion of a relatively macroscopic object provides a way to probe both the mechanics and the electronics with extraordinary sensitivity. We address three problems based on monitoring either the electronic or mechanical component to measure quantum properties of the coupled system. First, we study the full charge transfer statistics and correlations in a tunnel junction coupled to a mechanical oscillator, viewing the current measured through the junction as a detector of the oscillator position. We find several surprising results that are not obtained in a study of only the average and variance of tunneled charge. Even when the oscillator is weakly coupled to the tunnel junction, it can lead to highly non-Gaussian tunneling statistics; moreover, non-Gaussian correlations between the oscillator motion and transferred charge show that the backaction of tunneling electrons on the oscillator cannot be fully described as coupling the oscillator to an effective thermal bath. Second, we use a general scattering approach to study the backaction of a quantum point contact position detector on a mechanical oscillator. Our results remain valid far from the tunneling limit, an important experimental regime and where previous calculations of backaction break down. We obtain the backaction damping and heating directly in terms of the scattering matrix, and find that not only the transmission but also the scattering phases play an important role. Finally, we study a quantum dot capacitively coupled to an oscillating cantilever. In this case, the damping of the mechanical oscillator is monitored to measure quantum electronic properties of the dot. For weak electromechanical coupling, we f
Nous abordons des questions théoriques dans le domaine des systèmes quantiques nanoélectromécaniques. Ceux sont les systémes où un oscillateur mécanique est couplé à un conducteur dans lequel les électrons individuels ou la cohérence quantique des électrons joue un rôle important. L'interaction entre un dispositif électronique dans le régime quantique avec le mouvement d'un objet macroscopique fournit un moyen de sonder à la fois les degrés de liberté méchaniques et électroniques avec une sensibilité extraordinaire. Nous réglons trois problèmes basées sur la surveillance soit la composante électronique ou mécanique pour mesurer les propriétés quantiques du systéme couplé. D'abord, nous étudions les statistiques complète de transfert de charge et les corrélations dans une jonction tunnel couplé à un oscillateur mécanique, en traitant le courant mesuré à travers la jonction comme un détecteur de la position de l'oscillateur. Nous trouvons plusieurs résultats surprenants qui ne sont pas obtenus dans un étude de seulement la moyenne et la variance de la charge qui tunnel. Même lorsque l'oscillateur est faiblement couplé à la jonction tunnel, il peut produire des statistiques fortement non-Gaussian; d'ailleurs, les corrélations non-Gaussian entre le mouvement de l'oscillateur et la charge transférée montrent que le backaction associé avec l'effet tunnel des électrons sur l'oscillateur ne peut être entièrement décrit du point de vue du couplage de l'oscillateur à un bain thermique effectif. Deuxièmement, nous utilisons une approche générale pour étudier le backaction quantique sur un oscillateur mécanique causé par un détecteur de position, comprenant un point contact quantique. Nos résultats restent applicables loin de la limite de tunnel; un régime expérimental important et où les calculs précédents du backaction ne s'appliquent pas. Nous obtenons le backaction d'amortisseme

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Jiang, Jun. "A generalized quantum chemical approach for nano- and bio-electronics." Licentiate thesis, Stockholm, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-286.

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Yang, Weidong. "Electronic structure and optical properties of self-assembled InAs quantum dots /." view abstract or download file of text, 1999. http://wwwlib.umi.com/cr/uoregon/fullcit?p9947989.

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Thesis (Ph. D.)--University of Oregon, 1999.
Typescript. Includes vita and abstract. Includes bibliographical references (leaves 150-156). Also available for download via the World Wide Web; free to University of Oregon users. Address: http://wwwlib.umi.com/cr/uoregon/fullcit?p9947989.

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Shah, Dipal. "Design of Regular Reversible Quantum Circuits." PDXScholar, 2010. https://pdxscholar.library.pdx.edu/open_access_etds/129.

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The computing power in terms of speed and capacity of today's digital computers has improved tremendously in the last decade. This improvement came mainly due to a revolution in manufacturing technology by developing the ability to manufacture smaller devices and by integrating more devices on a single die. Further development of the current technology will be restricted by physical limits since it won't be possible to shrink devices beyond a certain size. Eventually, classical electrical circuits will encounter the barrier of quantum mechanics. The laws of quantum mechanics can be used for building computing systems that work on the principles of quantum mechanics. Thus quantum computing has drawn the interest of many top scientists in the world. Ion Trap technology is one of the most promising prospective technologies for building quantum computers. This technology allows the placement of qubits - ions in 1-, 2- and 3-dimensional regular structures. Development of efficient algorithms and methodologies for designing reversible quantum circuits is one of the most rapidly growing areas of research. All existing algorithms for synthesizing quantum circuits use multi-input Toffoli gates that have very high quantum cost in terms of electromagnetic pulses. They also do not use the opportunity of regular structures provided by the Ion Trap technology. In this thesis I present a completely new methodology for synthesizing quantum circuits that use only small (3x3) Toffoli gates and new gate families that have similar properties and use regular structures. These methods are for both binary and multiple valued quantum circuits. All my methods require adding some limited number of ancilla qudits [sic] but dramatically decrease the quantum cost of the synthesized circuits. I also present a new family of gates called "D-gates" that allows synthesis of quantum and reversible logic functions using structures called layered diagrams. The designed circuits can be directly mapped to a Quantum Logic Array implemented using the Ion Trap technology.

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Yang, Liu. "Quantum chemistry studies of molecular electronic devices from electronic structure to current flow /." Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file, 156 p, 2010. http://proquest.umi.com/pqdweb?did=1997524081&sid=7&Fmt=2&clientId=8331&RQT=309&VName=PQD.

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Bowman, John V. "Transport in a confined two-dimensional electron gas with longitudinal potential variations." Virtual Press, 1995. http://liblink.bsu.edu/uhtbin/catkey/958798.

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Since the discovery of conductance quantization within a nanostnucture, investigations have sought out causes to conductance fluctuations beyond the established plateaus. The focus of this work is to show the fundamental effects upon conductance due to longitudinal potentials and double quantum boxes when confined by hardwall boundaries. A theoretical model based upon a tight-binding recursive tureen's function methodology was modified to incorporate potential barrier variations. A qualitative evaluation, as well as, explanation of the model's results and limitations is discussed.
Department of Physics and Astronomy

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Al-Taie, Haider. "Cryogenic on-chip multiplexer for the statistical study of quantum transport in low-dimensional devices." Thesis, University of Cambridge, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.708580.

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Phillips, Mark Christopher. "Electromagnetically induced transparency in semiconductors /." view abstract or download file of text, 2002. http://wwwlib.umi.com/cr/uoregon/fullcit?p3072603.

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Thesis (Ph. D.)--University of Oregon, 2002.
Typescript. Includes vita and abstract. Includes bibliographical references (leaves 162-166). Also available for download via the World Wide Web; free to University of Oregon users.

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Alhagi, Nouraddin. "Synthesis of Reversible Functions Using Various Gate Libraries and Design Specifications." PDXScholar, 2010. https://pdxscholar.library.pdx.edu/open_access_etds/366.

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This dissertation is devoted to efficient automated logic synthesis of reversible circuits using various gate types and initial specifications. These Reversible circuits are of interest to several modern technologies, including Nanotechnology, Quantum computing, Quantum Dot Cellular Automata, Optical computing and low power adiabatic CMOS, but so far the most important practical application of reversible circuits is in quantum computing. Logic synthesis methodologies for reversible circuits are very different than those for classical CMOS or other technologies. The focus of this dissertation is on synthesis of reversible (permutative) binary circuits. It is not related to general unitary circuits that are used in quantum computing and which exhibit quantum mechanical phenomena such as superposition and entanglement. The interest in this dissertation is only in logic synthesis aspects and not in physical (technological) design aspects of reversible circuits. Permutative quantum circuits are important because they include the class of oracles and blocks that are parts of oracles, such as comparators or arithmetic blocks, counters of ones, etc. Every practical quantum algorithm, such as the Grover Algorithm, has many permutative circuits. These circuits are also used in Shor Algorithm (integer factorization), simulation of quantum systems, communication and many other quantum algorithms. Designing permutative circuits is therefore the major engineering task that must be solved to practically realize a quantum algorithm. The dissertation presents the theory that leads to MP (Multi-Path) algorithm, which is currently the top minimizer of reversible circuits with no ancilla bits. Comparison of MP with other 2 leading software tools is done. This software allows to minimize functions of more variables and with smaller quantum cost that other CAD tools. Other software developed in this dissertation allows to synthesize reversible circuits for functions with "don't cares" in their initial specifications. Theory to realize functions from relational representations is also given. Our yet other software tool allows to synthesize reversible circuits for new types of reversible logic, for which no algorithm was ever created, using the so-called "pseudo-reversible" gates called Y-switches.

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Mahajumi, Abu Syed. "InAs/GaSb quantum well structures of Infrared Detector applications. : Quantum well structure." Thesis, IDE, Microelectronics and Photonics, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:hh:diva-3848.

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The detection of MWIR (mid wavelength infrared radiation) is the important for industrial, biomedical and military applications.desirable for the radiation detector to operate in the middle wavelength IR (MWIR) band corresponding to a wavelength band ranging from about 3 microns to about 5 microns.Such MWIR detectors allow forobjects having a similar thermal signature. In addition, MWIR detectors may be used in low power applications such as in night vision for surveillance of personnel.

Now a day commercially available uncooled IR sensors operating in MWIR region (2 – 5 μm) use microbolometric detectors which are inherently slow. The novel detector of InAs/GaSb quantum well structures overcomes this limitation. However, third-generation high-performance IR FPAs are already an attractive proposition to the IR system designer. They covered such as multicolour (at least two, and maybe more different spectral bands) with the possibility of simultaneous detection in both space and time, and ever larger sizes of, say, 2000 × 2000, and operating at higher temperatures, even to room temperature, for all cut-off wavelengths.These hetero structures have a type-II band alignment such that the conduction band of InAs layer is lower than the valence band of GaSb layer. The effective bandgap of thesestructures can be adjusted from 0.4 eV to values below 0.1 eV by varying the thickness of constituent layers leading to an enormous range of detector cutoff wavelengths (3-20 This work is focused on the various key characteristics the optical (responsivity and detectivity) and electrical (surface leakage & dark current) of infrared detector and proof of concept is demonstrated on infrared P-I-N photodiodes based on InAs/GaSb superlattices with ~8.5 μm cutoff wavelength and bandgap energy ~150 meV operating at 78 K where supression of surface leakage currents is observed. In certain military applications, it isthermal imaging of airplanes, artillery tanks and otherμm).

Nice research work at Halmstad University

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Laroche, Dominique. "Coulomb drag in vertically-integrated one-dimensional quantum wires." Thesis, McGill University, 2014. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=121182.

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Understanding the physics taking place in coupled one-dimensional systemsis one of the many challenges of modern day condensed matter physics and nanoelectronics. While experimental studies in coupled quantum wires have recently confirmed some of the most striking predictions of Luttinger liquid theory such as spin-charge separation and charge partitioning, much remains to be done prior a complete understanding of one-dimensional physical phenomenons is achieved, especially in the field of one-dimensional Coulomb drag. In this thesis, I report our experimental study of one-dimensional Coulomb drag between quantum wires coupled at the nanoscale. The quantum wires are coupled in a vertical geometry, allowing the wires to be separated by a hard barrier only 15 nm wide and providing us with the possibility to study Coulomb drag in a regime never achieved previously. Our study of the 1D subband dependency of Coulomb drag shows an oscillation of the drag resistance (RD) with 1D subband occupancy. Peaks in the drag signal are observed concomitant with the opening of 1D subbands in either wire, regardless of 1D subband alignment between the wires, and a novel high electronic density re-entrant negative regime for RD is observed. These findings are not fully understood within the current theoretical momentum-transfer models for Coulomb drag between quantum wires. However, some of the predictions of a charge-fluctuation induced model for 1D Coulomb drag in mesoscopic circuits are consistent with our observations and raise questions as to whether mesoscopic physics play an important role in one-dimensional Coulomb drag. The temperature dependence of the drag resistance is also presented in the one-dimensional regime where both wires have no more than a single 1D subband occupied. As the temperature is reduced below the Fermi temperature TF , a decrease in RD and a subsequent upturn is observed in three different devices at T* ∼ 1.6 K, flagging a regime where RD increases with decreasing T (verified down to ∼ 75 mK). This upturn in the drag resistance andthe diverging drag resistance at the lowest temperatures is consistent with expectations from Tomonaga-Luttinger liquid models of 1D quantum wires ,potentially validates models including forward scattering corrections and is a strong sign that interaction effects and momentum-transfer play an important role in one-dimensional Coulomb drag. A crucial step for the future of electronic nano-devices is the development of doped shallow two-dimensional electron gases (2DEGs). In an effort towards this goal, we have also studied scattering mechanisms in shallow 2DEGs in parallel to our Coulomb drag experiment. In this endeavor, we achieved the fabrication of 2DEGs as shallow as 60 nm deep with a mobility in excess of 1×10^5 cm^2/ V · s and determined that scattering of intentional remote charged impurities is the dominant scattering mechanism in samples 130 nm deep and shallower.
La compréhension de la physique prenant place dans les sysèmes unidimensionels couplés est un des nombreux défis auxquels la physique de la matière condensée moderne et la nano-électronique sont confrontées. En dépit du fait que certaines études portant sur des fils quantiques couplés aient confirmé certaines des prédictions les plus fascinantes de la théorie des liquides de Luttinger tels que la séparation des spins et des charges ainsi que la partition des charges, beaucoup reste à faire avant qu'une compréhension complète des phénomènes prenant naissance dans les systèmes unidimensionels ne soit atteinte, surtout en ce qui a trait à la traînée de Coulomb unidimensionnelle. Dans cette thèse, nous rapportons l'étude expérimentale de la traînée de Coulomb unidimensionnelle entre des fils quantiques couplés à l'échelle nanométrique. Les fils quantiques sont couplés dans une géométrie verticale permettant aux fils d'être séparés par une barrière large de seulement 15 nm, nous donnant ainsi l'occasion d'étudier la traînée de Coulomb dans un régime jamais exploré auparavant. Les résultats de notre étude de la dépendance de la traînée de Coulomb avec le niveau d'occupation des sous-bandes unidimensionnelles des fils quantiques montrent une oscillation de la résistance de la traînée de Coulomb (RD) en fonction du nombre de sous-couches occupées dans les fils quantiques. Des maximums dans RD sont observés simultanément à l'ouverture de sous-bandes unidimensionnelles dans l'un ou l'autre des fils et un nouveau régime de RD négatif et résurgent est observé à haute densité. Ces observations ne sont pas complètement expliquées par les modèles actuels expliquant la traînée de Coulomb par un transfer de quantité de mouvement. Toutefois, certaines prédictions des modèles expliquant l'émergence de la traînée de Coulomb unidimensionnelle par un échange de fluctuations sont en accord avec nos résultats et soulèvent des doutes à savoir si la physique mésoscopique joue un role dans l'émergence de la traînée de Coulomb unidimensionnelle. La dépendence en température de la résistance de la traînée de Coulomb est également présentée dans le régime unidimentionel où les fils ont au plus une seule sous-bande de populée. Alors que la température est abaissée sous la température de Fermi, une diminution de RD est observée, suivie d'un renversement de cette tendence. Ce renversement est observé dans trois dispositifs distincts à une température T* ∼ 1.6 K et marque une transition vers un régime où RD augmente alors que la température diminue (mesuré jusqu'à ∼75 mK). La présence de ce renversement et d'une divergence de RD à basse température est en accord avec les prédictions de la théorie des liquides de Tomonoga-Luttinger pour des fils quantiques unidimentionels, confirment potentiellement les modèles incluant des corrections pour des faibles valeurs de transfer de quantité de mouvement et suggèrent fortement que les interactions et le transfer de quantité de mouvement sont importants dans l'émergence de la traînée de Coulomb unidimensionnelle. Une étape cruciale pour le futur développement de nano-dispositifs électroniques est la création de puits quantiques peu profonds et dopés. Dans l'espoir d'atteindre cet objectif, nous avons étudié les mécanismes de diffusion dans des puits quantiques peu profonds en parallèle à notre étude de la traînée de Coulomb unidimensionnelle. Au cours de cet effort, nous sommes parvenus à fabriquer des puits quantiques situés seulement 60 nm sous la surface avec une mobilité supérieure à 1 × 10^5 cm^2/ V · s et nous avons déterminé que ladiffusion par les impuretés ionisées volontairement insérées dans la structure est le mode de diffusion dominant dans les puits quantiques profonds de moins de 130 nm.

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Ahmed, Imtiaz. "Radio-frequency capacitive gate-based sensing for silicon CMOS quantum electronics." Thesis, University of Cambridge, 2019. https://www.repository.cam.ac.uk/handle/1810/284933.

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This thesis focuses on implementing radio frequency (rf) reflectometry techniques for dispersive detection of charge and spin dynamics in nanoscale devices. I have investigated three aspects of rf reflectometry using state-of-the-art silicon (Si) complementary metal-oxide-semiconductor (CMOS) nanowire field effect transistors (NWFETs). First, a high-sensitivity capacitive gate-based charge sensor is developed by optimising the external matching circuit to detect capacitive changes in the high frequency resonator. A new circuit topology is used where superconducting niobium nitride (NbN) inductor is connected in parallel with a single-gate Si NWFET resulting in resonators with loaded Q-factors in the 400-800 range. For a resonator operating at 330 MHz, I have achieved a charge sensitivity of 7.7 $\mu e/\sqrt{\text{Hz}}$ and, when operating at 616 MHz, I get 1.3 $\mu e/\sqrt{\text{Hz}}$. This gate-based sensor can be used for fast, accurate and scalable techniques for quantum state readout in Si CMOS based quantum computing. Second, this new circuit topology for the resonator is used with a dual-gate Si NWFET. This dual-gate device geometry provides access to a double quantum dot (DQD) system in few electron regime. The spin-state of the two-electron DQD system is detected dispersively using Pauli spin blockade between joint singlet S(2,0) and triplet T$_-$(1,1) states in a finite magnetic field $B$. The singlet-triplet relaxation time $T_1$ at $B=4.5$~T is measured to be $\sim$1 ms using standard homodyne detection technique. Third, I expand the range of applications of gate-based sensing to accurate temperature measurements. I have experimentally demonstrated a primary thermometer by embedding a single-gate Si NWFET with the rf capacitive gate-based sensor. The thermometer, termed as gate-based electron thermometer (GET), relies on cyclic electron tunneling between discrete energy levels of a quantum dot and a single electron reservoir in the NWFET. I have found that the full-width-half-maximum (FWHM) of the resonator phase response depends linearly with temperature via well known physical law by using the ratio $k_\text{B}/e$ between the Boltzmann constant and the electron charge. The GET is also found to be magnetic field independent like other primary thermometers such as Coulomb blockade and shot noise thermometers.

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Benhsaien, Abdessamad. "Self-assembled quantum dot semiconductor nanostructures modeling: Photonic device applications." Thesis, University of Ottawa (Canada), 2006. http://hdl.handle.net/10393/27225.

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A microscopic analysis of a vertical stack of self-assembled InAs/GaAs lens-shaped quantum dot nanostructures is presented. The analysis revolves around a rigorous Hamiltonian formulation of an eight-band k.p. perturbation to account for the lattice-mismatch strain endured by the islands. The numerical implementation yields the effective bandgap energy and electronic structure of an InAs/GaAs quantum dot. Within the framework of a resonant two-level energy system, material gain and absorption spectra are calculated up to a third-order susceptibility to include nonlinearity. The material gain polarization dependence is expressed in the dipole transition strength. Polarization-dependent anisotropy factors corresponding to different interband transitions are derived and shown to satisfy a momentum conservation rule. Modal analysis of a rectangular core waveguide realized by imbedding the active quantum dot layer(s) into a cladding medium with lower refractive index is presented. Polarization-independent modal gain is achieved by optimizing the width of the rectangular core waveguide. In illustration of a quantum dot device, a realistic semiconductor optical amplifier model accounting for both stimulated and spontaneous emission is considered. The calculated carrier density longitudinal profile yields other parameters characterizing the amplifier performance.

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Jain, Siddharth R. "Hybrid Silicon Photonic Integration using Quantum Well Intermixing." Thesis, University of California, Santa Barbara, 2013. http://pqdtopen.proquest.com/#viewpdf?dispub=3559799.

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With the push for faster data transfer across all domains of telecommunication, optical interconnects are transitioning into shorter range applications such as in data centers and personal computing. Silicon photonics, with its economic advantages of leveraging well-established silicon manufacturing facilities, is considered the most promising approach to further scale down the cost and size of optical interconnects for chip-to-chip communication. Intrinsic properties of silicon however limit its ability to generate and modulate light, both of which are key to realizing on-chip optical data transfer. The hybrid silicon approach directly addresses this problem by using molecularly bonded III-V epitaxial layers on silicon for optical gain and absorption. This technology includes direct transfer of III-V wafer to a pre-patterned silicon-on-insulator wafer. Several discrete devices for light generation, modulation, amplification and detection have already been demonstrated on this platform.

As in the case of electronics, multiple photonic elements can be integrated on a single chip to improve performance and functionality. However, scalable photonic integration requires the ability to control the bandgap for individual devices along with design changes to simplify fabrication. In the research presented here, quantum well intermixing is used as a technique to define multiple bandgaps for integration on the hybrid silicon platform. Implantation enhanced disordering is used to generate four bandgaps spread over 120+ nm. By combining these selectively intermixed III-V layers with pre-defined gratings and waveguides on silicon, we fabricate distributed feedback, distributed Bragg reflector, Fabry-Pérot and mode-locked lasers along with photodetectors, electro-absorption modulators and other test structures, all on a single chip. We demonstrate a broadband laser source with continuous-wave operational lasers over a 200 nm bandwidth. Some of these lasers are integrated with modulators with a 3-dB bandwidth above 25 GHz, thus demonstrating coarse wavelength division multiplexing transmitter on silicon.

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Tam, Hoi Lam. "Active textured metallic microcavity." HKBU Institutional Repository, 2004. http://repository.hkbu.edu.hk/etd_ra/613.

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27

Bessis, Charlotte. "Quantum interference and thermoelectric effects in molecular junctions." Thesis, Sorbonne Paris Cité, 2016. http://www.theses.fr/2016USPCC140/document.

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Cette thèse rapporte les mesures de transport réalisées sur des jonctions moléculaires à l'état solide large échelle, mettant en évidence des effets d'interférence quantique. Le premier chapitre pose les bases théoriques de ce phénomène et introduit le formalisme des fonctions de green hors équilibre adapté à la description du couplage molécules/interfaces métalliques. Le second chapitre présente l'état de l'art expérimental dans ce domaine et résume les principales expériences ayant permis de mettre en évidence des effets d'interférences à l'échelle moléculaire. Le troisième chapitre décrit les étapes de fabrication mises en place pour construire les dispositifs mesures pendant ce travail de thèse. Les résultats expérimentaux obtenus sur les mesures de conductance des jonctions moléculaires sont décrits dans le quatrième chapitre et compares a plusieurs modèles théoriques qui confirme la présence d'interférences quantiques. Le dernier chapitre aborde les effets de thermoélectricité qui peuvent avoir lieu dans ces jonctions en présence d'interférence
This thesis reports the transport measurement performed on large scale solid state molecular junctions, highlighting quantum interference effect. First chapter set the theoretical basis of such a phenomenon and introduces the out of equilibrium green's functions formalism which is adapted to the description of coupling molecules/metallic interfaces. Second chapter presents the corresponding experimental state of the art and summarizes the experiments that have contributed to highlight interference effect at the molecular scale. Third chapter describes the fabrication steps optimized to build the devices measured during the thesis work. Experimental results obtained on conductance measurements are described and compared to several theoretical models that confirm the presence of quantum interference. Last chapter deals with thermoelectric effect that can occur in presence of interference

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Oza, Neal N. "Engineering Photonic Switches for Quantum Information Processing." Thesis, Northwestern University, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3669298.

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In this dissertation, we describe, characterize, and demonstrate the operation of a dual-in, dual-out, all-optical, fiber-based quantum switch. This "cross-bar" switch is particularly useful for applications in quantum information processing because of its low-loss, high-speed, low-noise, and quantum-state-retention properties.

Building upon on our lab's prior development of an ultrafast demultiplexer ^[1-3] , the new cross-bar switch can be used as a tunable multiplexer and demultiplexer. In addition to this more functional geometry, we present results demonstrating faster performance with a switching window of ≈45 ps, corresponding to >20-GHz switching rates. We show a switching fidelity of >98%, i. e., switched polarization-encoded photonic qubits are virtually identical to unswitched photonic qubits. We also demonstrate the ability to select one channel from a two-channel quantum data stream with the state of the measured (recovered) quantum channel having >96% relative fidelity with the state of that channel transmitted alone. We separate the two channels of the quantum data stream by 155 ps, corresponding to a 6.5-GHz datastream.

Finally, we describe, develop, and demonstrate an application that utilizes the switch's higher-speed, lower-loss, and spatio-temporal-encoding features to perform quantum state tomographies on entangled states in higher-dimensional Hilbert spaces. Since many previous demonstrations show bipartite entanglement of two-level systems, we define "higher" as d > 2 where d represents the dimensionality of a photon. We show that we can generate and measure time-bin-entangled, two-photon, qutrit (d = 3) and ququat (d = 4) states with >85% and >64% fidelity to an ideal maximally entangled state, respectively. Such higher-dimensional states have applications in dense coding ^[4] , loophole-free tests of nonlocality ^[5] , simplifying quantum logic gates ^[6] , and increasing tolerance to noise and loss for quantum information processing ^[7] .

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Perkins, Brian. "Hot carrier transport in short diodes and nanotubes /." View online version; access limited to Brown University users, 2005. http://wwwlib.umi.com/dissertations/fullcit/3174658.

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Graham, Luke Alan. "Observation of enhanced spontaneous emission in dielectrically apertured microcavities /." Digital version accessible at:, 1999. http://wwwlib.umi.com/cr/utexas/main.

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31

Dhirhe, Devnath. "Monolithic tuneable quantum cascade lasers." Thesis, University of Glasgow, 2013. http://theses.gla.ac.uk/4604/.

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This thesis is concerned with the design, fabrication and characterisation of monolithic tuneable quantum cascade lasers (QCLs), which are suitable for tuneable diode laser based absorption spectroscopy and polarisation dependent spectroscopy in the mid-infrared wavelength range. All investigations and device development work were carried out using the QCL structure based on strain-compensated Ga0.331In0.669As/Al0.659In0.341As grown on an InP substrate that emits light around 4500 nm wavelength. To make the QCLs electrically tuned, two laser designs were investigated: the double ring quantum cascade laser based on the Vernier-tuning effect, and the integrated tuneable birefringent waveguide utilising current controlled birefringence in quantum-wells. The key advantage of the Vernier tuning effect based the double ring laser design is that it can facilitate both a single mode and wide-tuning range operation. The Vernier tuning enhancement factor associated with the coupled waveguide is responsible for a wide-tuning range observed in double ring configuration. However, the tuning range is limited by the available gain bandwidth (i.e. FWHM of spontaneous spectra) in the material and the maximum obtainable index change of the tuner ring. Theoretically, the tuning range of 155 nm was estimated for the double ring quantum cascade laser (DRQCL) design employed in this thesis. However, experimentally, a single mode (~19 dB single sideband suppression ratio) and tuning range of 59 nm which covers almost half the bandwidth were observed. For the first time in the history of the QCL, a research into the design, fabrication and characterisation of integrated polarisation mode convertors (PMCs) has been carried out. The PMC design is based upon etching trenches, using the RIE lag effect, of sub-wavelength dimensions into one side of a waveguide in order to achieve an asymmetric cross-sectional profile, resulting in a waveplating effect. This thesis presents such PMCs integrated with QCLs that emit 69% TE light with the polarisation angle of 65 degree from one facet and a pure TM light emitted from the other facet using a 256 μm long PMC design (design D2). An integrated tunable birefringent waveguide (ITBW) consisting of two PMCs with a differential phase shift (DPS) section between them. To probe the birefringence operation, a sub-threshold electroluminescence was employed to investigate the single pass operation of the ITBW. A theory based on the electro-optic properties of birefringence in QCL waveguides was used combined with a Jones-matrix based description to gain an understanding of the electroluminescence results. With the QCL operating above threshold, polarisation and wavelength tuning of the signal output was demonstrated. By comparing the sub-threshold electroluminescence and active polarisation angle measurement result with the Jones matrix model, the material birefringence (no DPS current), 4n, was estimated to be around 0.005 for the QCL employed in this work. However, single mode emission was not observed and 24 nm discontinuous tuning was recorded. Despite this, using a QCL incorporating an ITBW device, active polarisation control over 45 degree was demonstrated, and currently, to the best of the authors knowledge there has been no other QCL device that is capable of electronically controlling the output polarisation.

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Rezaee, Amirabbas, and amirabbas rezaee@rmit edu au. "Phase-Periodic Quantum Structures and Perturbed Potential Wells." RMIT University. Electrical and Computer Engineering, 2009. http://adt.lib.rmit.edu.au/adt/public/adt-VIT20091218.160522.

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The restrictions of micro-scale systems are approaching rapidly. In anticipation of this development, nano-scale electronics has become the focus of many researchers and engineers in academia and industry since early 1990s. The basic building blocks of modern integrated circuits have been diodes and transistors with their current-voltage I-V characteristics being of prime significance for the design of complex signal processing and shaping devices and systems. Classical and semi-classical physical principles are no longer powerful enough or even valid to describe the phenomena involved. The application of rich and powerful concepts in quantum theory has become indispensable. These facts have been influential in undertaking this research project. This research is built upon the determination of the Eigenpairs of one and two dimensional positive differential operators with periodic boundary conditions. The Schrödinger equation was solved for positive operators in both one and two dimensions. Fourier series were used to express the derivatives as the summation of Fourier terms. This led to a novel approach for the calculation of the eigenmodels of a perturbed potential well. The perturbation can be done via an electric field applied to the potential well. The research in this thesis includes a thorough understanding of quantum mechanics fundamentals, mastering of different approximation techniques such as the variational technique and results that have been generated and published using the novel techniques.

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33

Yao, Tianfu. "Fibre laser sources with low quantum defect." Thesis, University of Southampton, 2014. https://eprints.soton.ac.uk/372830/.

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High power fibre lasers with efficient amplification of optical signals have been widely used for various applications for many years. A great advantage of fibres is their thermal properties, which are so good that fibres are often quoted as being immune to thermal degradation. However, powers keep increasing and the fibre geometry is becoming more similar to their “bulk” (non-waveguiding) counterparts. Thermal problems are severe in bulk lasers, and are, unsurprisingly, becoming more important also for fibres. Heating is inevitable in the laser cycle due to the energy difference(quantum defect) between pump and signal photons. Nevertheless, there is substantial room to reduce the heating by minimising the quantum defect, with pump and signal wavelengths as close as possible. In this thesis, I demonstrate low-quantum-defect fibre amplifiers and lasers based on two different energy conversion processes, i.e., in high-brightness (tandem) pumped ytterbium-doped fibre and in short-wavelength-pumped Raman fibre. The latter approach increases the photon energy relative to the thermal energy and vibrational energy of the host. Firstly, as it comes to tandem-pumping of ytterbium-doped fibre amplifiers, the challenge is to make the pump and signal wavelengths as close as possible, while keeping the signal gain and pump absorption sufficiently high. With the optimum average ytterbium excitation level and high pump brightness, the quantum defect can be as small as 0.6% when pumped at 1030 nm, according to theoretical calculations. Subsequently, an experimental amplifier core-pumped by a single-mode laser source is presented. A 2% quantum defect is reached, with pump and signal wavelengths at 1030 nm and 1050 nm, respectively. The slope efficiency reaches 95% to 96%. Initial investigations show low photodarkening with tandem-pumping, with some dependence on the dependence on ytterbium ions concentration. Secondly, Raman conversion of pulses in a diode-pumped highly nonlinear fibre is studied in a ring-laser cavity configuration. The quantum defect is 3.5% with 806 nm pump and 835 nm Stokes wavelengths. A slope efficiency of 65% is obtained with 600 m long fibre and 100 ns pulse width. Then, I study experimentally and theoretically 975 nm continuous-wave-pumped fibre Raman lasers based on a graded-index and a double-clad fibre. Both lasers emit at 1019 nm (4.3% quantum defect) with improved brightness. A record laser output of 6 W and 19% slope efficiency from the double-clad Raman fibre and 20 W from the graded-index fibre shows further scaling of singlemode power is possible with improved cavity and fibre design.

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34

Zhang, Zheshen. "New techniques for quantum communication systems." Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/42843.

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Although mathematical cryptography has been widely used, its security has only been proven under certain assumptions such as the computational power of opponents. As an alternative, quantum communication, in particular quantum key distribution (QKD) can get around unproven assumptions and achieve unconditional security. However, the key generation rate of practical QKD systems is limited by device imperfections, excess noise from the quantum channel, limited rate of true random-number generation, quantum entanglement preparation, and/or post-processing efficiency. This dissertation contributes to improving the performance of quantum communication systems. First, it proposes a new continuous-variable QKD (CVQKD) protocol that loosens the efficiency requirement on post-processing, a bottleneck for long-distance CVQKD systems. It also demonstrates an experimental implementation of the proposed protocol. To achieve high rates, the CVQKD experiment uses a continuous-wave local oscillator (CWLO). The excess noise caused by guided acoustic-wave Brillioun scattering (GAWBS) is avoided by a frequency-shift scheme, resulting in a 32 dB noise reduction. The statistical distribution of the GAWBS noise is characterized by quantum tomography. Measurements show Gaussian statistics upto 55 dB of dynamical range, which validates the security calculations in the proposed CVQKD protocol. True random numbers are required in quantum and classical cryptography. A second contribution of this thesis is that it experimentally demonstrates an ultrafast quantum random-number generator (QRNG) based on amplified spontaneous emission (ASE). Random numbers are produced by a multi-mode photon counting measurement on ASE light. The performance of the QRNG is analyzed with quantum information theory and verified with NIST standard random-number test. The QRNG experiment demonstrates a random-number generation rate at 20 Gbits/s. Theoretical studies show fundamental limits for such QRNGs. Quantum entanglement produced in nonlinear optical processes can help to increase quantum communication distance. A third contribution is the research on nonlinear optics of graphene, a novel 2D material with unconventional physical properties. Based on a quantum-dynamical model, optical responses of graphene are derived, showing for the first time a link between the complex linear optical conductivity and the quantum decoherence. Nonlinear optical responses, in particular four-wave mixing, is studied for the first time. The theory predicts saturation effects in graphene and relates the saturation threshold to the ultrafast quantum decoherence and carrier relaxation in graphene. For the experimental part, four-wave mixing in graphene is demonstrated. Twin-photon production in graphene is under investigation.

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Baker, Luke James. "Superconducting nanowire devices for optical quantum information processing." Thesis, University of Glasgow, 2018. http://theses.gla.ac.uk/8440/.

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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.

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Donehoo, Brandon. "A superconducting investigation of nanoscale mechanics in niobium quantum point contacts." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/24784.

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Thesis (Ph.D.)--Physics, Georgia Institute of Technology, 2008.
Committee Chair: Alexei Marchenkov; Committee Member: Bruno Frazier; Committee Member: Dragomir Davidovic; Committee Member: Markus Kindermann; Committee Member: Phillip First

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37

Rauf, Sakandar. "Quantum dot encoded magnetic beads for multiplexed fluorescence biosensing." Thesis, University of Glasgow, 2010. http://theses.gla.ac.uk/1647/.

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In recent years, the use of encoded beads has received considerable attention due to their potential for measuring multiple analytes in solution.(1-4) This can be achieved without the need for knowledge of their spatial position, as in the case of microarray technology. Encoded bead technology also relies on the solution kinetics rather than diffusion to a fixed surface as in the case of microarray technology, offering the possibility of developing rapid high throughput screening methods. This thesis describes the production, characterisation and application of quantum dot encoded beads prepared using layer-by-layer assembly of different colour quantum dots around a magnetic bead. To achieve this, two different strategies were used to make “coloured” barcodes. The first strategy used thiol chemistry to immobilise quantum dots in a layer-by-layer assembly onto magnetic beads whereas the second strategy uses the interaction between quantum dot-biotin and quantum dot-streptavidin conjugates to create constructs on the magnetic bead surface. The development of both of these immobilisation strategies was characterisation using X-ray photoelectron spectroscopy and fluorescence spectroscopy of immobilised quantum dot structures onto a plain glass substrate. After the preparation of encoded beads, they were characterised using single bead fluorescence spectroscopy. It was found that attempts to prepare barcodes by layer-by-layer assembly of CdSe/ZnS quantum dots using thiol chemistry onto magnetic beads did not comply with the necessary barcode characteristics i.e., different colour coded beads could not be distinguished from each other. However, the encoded beads prepared using layer-by-layer assembly of quantum dot-biotin and quantum dot-streptavidin conjugates onto streptavidin coated magnetic beads gave distinct multicolour coded bead spectra. These barcodes were characterised in terms of different spectral responses, stability at raised temperatures, stability in biotin solutions, and long-term stability after storage. Encoded beads prepared using layer-by-layer assembly of quantum dot-biotin and quantum dot-streptavidin conjugates onto streptavidin coated magnetic beads were then used to develop multiplexed immunoassays. Four different barcodes were prepared and used to perform model multiplexed immunoassays. The barcodes were identified upon the basis of different spectral response measured using single bead fluorescence spectroscopy. Finally, a quantitative immunoassay for human IgG was performed using these barcodes, which showed that different concentrations of human IgG can be determined in solution.

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Kirkwood, Robert A. "Superconducting single photon detectors for quantum information processing." Thesis, University of Glasgow, 2017. http://theses.gla.ac.uk/8136/.

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Single photon detectors are a vital part of many emerging technologies which harness the quantum properties of light to benefit the fields of communication, computation and sensing. Superconducting nanowire single photon detectors (SNSPDs) offer high detection efficiency, low dark count rates, low timing jitter, and infrared sensitivity that are required by the most demanding single photon counting applications. This thesis presents SNSPDs fabricated and tested at the University of Glasgow that are integrated with optical structures which enable enhanced detection efficiency and integration with waveguide circuit technology. The monolithic integration of waveguide circuit components presents a route towards realisation of an optical quantum information processor that has the stability and scalability to perform the demanding tasks of quantum computation. A novel process is introduced for incorporating superconducting detectors with single mode gallium arsenide waveguides and quantum dot single photon sources. Together these elements would enable the generation of quantum states of light which could be manipulated and detected on a single chip. Detectors are patterned in NbTiN thin superconducting films on to suspended nanobeam waveguides with better than 50 nm alignment accuracy. Low temperature electrical and optical testing confirms the detectors’ single photon sensitivity under direct illumination as well as to waveguide coupled light. Measured detectors were found to have internal registering efficiencies of 6.8 ± 2.4%. Enhancing absorption of photons into thin superconducting films is vital to the creation of high efficiency superconducting single photon detectors. Fabricating an SNSPD on a dielectric mirror creates a partial cavity that can be tailored to enhance detection of light at specific wavelengths. Devices have been fabricated and tested in this thesis with enhanced detection efficiency at infrared and visible wavelengths for quantum cryptography, remote sensing and life science applications. Detectors fabricated in NbTiN on GaAs/AlGaAs Bragg mirrors exhibited a system detection efficiency of 1.5% at 1500 nm wavelength for the best device measured. SNSPDs were also fabricated in NbN on aperiodic dielectric mirrors with a range of different bandwidths. A peak system detection efficiency of 82.7% at 808 nm wavelength was recorded.

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Zhang, Fan. "QUANTUM EFFICIENCY ENHANCEMENT FOR GAN BASED LIGHT-EMITTING DIODES AND VERTICAL CAVITY SURFACE-EMITTING LASERS." VCU Scholars Compass, 2014. http://scholarscompass.vcu.edu/etd/3655.

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This thesis explores the improvement of quantum efficiencies for InGaN/GaN heterostructures and their applications in light-emitting diodes (LEDs) and vertical cavity surface-emitting lasers (VCSELs). Different growth approaches and structural designs were investigated to identify and address the major factors limiting the efficiency. (1) Hot electron overflow and asymmetrical electron/hole injection were found to be the dominant reasons for efficiency degradation in nitride LEDs at high injection; (2) delta p-doped InGaN quantum barriers were employed to improve hole concentration inside the active region and therefore improve hole injection without sacrificing the layer quality; (3) InGaN active regions based on InGaN multiple double-heterostructures (DHs) were developed to understand the electron and hole recombination mechanisms and achieve high quantum efficiency and minimal efficiency droop at high injection; (4) the effect of stair-case electron injectors (SEIs) has been investigated with different active region designs and SEIs with optimized thickness greatly mitigated electron overflow without sacrificing material quality of the active regions. The active regions showing promising performance in LEDs were incorporated into VCSEL designs. Hybrid VCSEL structures with bottom semiconductor AlN/GaN and a top dielectric SiO2/SiNx DBRs have been investigated, and quality factors as high as 1300 have been demonstrated. Finally, VCSEL structures with all dielectric DBRs have been realized by employing a novel ELO-GaN growth method that allowed integration of a high quality InGaN cavity active region with a dielectric bottom DBR without removal of the substrate while forming a current aperture through the ideally dislocation-free region. The full-cavity structures formed as such exhibited quality factors 500 across the wafer.

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Tandoi, Giuseppe. "Monolithic high power mode locked GaAs/AlGaAs quantum well lasers." Thesis, University of Glasgow, 2011. http://theses.gla.ac.uk/2721/.

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In this thesis, approaches for increasing the output power in monolithically integrated semiconductor mode locked (ML) lasers were investigated. The wavelength range considered is the range of operation of low temperature grown GaAs photomixers, devices commonly used for THz generation. In particular, two GaAs/AlGaAs quantum well laser epistructures (operating at 830 nm and 795 nm) were considered, both with reduced optical confinement and elongated vertical optical mode size. In this work, such laser epistructures, commonly used by high power semiconductor laser manufacturers, were successfully employed, for the first time, for producing passively ML devices. Improved average powers (up to 48 mW) under ML operation were demonstrated, around ten times higher than values previously reported in monolithic GaAs/AlGaAs ML lasers. In continuous wave operation, the output power was limited by the catastrophic damage of the laser facets at around 50 mW. For this reason, facet passivation techniques were investigated, allowing for powers up to 124 mW to be achieved. In ML regime, the output power was instead limited by the catastrophic damage of the reverse biased section of the laser. This failure mechanism was investigated and explained considering thermal effects on the reverse biased section. Such effects limited the output power to around 27 mW in 830 nm devices, which was then improved by 70% in 795 nm devices with a 70% larger optical mode area. The larger mode size, combined to a small duty-cycle laser geometry, enabled a record peak power of 9.8 W to be achieved at 6.83 GHz. This particular repetition rate was specifically designed for coherent population trapping experiments in 87Rb vapors. Sub-picosecond transform limited pulses were achieved in both the laser materials considered, with a minimum duration of 0.43 ps at 126 GHz. With the values of peak power achieved, the developed devices may also be directly used for two-photon microscopy applications.

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Ledwosinska, Elzbieta. "Design, fabrication, and characterization of a floating-gate double quantum well far-infrared photoconductor." Thesis, McGill University, 2009. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=66787.

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The design, fabrication, and characterization of a GaAs/AlGaAs double quantum well long-infrared photoconductor are reported. The double quantum well structure was designed using a 1-D Schrodinger-Poisson solver to fine-tune the material composition, thickness, and doping to control the 2-D electron gas (2DEG) subband energies, positions, carrier densities, and lifetimes of transitions. The wafer was grown commercially and devices were fabricated using cleanroom facilities at McGill and the Universite de Montreal. The device has a floating gate architecture with perpendicular carrier transport, in contrast with the parallel transport scheme employed by almost all commercial detectors, where the photocurrent is measured directly. Perpendicular transport provides greater photoconductive gain than parallel transport, as seen in the high responsivities achieved by competing groups. The device reported herein was engineered to increase the coupling, and thus the responsivity, between the two wells by 40 % over the best previously reported results. The fabricated devices were tested under various temperature, biasing, and illumination conditions and fundamental properties of the device such as responsivity, sensitivity, and stability were characterized. The peak photoresponse has been observed for modulation frequencies from 20 Hz to 1 kHz, and is clearly discernible up to 30 K. The device exhibits a responsivity of 80-160 A/W. This result is in fair competition with QWIPs demonstrating responsivities of mA/W -10 A/W. The device shows a NEP = 4.7e-11 W/sqrt(Hz), and D* = 1.7e8 cm sqrt(Hz)/W.
La planifcation, fabrication et characterization d'un double puit quantique photoconducteuren longue-infrarouge de GaAs/AlGaAs sont presentes. La structure dudouble puit quantique a ete creee avec le programme 1-D Schrodinger-Poisson solverafin de bien ajuster la composition des materiaux, les epaisseurs et le dopage afin decontroler les energies des sous-bandes, les positions, les densites de transporteurs descharge et les dures de vies de transitions du gaz d'electrons bi-dimensionel (2DEG).La plaquette a ete fabriquee commercialement et les dispositifs ont ete fabriques dansles salles blanches de l'Universite McGill et l'Universite de Montreal. Le dispositifa une architecture a la grille flottante avec transport de charge perpendiculaire contrairementau concept de transport parallele utilise par presque tous les detecteurscommerciaux ou le photocourant est mesure directement. Le transport perpendiculairedonne un gain photoconductif plus grand que dans la geometrie parallele commedemontre par les plus grandes responsivites produites par les competiteurs. Le dispositifa ete cree afin d'augmenter le couplage et ameliore donc la responsivite entreles deux puits de 40 % de plus que les meilleurs resultats publies. Les dispositifsfabriques ont ete testes a differentes temperatures, differents voltages et differentesilluminations. Les proprietees fondametales du dispositif comme la responsivite, lasensibilite et la stabilite ont ete mesurees. La photoreponse maximale a ete observeesous des frequences modulatrices entre 20 Hz et 1 kHz et est clairement visible jusqu'a30 K. Le dispositif demontre une responsivite de 80-160 A/W. Ce resultat est a lahauteur avec QWIPs qui demontre des responsivite allant de mA/W-10 A/W. Cedispositif demontre NEP = 4.7e11 W/sqrt(Hz) et D* = 1.7e8 cm sqrt(Hz)/W.

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Curtis, Kellye Suzanne. "Sub-10-nanometre metallic gaps for use in molecular electronics." Thesis, University of Cambridge, 2012. https://www.repository.cam.ac.uk/handle/1810/243618.

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This thesis presents the development of a selective-etch fabrication process to create sub-10 nanometre metallic gaps and the subsequent use of the gaps to study the electronics of nanocrystals and molecules. A complete picture of the success of the process required both examination by scanning electron microscopy as well as probing the current response to an applied bias at low temperature. The empty gaps were fully characterised before self-assembling 7 nm CdSe nanocrystals onto the metal with the help of linker molecules. The I-V characteristics of the empty gaps showed a reduction of the tunnelling barrier height from the expected value (~5.1 eV, the work function of Au) when the results were fitted to the Simmons tunnelling model for a metal-insulator-metal system. Results indicate that after the barrier height is surpassed, a transition from direct to field-effect (Fowler-Nordheim) tunnelling occurs. After CdSe assembly, the collected I-V characteristics of the system at 77 K showed varied results. Many devices displayed conductance peaks at low voltages comparable to the results of the shadow evaporation process for 4.2 nm nanocrystals (also documented in this thesis). Several devices revealed switching between multiples of fundamental curves, suggesting conduction through multiples of nanocrystals.

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Prineas, John Paul. "Pronounced light-matter coupling in periodic semiconductor quantum wells." Diss., The University of Arizona, 2000. http://hdl.handle.net/10150/284160.

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The development of advanced technological methods for growth of semiconductors, such as molecular beam epitaxy, have allowed growth of layered semiconductor structures with precision to a single atomic layer. One important structure is the semiconductor quantum well, consisting of a thin layer of a smaller bandgap semiconductor grown between layers of thicker, larger bandgap semiconductor. Quantum wells are, for example, largely responsible for making the semiconductor laser a practical device. By increasing the binding energy of excitons (hydrogen-like, bound electron-hole pairs in semiconductors), and allowing them to couple to the continuum of vacuum photon modes, semiconductor quantum wells have made excitons the focus of numerous fundamental optical studies. Stacks of periodically grown quantum wells, grown far enough apart such that electronic tunneling between quantum wells is unimportant, can still be coupled by light. N light-coupled quantum wells have N exciton-light, or exciton-polariton, eigenmodes, each characterized by a distinct energy and radiative lifetime dependent on the periodicity of the quantum wells. By adjusting the periodicity of the quantum wells and the material parameters, engineering of the light-matter interaction of these one-dimensional mesoscopic crystals is possible. The interesting new properties of these structures open the possibility for new devices. Periodic multiple quantum wells with a period in the vicinity of half the exciton resonance wavelength are studied in linear measurements of reflection, transmission and absorption. The optical properties are dominated by the eigenmodes of the light-coupled quantum wells. At Bragg periodicity, where the oscillator strengths of all quantum well excitons are concentrated into one superradiant mode, a photonic band gap grows in amplitude and linearly in energy width with increasing number of quantum wells N. A corresponding N times increased radiative damping rate compared to a single quantum well is observed, originating from expulsion of the light character of the superradiant mode from the photonic bandgap structure. The slope of linewidth versus N gives the radiative linewidth of the exciton. For periods away from Bragg condition, all normal modes become optically active, and are observed in reflection and absorption experiments. Because light-coupling alters the photon density of states, formation of the N exciton-polariton eigenmodes is also evidenced in photoluminescence after nonresonant excitation into the free carrier continuum. The strongly modified light-matter interaction for photons in the photonic gap at Bragg periodicity is also manifest in the inhibited emission from the superradiant mode, a surprising result explained by a consideration of the linear properties. The temporal dynamics of Rayleigh scattering of a resonant excitation pulse from disordered semiconductor multiple quantum wells has many interesting aspects, and has recently been the subject of much debate. The effect of light-coupling on resonant Rayleigh scattering from periodic semiconductor multiple quantum well structures is investigated both experimentally and theoretically. Polaritonic effects are found to dominate the Rayleigh scattered light temporal dynamics due to the simultaneous coexistence of several eigenmodes with different energy and radiative decay times for a given periodicity. They give rise to polarization beating between modes and determine rise and decay times of the resonance Rayleigh scattered signals.

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44

Kim, Dong Kwon. "Optical properties of asymmetric double quantum wells and optimization for optical modulators." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/22649.

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Thesis (Ph. D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2008.
Committee Chair: Citrin, David; Committee Member: Dupuis, Russell; Committee Member: Gaylord, Thomas; Committee Member: Rhodes, William; Committee Member: Zhang, Zhuomin.

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Underwood, Devin Lane. "Microwave cavity lattices for quantum simulation with photons." Thesis, Princeton University, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3686679.

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Historically our understanding of the microscopic world has been impeded by limitations in systems that behave classically. Even today, understanding simple problems in quantum mechanics remains a difficult task both computationally and experimentally. As a means of overcoming these classical limitations, the idea of using a controllable quantum system to simulate a less controllable quantum system has been proposed. This concept is known as quantum simulation and is the origin of the ideas behind quantum computing.

In this thesis, experiments have been conducted that address the feasibility of using devices with a circuit quantum electrodynamics (cQED) architecture as a quantum simulator. In a cQED device, a superconducting qubit is capacitively coupled to a superconducting resonator resulting in coherent quantum behavior of the qubit when it interacts with photons inside the resonator. It has been shown theoretically that by forming a lattice of cQED elements, different quantum phases of photons will exist for dierent system parameters. In order to realize such a quantum simulator, the necessary experimental foundation must rst be developed. Here experimental eorts were focused on addressing two primary issues: 1) designing and fabricating low disorder lattices that are readily available to incorporate superconducting qubits, and 2) developing new measurement tools and techniques that can be used to characterize large lattices, and probe the predicted quantum phases within the lattice.

Three experiments addressing these issues were performed. In the rst experiment a Kagome lattice of transmission line resonators was designed and fabricated, and a comprehensive study on the effects of random disorder in the lattice demonstrated that disorder was dependent on the resonator geometry. Subsequently a cryogenic 3-axis scanning stage was developed and the operation of the scanning stage was demonstrated in the final two experiments. The rst scanning experiment was conducted on a 49 site Kagome lattice, where a sapphire defect was used to locally perturb each lattice site. This perturbative scanning probe microscopy provided a means to measure the distribution of photon modes throughout the entire lattice. The second scanning experiment was performed on a single transmission line resonator where a transmon qubit was fabricated on a separate substrate, mounted to the tip of the scanning stage and coupled to the resonator. Here the coupling strength of the qubit to the resonator was mapped out demonstrating strong coupling over a wide scanning range, thus indicating the potential for a scanning qubit to be used as a local quantum probe.

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Kaldirim, Melih. "Dual And Single Color Mid-wavelength Infrared Quantum Well Photodetectors." Master's thesis, METU, 2008. http://etd.lib.metu.edu.tr/upload/2/12609900/index.pdf.

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Quantum Well Infrared Photodetector (QWIP) technology is promising for the development of large format low cost single and dual/multi color infrared sensor arrays. Thanks to the mature III-V semiconductor technology, QWIP focal plane arrays (FPAs) provide high uniformity and excellent noise equivalent temperature difference (NETD) in both long wavelength infrared (LWIR 8-12 &
#61549
m) and mid wavelength infrared (MWIR 3-5 &
#61549
m) bands. This thesis work focuses on the development of large format single and dual color MWIR QWIP FPAs. For single band MWIR detection, we report QWIP FPAs on InP substrate as an alternative to the GaAs based MWIR QWIPs suffering from the degrading effects of lattice mismatched epitaxy. In the course of this work, epitaxial growth conditions of the device structure were optimized and 640×
512 AlInAs/InGaAs QWIP FPAs on InP substrate have been fabricated yielding NETD of 22 mK (f/1.5) and background limited performance (BLIP) temperature as high as 115 K In the second part, we report the first voltage tunable 640×
512 dual color MWIR QWIP FPA. After optimizing epitaxial growth of AlGaAs/InGaAs material system, we have designed and implemented the device structure to yield voltage tunable spectral response in two different windows in the MWIR band. The FPA provides NETDs of 60 and 30 mK (f/1.5) in colors 1 and 2. The results are very encouraging for the development of low cost dual/multi color FPAs since our approach utilizes one In bump per pixel allowing fabrication of dual color FPAs with the same process steps for single color FPAs.

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Alsina, Leal Daniel. "Multipartite entanglement and quantum algorithms." Doctoral thesis, Universitat de Barcelona, 2017. http://hdl.handle.net/10803/459120.

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Quantum information science has grown from being a very small subfield in the 70s until being one of the most dynamic fields in physics, both in fundamentals and applications. In the theoretical section, perhaps the feature that has attracted most interest is the notion of entanglement, the ghostly relation between particles that dazzled Einstein and has provided fabulous challenges to build a coherent interpretation of quantum mechanics. While not completely solved, we have today learned enough to feel less uneasy with this fundamental problem, and the focus has shifted towards its potential powerful applications. Entanglement is now being studied from different perspectives as a resource for performing information processing tasks. With bipartite entanglement being largely understood nowadays, many questions remain unanswered in the multipartite case. The first part of this thesis deals with multipartite entanglement in different contexts. In the first chapters it is studied within the whole corresponding Hilbert space, and we investigate several entanglement measures searching for states that maximize them, including violations of Bell inequalities. Later, focus is shifted towards hamiltonians that have entangled ground states, and we investigate entanglement as a way to establish a distance between theories and we study frustration and methods to efficiently solve hamiltonians that exhibit it. In the practical section, the most promised upcoming technological advance is the advent of quantum computers. In the 90s some quantum algorithms improving the performance of all known classical algorithms for certain problems started to appear, while in the 2000s the first universal computers of few atoms began to be built, allowing implementation of those algorithms in small scales. The D-Wave machine already performs quantum annealing in thousands of qubits, although some controversy over the true quantumness of its internal workings surrounds it. Many countries in the planet are devoting large amounts of money to this field, with the recent European flagship and the involvement of the largest US technological companies giving reasons for optimism. The second part of this thesis deals with some aspects of quantum computation, starting with the creation of the field of cloud quantum computation with the appearance of the first computer available to the general public through internet, which we have used and analysed extensively. Also small incursions in quantum adiabatic computation and quantum thermodynamics are present in this second part.
La informació quàntica ha crescut des d'un petit subcamp als anys setanta fins a esdevenir un dels camps més dinàmics de la física actualment, tant en aspectes fonamentals com en les seves aplicacions. En la secció teòrica, potser la propietat que ha atret més interès és la noció d'entrellaçament, la relació fantasmagòrica entre partícules que va deixar estupefacte Einstein i que ha suposat un enorme desafiament per a construir una interpretació coherent de la mecànica quàntica. Sense estar totalment solucionat, hem après prou per sentir-nos menys incòmodes amb aquest problema fonamental i el focus s'ha desplaçat a les seves aplicacions potencials. L'entrellaçament s'estudia avui en dia des de diferents perspectives com a recurs per realitzar tasques de processament de la informació. L'entrellaçament bipartit està ja molt ben comprès, però en el cas multipartit queden moltes qüestions obertes. La primera part d'aquesta tesi tracta de l'entrellaçament multipartit en diferents contextos. Estudiem l'hiperdeterminant com a mesura d'entrellaçament el cas de 4 qubits, analitzem l'existència i les propietats matemàtiques dels estats absolutament màximament entrellaçats, trobem noves desigualtats de Bell, estudiem l'espectre d'entrellaçament com a mesura de distància entre teories i estudiem xarxes tensorials per tractar eficientment sistemes frustrats. En l'apartat pràctic, el més prometedor avenç tecnològic del camp és l'adveniment dels ordinadors quàntics. La segona part de la tesi tracta d'alguns aspectes de computació quàntica, començant per la creació del camp de la computació quàntica al núvol, amb l'aparició del primer ordinador disponible per al públic general, que hem usat extensament. També fem petites incursions a la computació quàntica adiabàtica i a la termodinàmica quàntica en aquesta segona part

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Zou, Yu. "Strained Semiconductor Quantum Dots - Electronic Band Structure and Multilayer Correlation." Akron, OH : University of Akron, 2009. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=akron1248029992.

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Thesis (M.S.)--University of Akron, Dept. of Electrical and Computer Engineering, 2009.
"August, 2009." Title from electronic thesis title page (viewed 10/7/2009) Advisor, Ernie Pan; Co-Advisor, Nathan Ida; Committee members, Malik Elbuluk, Igor Tsukerman; Department Chair, Alex De Abreu Garcia; Dean of the College, George K. Haritos; Dean of the Graduate School, George R. Newkome. Includes bibliographical references.

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Shen, Jianqi. "Quantum Coherence and Quantum-Vacuum Effects in Some Artificial Electromagnetic Media." Doctoral thesis, KTH, Elektroteknisk teori och konstruktion, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-10074.

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The author of this thesis concentrates his attention on quantum optical properties of some artificial electromagnetic media, such as quantum coherent atomic vapors (various multilevel electromagnetically induced transparency vapors) and negative refractive index materials, and suggests some possible ways to manipulate wave propagations inside the artificial electromagnetic materials based on quantum coherence and quantum vacuum effects. In Chapters 1 and 2, the author reviews the previous papers on quantum coherence as well as the relevant work such as electromagnetically induced transparency (EIT), atomic population trapping and their various applications. The basic concepts of quantum coherence (atomic phase coherence, quantum interferences within atomic energy levels) and quantum vacuum are introduced, and the theoretical formulations for treating wave propagations in quantum coherent media are presented. In Chapter 3, the author considers three topics on the manipulation of light propagations via quantum coherence and quantum interferences: i) the evolutional optical behaviors (turn-on dynamics) of a four-level N-configuration atomic system is studied and the tunable optical behavior that depends on the intensity ratio of the signal field to the control field is considered. Some typical photonic logic gates (e.g. NOT and NOR gates) are designed based on the tunable four-level optical responses of the N-configuration atomic system; ii) the destructive and constructive quantum interferences between two control transitions (driven by the control fields) in a tripod-type four-level system is suggested. The double-control quantum interferences can be utilized to realize some photonic devices such as the logic-gate devices, e.g., NOT, OR, NOR and EXNOR gates; iii) some new quantum coherent schemes (using EIT and dressed-state mixed-parity transitions) for realizing negative refractive indices are proposed. The most remarkable characteristic (and advantage) of the present scenarios is such that the isotropic left-handed media (with microscopic structure units at the atomic level) in the optical frequency band can be achieved. Quantum vacuum (the ground state of quantized fields) can exhibit many interesting effects. In Chapter 4, we investigate two quantum-vacuum effects in artificial materials: i) the anisotropic distribution of quantum-vacuum momentum density in a moving electromagnetic medium; ii) the angular momentum transfer between quantum vacuum and anisotropic medium. Such quantum-vacuum macroscopic mechanical effects could be detected by current technology, e.g., the so-called fiber optical sensor that can measure motion with nanoscale sensitivity. We expect that these vacuum effects could be utilized to develop sensitive sensor techniques or to design new quantum optical and photonic devices.In Chapter 5, the author suggests some interesting effects due to the combination of quantum coherence and quantum vacuum, i.e., the quantum coherent effects, in which the quantum-vacuum fluctuation field is involved. Two topics are addressed: i) spontaneous emission inhibition due to quantum interference in a three-level system; ii) quantum light-induced guiding potentials for coherent manipulation of atomic matter waves (containing multilevel atoms). These quantum guiding potentials could be utilized to cool and trap atoms, and may be used for the development of new techniques of atom fibers and atom chips, where the coherent manipulation of atomic matter waves is needed.In Chapter 6, we conclude this thesis with some remarks, briefly discuss new work that deserves further consideration in the future, and present a guide to the previously published papers by us.
QC 20100810

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Prosyk, Kelvin. "Power and spectral characterization of InGaAsP-InP multi-quantum well lasers." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape11/PQDD_0008/NQ42759.pdf.

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Dissertations / Theses on the topic 'Quantum electronics'

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