Orientador: Iakov Veniaminovitch Kopelevitch / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-13T05:43:34Z (GMT). No. of bitstreams: 1 GutierrezYatacue_DiegoFernando_M.pdf: 13276293 bytes, checksum: 707c38ac7116b9830e881ac37cc37b9e (MD5) Previous issue date: 2009 / Resumo: No intuito de verificar a existência de correlações supercondutoras em grafite, estudamos a possível existência do fenômeno conhecido como efeito de proximidade gigante em amostras de grafite pirolítica altamente orientada (HOPG). Medidas de magneto-transporte realizadas em amostras de HOPG com eletrodos supercondutores de In ou In-Pb revelaram a ocorrência de efeito de proximidade em uma escala muito maior que o comprimento de coerência dos eletrodos supercondutores, o que indica que a grafite pode ser considerada um supercondutor com flutuações de fase. Além disso, nossos estudos revelaram uma supressão do efeito de proximidade para campos magnéticos da ordem de 1 kOe aplicado perpendicularmente aos planos de grafite. Adicionalmente, realizamos estudos comparativos do efeito de proximidade em bismuto metálico. Discutimos os resultados obtidos em termos de modelos teóricos propostos para este assunto. / Abstract: In order to verify the existence of superconducting correlations in graphite, in this work we studied the possibility of the so-called giant proximity effect in highly oriented pyrolytic graphite (HOPG) samples. Magnetoresistance measurements performed on various thoroughly characterized HOPG samples with attached superconducting In or Pb-In electrodes revealed the occurrence of proximity effect on a scale much bigger than a coherence length of superconducting electrodes, indicating that graphite can be considered as a phase-fluctuating superconductor, indeed. Besides, our studies revealed a suppression of the proximity effect in magnetic field H ~ 1 kOe applied perpendicularly to graphene planes. Additionally, we performed comparative studies of the proximity effect in semimetallic bismuth. We discuss the obtained results in terms of available theoretical models. / Mestrado / Física da Matéria Condensada / Mestre em Física

Nesta dissertação nós revisamos a teoria básica que descreve a junção Josephson bosônica para uma e duas espécies partindo do modelo de Bose-Hubbard. Em seguida explicamos como é possível gerar campos de calibe artificiais em um sistema de átomos neutros, como é o caso do condensado de Bose-Einstein. Finalmente, utilizando os conhecimentos teóricos desenvolvidos anteriormente nós buscamos os estados estacionários de um sistema de pseudospin 1/2 submetido a um campo de calibre não-Abeliano artificial, que torna a dinâmica da junção muito mais complexa e rica. Nós também exploramos um novo desbalanceamento de população que surge no sistema, devido a presença do campo de calibre, com características similares as do Macroscopic Quantum Self-Trapping. / In this dissertation we review the basic theory that describes the bosonic Josephson junction for one and two species using the Bose-Hubbard model. Afterwards, we explain how it is possible to generate artificial gauge fields for neutral atoms, like a Bose-Einstein condensate. Finally, using this theoretical background we search for stationary states of a pseudospin 1/2 system subject to a non-Abelian artificial gauge field which turns the dynamic of the junction much more complex and rich. We also explore a possible new populational imbalance that appears on the system due to the presence of the gauge field, with similar features as the Macroscopic Quantum Self-Trapping.

Au cours de ma thèse, je me suis intéressé à deux aspects de l'interaction charge-rayonnement dans des jonctions tunnel. Premièrement j'ai étudié la dynamique des transferts de charge à travers une jonction tunnel normale lorsque celle-ci est couplée à un environnement dont l'impédance est comparable au quantum d'impédance (RK= h/e^2). Nous avons montré que les uctuations de courant portent des signatures de processus à un, deux et trois photons. Deuxièmement j'ai caractérisé le rayonnement émis par une jonction Josephson lorsque celle-ci est couplée à deux résonateurs de fréquences différentes, nu_1 et nu_2, et soumise à une tension 2eV = h nu_1+h nu_2. Nous avons montré que les photons sont émis pas paires, ce qui constitue un rayonnement non-classique violant l'inégalité de Cauchy-Schwarz.Nos résultats montrent que le blocage de Coulomb dynamique constitue une ressource pour la production de micro-ondes quantiques. / In this thesis work, I have focused on two aspects of the charge-light interaction for tunnel junctions. On one hand, I have investigated the influence of the coupling to electromagnetic radiation on the dynamics of charge transfer through a normal tunnel junction when the impedance of its environment gets comparable to the resistance quantum (RK=h/e^2). We showed that the current fluctuations bear signature of the processes where one, two or three photons are emitted.On the other hand, we used a Josephson junction, embedded in a circuit made of two resonators at different frequencies, nu_1 and nu_2, and biased at the voltage 2eV=h nu_1+h nu_2. We showed that the Josephson junction emits photon pairs corresponding to a non classical state of radiation which violates the Cauchy-Schwarz inequality.Our results extend the toolbox for performing quantum optics experiments in the microwave domain.

En circuit-QED, la technique la plus usuelle pour lire l'état d'un qubit est d'utiliser le couplage transverse entre le qubit et une cavité micro-onde dans la limite dispersive. Cependant, malgré d'importants progrès au cours de cette décennie, obtenir une lecture rapide, en un seul coup et hautement fidèle d'un qubit reste un défi majeur. En effet, la distinction de l'état d'un qubit est limitée par le compromis entre vitesse d'acquisition et précision. Cette limite a pour origine le couplage transverse qui impose deux importantes contraintes expérimentales : premièrement, augmenter les interactions pour lire plus rapidement restreint la durée de vie du qubit via l'effet Purcell. La seconde contrainte est sur la force du signal, qui est limitée pour éviter des transitions non voulues et induites par la mesure. Par conséquent, le défi expérimental à relever avec le couplage transverse est d'acquérir un signal faible en un temps court...Pour surmonter ces limitations, nous voulons changer de paradigme en introduisant un nouveau schéma de lecture qui se base sur un couplage cross-Kerr direct. Ce schéma est obtenu grâce à une molécule artificielle supraconductrice couplée à une cavité micro-onde 3D. La molécule est construite en couplant inductivement deux atomes transmons supraconducteurs. Elle manifeste alors deux modes propres : le mode symétrique qubit transmon et le mode antisymétrique ancilla. En insérant cette molécule dans la cavité de manière optimale, une hybridation transverse entre l'ancilla et la cavité conduit à deux résonateurs faiblement anharmoniques, appelés polaritons. Ces derniers possèdent un couplage cross-Kerr direct et large avec le qubit transmon. En mesurant le signal micro-onde transmis par un polariton, l'état du qubit peut être résolu.Théoriquement, dans ce nouveau paradigme, le qubit est immunisé contre les limitations du couplage transverse tel que l'effet Purcell. Cependant, pour les deux échantillons étudiés, un couplage transverse résiduel existe à cause d'imperfections expérimentales. Même faible, il limite pour l'instant la durée de vie du qubit et nos performances de lecture. Malgré cela, nous avons obtenu une lecture du qubit en un seul coup avec une fidélité allant jusqu'à 97.2 % en 500 ns par une mesure dite de verrouillage grâce à la non-linéarité du polariton. Dans une limite linéaire à faible nombre de photons, nous démontrons une fidélité atteignant 94.7 % en seulement 50 ns de lecture grâce à l'ajout d'un amplificateur paramétrique Josephson. Dans ce régime, les sauts quantiques sont résolus et le qubit est lu de manière non-destructive 99.2 % du temps. / Using the transverse coupling between a qubit and a microwave cavity in the dispersive limit is the most common technique in circuit-QED to readout a qubit state. However, despite important progress in the last decade, implementing a fast single shot high fidelity readout remains a major challenge. Indeed, inferring the qubit state is limited by the trade-off between speed and accuracy. The transverse coupling imposes two significant experimental limitations: firstly, increasing the interaction for faster readout leads to limited qubit lifetime via the Purcell effect. Secondly, the strength of the signal is limited to avoid unwanted measurement-induced transitions. Therefore, the experimental challenge with transverse coupling is to acquire a weak signal in a short time...To overcome these limitations, we want to change this coupling paradigm by introducing a new readout scheme relying on a direct cross-Kerr coupling. This scheme is obtained thanks to a superconducting artificial molecule coupled to a microwave 3D cavity. The molecule is built by inductively coupling two transmon artificial atoms, resulting in two eigenmodes: a symmetric mode, the transmon qubit and an antisymmetric mode, the ancilla. By optimal positioning of the molecule in the cavity, a transverse hybridization between ancilla and cavity leads to two weakly anharmonic resonators, called polaritons. The latter possess a large and direct cross-Kerr coupling with the transmon qubit. By driving one of the polariton, the qubit states can be resolved.Theoretically, in such a coupling scheme, the qubit is immune to the limitation of the transverse coupling such as the Purcell effect. However, for the two studied samples, a residual transverse coupling remains due to experimental imperfections. Even if it is weak, it limits for now the qubit lifetime and the readout performances. Despite this, we observe single shot qubit readout performance with fidelity as high as 97.2 % in a 500 ns latching measurement using the non-linearity of the polariton. In a low photons number linear regime, we report fidelity as high as 94.7 % in only 50 ns thanks to the addition of a Josephson parametric amplifier. In this regime, quantum jumps are resolved and the qubit is measured non-destructively 99.2 % of the time.

We address the excitation of quantum breathers in small nonlinear networks of two and three degrees of freedom, in order to study their properties. The invariance under permutation of two sites of these networks substitutes the translation invariance that is present in nonlinear lattices, where (classical) discrete breathers are time periodic space localized solutions of the underlying classical equations of motion. We do a systematic analysis of the spectrum and eigenstates of such small systems, characterizing quantum breather states by their tunnelling rate (energy splitting), site correlations, fluctuations of the number of quanta, and entanglement. We observe how these properties are reflected in the time evolution of initially localized excitations. Quantum breathers manifest as pairs of nearly degenerate eigenstates that show strong site correlation of quanta, and are characterized by a strong excitation of quanta on one site of the network which perform slow coherent tunnelling motion from one site to another. They enhance the fluctuations of quanta, and are the least entangled states among the group of eigenstates in the same range of the energy spectrum. We use our analysis methods to consider the excitation of quantum breathers in a cell of two coupled Josephson junctions, and study their properties as compared with those in the previous cases. We describe how quantum breathers could be experimentally observed by employing the already developed techniques for quantum information processing with Josephson junctions.

Quasicrystals are systems that are ordered but not periodic. They do however still have long-range order and well-defined diffraction peaks. This leads to interesting properties, like critical states which are neither extended nor localized, and to topological invariants and edge states. We study how these peculiar properties impact superconductivity in an SNS-junction, by attaching superconducting leads to a quasicrystal nanowire. We choose to investigate proximitzed superconductivity in Fibonacci quasicrystals, since their normal state has been thoroughly studied and understood. Using the Bogoliubov-de Gennes method and solving the order parameter self-consistently, we calculate the proximity effect as well as the Josephson current. We find that quasicrystals can enhance the proximity effect and significantly enhance the Josephson currents.

Au cours des dernières années, le développement de l’amplificateur paramétrique Josephson a permis de grandement accroître la fidélité de la mesure des qubits supraconducteurs. De plus, ce nouvel outil a ouvert un éventail de possibilités expérimentales telles la mesure de sauts quantiques et la compression de la lumière. Par contre, de récents résultats expérimentaux [Murch et al. Nature 499, 62 (2013), Zhong et al. NJP 15, 125013 (2013)] démontrent que, malgré les bonnes propriétés de cet amplificateur à faible bruit, des imperfections limitent ses performances. Dans le cadre de ce mémoire, une modélisation de l’amplificateur paramétrique Josephson (JPA) au-delà des approximations standards est présentée afin de mieux comprendre et caractériser ces imperfections. Pour la première fois, le JPA à pompe de courant monochromatique, le JPA à pompe de courant bichromatique et le JPA à pompe de flux sont comparés dans un même formalisme. Ceci permet une analyse des avantages et inconvénients de chacun au-delà de ce qui se trouve dans la littérature. En particulier, l’analyse démontre que ces différentes pompes mènent à des corrections d’ordres supérieurs différentes. À l’aide d’outils numériques et analytiques, on caractérise le champ en sortie de l’amplificateur. On démontre alors que le niveau de compression de ce champ est limité par les corrections d’ordres supérieurs généralement négligées dans la modélisation du JPA. Ce résultat numérique est comparé aux résultats expérimentaux obtenus par nos collaborateurs du laboratoire de nanoélectronique quantique de l’Université de Berkeley. Finalement, les cumulants de troisième et quatrième ordres sont calculés et démontrent que le caractère non gaussien du champ en sortie d’un amplificateur paramétrique Josephson augmente avec le gain et la non-linéarité.

No description available.

Thesis (PhD)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: Analog-to-digital converters (ADCs) are an integral part of the interface between the analog and digital realms. This dissertation presents the design and simulation of a Complementary Output Switching-Logic (COSL) compatible, voltage state, switching logic comparator and a flash ADC for high speed applications with multi-GHz input bandwidth. Josephson technology and the COSL family of gates were utilized for this purpose. A detailed design for the switching logic comparator is first provided. The design is verified with simulations to obtain a functional comparator. The comparator is then optimized utilizing an optimization tool developed using the scripting facilities of WRSpice. Incorporated in this tool is a Monte Carlo capability to randomly vary the component values according to Gaussian distributions, and trimming facilities to be able to trim a non-functional comparator to restore functionality. The design component values are then optimized by maximizing the yield of a comparator. The optimized comparator is incorporated into the construction of a4-bit quantizer of an ADC. The output from the quantizer section yields a switching-logic Gray-code output. A Gray-to- Binary converter is designed with COSL gates to convert the Gray output from the quantizer into Binary code for further processing. The functionality, linearity, maximum input bandwidth and dynamic range of the 4-bit ADC is verified by simulation. A number of special input waveforms are used for this purpose. The performance of the comparator and the 4-bit ADC is also evaluated with thermal noise incorporated into simulation. Beat frequency simulations and Fourier spectra were also used in the evaluation of the ADC performance. A fully functional 4-bit ADC, with a maximum input bandwidth of 10 GHz for a clock speed of 20 GHz was achieved through simulations. Beat frequency simulations revealed that the comparators have an input bandwidth greater than 19 GHz with sufficient dynamic range for an ADC of greater than 6 bits of resolution. Due to the fact that the aperture time for the ADC is dependant on the rise time of the sampling pulse and not the width of the pulse, a much smaller aperture time is obtained which directly translates to higher input bandwidth. Finally, a layout of a 4-bit sampler circuit was done according to the Hypres manufacturing process to enable the high-speed testing of the comparator circuits. / AFRIKAANSE OPSOMMING: Analoog-na-Digitale Omsetters (ADOs) vorm 'n integrale deel van die koppelvlak tussen die analoog en digitale wêrelde. Hiedie proefskrif stel die ontwerp en simulasie van 'n Komplementêre Uittree Geskakelde Logika (COSL) aanpasbare, spanningstoestand, geskakelde logika vergelyker en ADO bekend. Hierdie ADO kan vir hoë spoed toepassings waar multi-GHz intree-bandwydte benodig word, aangewend word. Josephson tegnologie en die Komplementêre Uittree Geskakelde Logika (COSL) familie van hekke word vir hierdie doel gebruik. Die volledige ontwerp vir die geskakelde logika vergelyker word eerstens gegee. Die ontwerp word met behulp van simulasies bevestig om sodoende 'n ten volle funksionele vergelyker te verkry. Die vergelyker word verder geëptimeer deur middel van 'n proses wat met behulp van programmering in WRSpice ontwikkel is. Hierdie optimeringsproses sluit 'n Monte Carlo proses in wat die komponentwaardes van die vergelyker onwillekeurig volgens 'n Gaussiese verspreiding verander, sowel as 'n verstellingsmeganisme waarmee 'n nie-funksionerende vergelyker verstel kan word totdat dit weer ten volle funksioneer. Die komponentwaardes word dan geëptimeer vir maksimale opbrengs van 'n vergelyker. Die geëptimeerde vergelyker word gebruik in die konstruksie van 'n 4-bis kwantifiseerder vir 'n ADO. Die uittree van die 4-bis kwantifiseerder is in Gray kode. 'n Gray-na-Binêre kode omsetter word vir hierdie doelontwerp deur van COSL hekke gebruik te maak. Die volle ADO word voorts gesimuleer om die funksionalitet, lineariteit, maksimum intreebandwydte en dinamiese bereik te verifieer. 'n Verskeidenheid van intreeseine is vir hierdie doel gebruik. Die vergelyker en die 4-bis ADO is ook gesimuleer met termiese ruis om die effek daarvan te bepaal. Fourier spektra en ''verskilfrekwensie'' (Beat Frequency) simulasies word ook gebruik in die evaluering van die vergelyker en die ADO. Die korrekte werking van 'n 4-bis ADO met intreebandwydte van 10 GHz met 'n klokspoed van 20 GHz is deur simulasie bevestig. Verskilfrekwensie simulasies dui aan dat die vergelykers 'n intreebandwydte van groter as 19 GHz het, met voldoende dinamiese bereik vir 6 bis resolusie. Aangesien die vergelykers se venstertydperk bepaal word deur die stygende helling van die monsterpuls en nie deur die pulswydte nie, maak dit voorsiening vir 'n baie klein venstertydperk. 'n Klein venstertydperk is essensieel vir 'n hoë intreebandwydte. 'n Uitleg van 'n 4-bis vergelyker stadium is gedoen vir die Hypres vervaardigingsproses om die vergelyker teen hoë spoed te kan toets.

In this research we looked at how fluxons propagate in an annular Josephson junction containing a microshort. We studied this from a theoretical stance and looked at how a single fluxon based on the sine-Grodon soliton equation propagates in this type of junction. It has been seen from a variety of studies that fluxons have many applications through the use of Josephson junctions. The aim of this thesis was to see whether a fluxon will show new properties whilst coming into contact with a microshort located in the junction. We also explored the different geometries a Josephson junction can have and whether that would show the fluxon to present new phenomena. We will also examine point particle systems. With this in mind we took a keen interest in how the interaction between two of these particles in a double well potential would present itself and whether a relationship would become apparent. Alongside the point particle system we modelled fluxons in a double well potential and comment on the similarities with the point particle system. With the aid of the computer programmes Mathematica and COMSOL Multiphysics we were able to compute these different theoretical models and present the work in a logical order with a progression from a single point particle in a double well potential to a fluxon in a heart-shaped Josephson junction. We have looked at current theories and ideas present in this area of condensed matter physics and have explained these in the subsequent thesis.