Orientador: Antonio Vidiella Barranco / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica "Gleb Wataghin" / Made available in DSpace on 2018-08-06T16:53:55Z (GMT). No. of bitstreams: 1 Silva_FernandoLuisSemiaoda_D.pdf: 1615501 bytes, checksum: 370145b0056b0b8da7cf94fb9d01bc25 (MD5) Previous issue date: 2006 / Resumo: A presente tese é dedicada à utilização de conhecidos sistemas quânticos em aplicações de interesse em óptica e informação quântica. Motivados pelos recentes avanços experimentais em sistemas formados por íons aprisionados interagindo com lasers e na eletrodinâmica quântica de cavidades, nós focamos grande parte de nossas propostas nestes sistemas. Mais especificamente, nós estudamos a interação de íons e campos quantizados na chamada eletrodinâmica quântica de cavidades com íons aprisionados. Neste contexto, iniciamos nossos trabalhos com uma proposta de geração de superposições mesoscópicas no movimento do íon. Uma vez que tais superposições são muito sensíveis à decoerência, incluímos perdas na cavidade para tratar uma situação mais realista. Através da observação de quantum jumps, ou fóton-contagens fora da cavidade, mostramos um esquema de geração de estados com características quânticas muito similares aos encontrados no caso da cavidade ideal, sem perdas. Neste aspecto, encontramos um modo de usar a dissipação a nosso favor, fato de grande interesse experimental devido às imperfeições dos espelhos reais. Apresentamos também uma proposta de implementação de uma interação do tipo Kerr em íons como uma alternativa ao uso de cristais não-lineares que apresentam baixíssima eficiência para esse tipo de efeito. Essa proposta abre novas possibilidades para o uso de íons em medidas não demolidoras e computação quântica. Nossos estudos na área de eletrodinâmica quântica com íons aprisionados terminam com a análise dos efeitos do movimento do íon na dinâmica das transições multi-fotônicas. Esse é um estudo mais fundamental e está relacionado com o entendimento da interação da radiação com a matéria. Na última parte desta tese são apresentados resultados sobre o uso de sistemas de muitos corpos para a distribuição de informação quântica. O objetivo de se estudar estes sistemas mais complexos é a busca de implementação de protocolos quânticos em larga escala. Neste sentido, poderíamos pensar numa cadeia de osciladores harmônicos acoplados como ocorre em sistemas típicos da física da matéria condensada. Em particular, nós estudamos como aumentar a eficiência na transmissão de emaranhamento nestas cadeias. Propusemos um esquema que funciona como um tipo de quantum data bus, ou ônibus quântico para transportar e distribuir emaranhamento com alta eficiência / Abstract: This thesis is concerned with the use of firmly established quantum systems for applications in quantum optics and quantum information. Having been driven by recent experimental advances in laser-manipulated trapped ions and cavity quantum electrodynamics, we concentrated more on proposals to be implemented in those systems. Being more specific, we have studied the interaction between trapped ions and quantized fields in the so-called cavity quantum electrodynamics with trapped ions. In this context, we began with a proposal to generate mesoscopic superpositions in the motion of the ion. Since these superpositions are extremely sensitive to decoherence, we have included cavity losses in order to make the situation slightly more realistic. We showed that the observation of quantum jumps, or photon detection outside the cavity, would generate quantum states with properties close to that generated in the ideal lossless case. In spite of the normally destructive effect of dissipation, we found a way to use it in our favor which turns out to be of great experimental importance due to always present mirror imperfections. We also showed how to mimic cross-Kerr nonlinearities in the cavity-ion system as a feasible alternative to the use of nonlinear crystals whose intensity of that non-linearity is too weak. This proposal opens up new possibilities for the use of trapped ions in non-demolition measurements and quantum computing. We finish our work in cavity electrodynamics with trapped ions with the study of the effect of the ionic motion on the dynamics of multiphotonic transitions. This is a more fundamental issue that is related to the understanding of matter-field interaction. In the last part of this thesis, we present results on the use of many-body systems for quantum information distribution. It was our goal to study more complex systems for the implementation of quantum protocols in large scale. In this sense, one could think of a chain of coupled harmonic oscillators as commonly found in condensed matter physics. Particularly, we dealt with the efficiency of entanglement transmission through the chain, trying to improve it. We ended up with a scheme which acts as a quantum data bus able to transport and distribute entanglement around quite efficiently / Doutorado / Física / Doutor em Ciências

M.Sc. (Applied Mathematics) / Entanglement is a quantum resource with applications in quantum communication as well as quantum computing amongst others. Since quantum entanglement is such an abstract concept numerous mathematical measures exist. Some of these have a purely theoretic purpose whereas others play a role in describing the magnitude of entanglement of a system. In quantum systems energy level crossing may occur. Energy levels in quantum systems tend to repel each other so when any type of degeneracy occurs where the energy levels coalesce or cross it is of interest to us. Two such points of degeneracy are exceptional and diabolic points. When these occur it is useful to investigate these points in specific systems and observe level crossing. In this thesis we mainly investigate the relationship between entanglement, energy level crossing and symmetry as well as the exceptional and diabolic points of specific systems. We are especially interested in systems described by spin and Fermi operators.

Orientador: Antonio Vidiella Barranco / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-31T02:56:15Z (GMT). No. of bitstreams: 1 Decordi_GustavoLazero_D.pdf: 2456755 bytes, checksum: 42a17eacb2e1a86e81888e48272ed08e (MD5) Previous issue date: 2016 / Resumo: Estudamos nesta tese, sistemas quânticos compostos sob a influência de ambientes externos. Na primeira parte do trabalho, investigamos um sistema de dois qubits interagentes, estando um deles isolado e o outro acoplado a um banho térmico. Analisamos os efeitos da temperatura do banho sobre a dinâmica do sistema de dois qubits. Com essa finalidade, empregamos dois modelos distintos da interação sistema-ambiente: i) um modelo microscópico, no qual a equação mestra é obtida levando-se em conta o acoplamento entre os qubits na dedução do termo dissipativo, ii) um modelo fenomenológico, no qual o termo dissipativo é simplesmente adicionado ao termo unitário da equação de evolução do operador densidade. Obtemos soluções analíticas para os modelos, o que permitiu estudá-los em um intervalo considerável do acoplamento entre os qubits. Dedicamos a segunda parte do trabalho ao estudo de um sistema quântico em particular acoplado a um pequeno ambiente. Neste contexto, resolvemos exatamente o modelo da interação radiação-matéria conhecido como modelo de Tavis-Cummings a dois átomos. De posse das soluções, obtidas em circunstâncias bastante gerais e até então não encontradas na literatura, investigamos os efeitos oriundos da interação de um pequeno ambiente (átomo em estado de mistura estatística) sobre a dinâmica do subsistema composto pelo outro átomo acoplado ao modo do campo eletromagnético. Nós mostramos que propriedades não-clássicas associadas ao sistema principal podem ser significativamente degradadas pela ação do ambiente quando o átomo 2 está acoplado de maneira resonante ao campo. Encontramos que o comportamento não-clássico do sistema pode ser restaurado a medida que dessintonizamos o campo da frequência de transição do átomo 2, o ambiente / Abstract: We study in this thesis composite quantum systems under the influence of external environments. In the first part of this work, we investigate a two qubit interacting system having one of them isolated and the other coupled to a thermal bath. We analyze the effect of the temperature of the bath on the dynamics of the two qubit system. In order to do that, we consider two different models of system-reservoir interaction: i) a "microscopic" model, in which the master equation is derived taking into account the interaction between the two subsystems (qubits), ii) a "phenomenological" approach, in which the master equation consists of a dissipative term added to the unitary evolution term. We show that in the strong coupling regime between the subsystems (qubits), the expected thermal equilibrium steady state for the two-qubit system naturally arises in the framework of the microscopic model, while in the phenomenological approach it is obtained a steady state density operator which is not correct. Furthermore, the differences are even more profound in the weak coupling regime, when the models give rise to opposite behaviors with regard to the linear entropy of qubit 1. At the context of quantum systems coupled to environments with few degrees of freedom, we solve analytically the matter-radiation interaction model known as two atom Tavis-Cummings Model. With the solutions at hand, achieved in general circumstances until the present not found in literature, in which the constituent atoms may be coupled with different strengths to the field and also have different frequency detunings, we study the effects that arise from the interaction of a small environment (atom in a statistical mixture state) with the other atom coupled to an oscillator (cavity mode). We show that nonclassical features associated to the main system may be significantly degraded by the action of the small environment, if atom 2 is resonantly coupled to the field. We also demonstrate that the nonclassical behaviour of the system may be restored if we detune the field from the transition frequency of atom 2, the environment / Doutorado / Física / Doutor em Ciências / 899872/2011 / CAPES

Le but de cette thèse est de développer des sources d’intrication photonique en vue d'applications en sciences information quantique. Dans ce contexte, nous présentons une source très performante et entièrement guidée permettant, au moyen d'une boucle de Sagnac, la génération d'états hyper-intriqués en polarisation et en énergie-temps. La configuration guidée rend le dispositif versatile, efficace et compatible avec une large bande spectrale, répondant ainsi au besoin des systèmes et réseaux de communication fibrés. À cette fin, nous avons distribué simultanément dans différents canaux télécoms des paires de photons hyper-intriqués au moyen de multiplexeurs en longueur d'onde à 5 canaux (DWDM), augmentant de fait le débit. La qualité de l'intrication est validée par la violation d'une inégalité de Bell étendue à un espace de Hilbert à 16 dimensions. Afin de pouvoir interfacer des photons aux longueurs d'ondes des télécommunications avec les bandes d'absorption des mémoires quantiques situées dans le visible, nous avons également développé une interface cohérente en longueur d'ondes. Un nouveau dispositif de métrologie quantique permettant la mesure avec une précision inégalée des effets de la dispersion chromatique dans les fibres optiques standards est également proposé. Notre approche "quantique" améliore la précision par un facteur 2.6 par rapport aux méthodes de mesures conventionnelles. Dans ce même contexte, nous avons aussi implémenté un nouveau protocole de métrologie de la phase de deux photons en ne détectant uniquement qu'un seul photon. Cette réalisation ouvre la voie à des applications potentielles simples s'appuyant sur peu de ressources au niveau de la détection. Finalement, dans la perspective de la miniaturisation de dispositifs quantiques, nous avons démontré un générateur d'intrication annoncée intégré sur puce qui trouve des applications en calcul et métrologie quantique. / The aim of this thesis was to develop photonic entanglement sources and study their implementation in the general field of quantum information technologies. To this end, a novel fully wave-guided, high performance photonic entanglement source is presented, able to generate hyper-entangled states in the observables of polarization and energy-time by means of a nonlinear Sagnac loop. The waveguide-based design makes it flexible, reliable, and adaptable to a wide spectral range, paving the way towards compact photonic entanglement generators, compatible with fiber-based communication systems and networks. This has been underlined by generating and distributing hyperentanglement in 5x2 dense wavelength division multiplexed channel telecom pairs, simultaneously, towards higher bit rates. The quality of the generated entanglement has been qualified by violating the Bell inequalities in a 16-dimension Hilbert space. Moreover, to adapt the wavelength of the entangled telecom photon pairs to the absorption wavelength of current quantum memory systems, a coherent wavelength converter is demonstrated. Furthermore, within the framework of quantum metrology, a new concept for a high-precision chromatic dispersion (CD) measurement in standard single mode fibers is introduced and demonstrated. In this demonstration, due to conceptual advantages enabled by quantum optics, an unprecedented 2.6 times higher accuracy on CD measurements is shown, compared to state-of-the-art techniques. In the same context, a new protocol for measuring two-photon phase shifts is performed using single photon detection only, promising scalable and potential real device applications with limited resources and simplified detection schemes. Finally, any potential application of quantum optics will be realized using small-scale devices. In this framework, an integrated on-chip heralded path entanglement generator is demonstrated, and shown to be adaptable to logic gate operations.

No description available.

Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: High-dimensional quantum entanglement offers an increase in information capacity per photon; a highly desirable property for quantum information processes such as quantum communication, computation and teleportation. As the orbital angular momentum (OAM) modes of light span an infinite-dimensional Hilbert space, they have become frontrunners in achieving entanglement in higher dimensions. In light of this, we investigate the potential of OAM entanglement of photons by controlling the parameters in both the generation and measurement systems. We show the experimental procedures and apparatus involved in generating and measuring entangled photons in two-dimensions. We verify important quantum tests such as the Einstein, Podolsky and Rosen (EPR) paradox using OAM and angle correlations, as well as a violation of a Bell-type inequality. By performing a full state tomography, we characterise our quantum state and show we have a pure, highly entangled quantum state. We demonstrate that this method can be extended to higher dimensions. The experimental techniques used to generate and measure OAM entanglement place an upper bound on the number of accessible OAM modes. As such, we investigate new methods in which to increase the spiral bandwidth of our generated quantum state. We alter the shape of the pump beam in spontaneous parametric down-conversion and demonstrate an effect on both OAM and angle correlations. We also made changes to the measurement scheme by projecting the photon pairs into the Bessel-Gaussian (BG) basis and demonstrate entanglement in this basis. We show that this method allows the measured spiral bandwidth to be optimised by simply varying the continuous radial parameter of the BG modes. We demonstrate that BG modes can be entangled in higher dimensions compared with the commonly used helical modes by calculating and comparing the linear entropy and fidelity for both modes. We also show that quantum entanglement can be accurately simulated using classical light using back-projection, which allows the study of projective measurements and predicts the strength of the coincidence correlations in an entanglement experiment. Finally, we make use of each of the techniques to demonstrate the effect of a perturbation on OAM entanglement measured in the BG basis. We investigate the self-healing property of BG beams and show that the classical property is translated to the quantum regime. By calculating the concurrence, we see that measured entanglement recovers after encountering an obstruction. / AFRIKAANSE OPSOMMING: Hoë-dimensionele kwantumverstrengeldheid bied ’n toename in inligtingskapasiteit per foton. Hierdie is ’n hoogs wenslike eienskap vir kwantum inligting prosesse soos kwantum kommunikasie, berekening en teleportasie. Omdat die orbitale hoekmomentum (OAM) modusse van lig ’n oneindig dimensionele Hilbertruimte beslaan, het dit voorlopers geword in die verkryging van verstrengeling in hoër dimensies. In die lig hiervan, ondersoek ons die potensiaal van OAM verstrengeling van fotone deur die parameters in beide die generering en meting stelsels te beheer. Ons toon die eksperimentele prosedures en apparaat wat betrokke is by die generering en die meet van verstrengelde fotone in twee dimensies. Ons verifieer kwantumtoetse, soos die Einstein, Podolsky en Rosen (EPR) paradoks vir OAM en die hoekkorrelasies, sowel as ’n skending van ’n Bell-tipe ongelykheid. Deur middel van ’n volledige toestand tomografie, karakteriseer ons die kwantum toestand en wys ons dat dit ’n suiwer, hoogs verstrengel kwantum toestand is. Ons toon ook dat hierdie metode uitgebrei kan word na hoër dimensies. Die eksperimentele tegnieke wat tydens die generasie en meet van OAM verstrengeling gebruik is, plaas ’n bogrens op die aantal toeganklik OAM modusse. Dus ondersoek ons nuwe metodes om die spiraal bandwydte van ons gegenereerde kwantum toestand te verhoog. Ons verander die vorm van die pomp bundel in spontane parametriese af-omskakeling en demonstreer die uitwerking daarvan op beide OAM en die hoekkorrelasies. Ons het ook veranderinge aan die meting skema gemaak deur die foton pare op die Bessel-Gauss (BG) basis te projekteer. Ons wys dat hierdie metode die gemeetde spiraal bandwydte kan optimeer deur eenvoudig die kontinue radiale parameter van die BG modes te verander. Ons demonstreer dat BG modusse verstrengel kan word in hoër dimensies as die heliese modusse, wat algemeen gebruik word, deur berekeninge te maak en te vergelyk met lineêre entropie en vir beide modusse. Ons wys ook dat kwantumverstrengling akkuraat nageboots kan word, met behulp van die klassieke lig terug-projeksie, wat die studie van projeksie metings toelaat en voorspel die krag van die saamval korrelasies in ’n verstrengeling eksperiment. Ten slotte, gebruik ons elk van die tegnieke om die effek van ’n storing op OAM verstrengling wat in die BG basis gemeet is, te demonstreer. Ons ondersoek die self-genesingseienskap van BG bundels en wys dat die klassieke eienskap vertaal na die kwantum-gebied. Deur die berekening van die konkurrensie (concurrence), sien ons dat die gemeetde verstrengeling herstel word nadat ’n obstruksie ondervind is.

Orientador: Marcos Cesar de Oliveira / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-09-24T17:07:35Z (GMT). No. of bitstreams: 1 Oliveira_ThiagoRodriguesde_D.pdf: 2410853 bytes, checksum: 48f52d2d48ef1be2ecb155881b8e16df (MD5) Previous issue date: 2008 / Resumo: Esta dissertação tenta contribuir ao entendimento das possíveis interconexões entre a Teoria de Informação Quântica e Matéria Condensada, um novo campo de pesquisa em amplo desenvolvimento. Mais especificamente, investigamos o papel do emaranhamento, ou correlações quânticas, em transições de fase quânticas contínuas. Enquanto o papel do primeiro na Teoria de Informação dispensa apresentação, as últimas são de grande interesse por exibir um comportamento universal, o qual se origina na divergência de um comprimento de correlação. É esta origem mútua em correlações de ambos os fenômenos que cria uma expectativa de uma possível relação entre estes. Nosso trabalho, embasado no estudo do modelo XY unidimensional em um campo transverso, aponta evidências de um favorecimento do emaranhamento multipartite em detrimento do bipartite na transição, e assim da importância do primeiro no estabelecimento de correlações de longo alcance. Nessa tarefa, acabamos por definir uma classe de medidas de emaranhamento multipartite, generalizando o Emaranhamento Global introduzido por Meyer e Wallach em2002. Mostramos que algumas destas classes provêem informações adicionais à do Emaranhamento Global, além de serem escritas de forma simples em termos de funções de correlação. Tal simplicidade permite o estabelecimento de uma relação formal entre uma dessas classes e transições de fase sinalizadas por divergências na energia. Ao final estudamos o papel da quebra de simetria no emaranhamento bipartite e multipartite, evidenciando, uma vez mais, a maior importância do último em relação ao primeiro. Em uma segunda parte, examinamos o uso de estados de produtos de matrizes na aproximação de estados fundamentais de sistemas críticos. Estes estados podem ser vistos como o ansatz utilizado no Grupo de Renormalização de Matriz Densidade (DMRG), quando este é encarado como um método variacional. Analisando o poder de aproximação de tais estados, agora no modelo de Ising, descobrimos que a "dimensão" do ansatz (ou número de graus de liberdade renormalizados) é uma variável relevante do grupo de renormalização de maneira análoga ao tamanho finito do sistema. Isto possibilita uma análise de escala em relação a essa "dimensão" dos estados de produto de matrizes, com uma possível obtenção de propriedades críticas a baixo custo computacional / Abstract: This thesis attempts to contribute to the understanding of possible connections between Quantum Information and Condensed Matter theories, a new field of research in broad development. Specifically, we investigated the role of entanglement, or quantumcorrelations, in continuous quantum phase transitions. While the importance of the first in the theory of Quantum Information is well known dispense presentation, the latter are of great interest as they exhibit a universal behavior, which descent fromthe divergence of the correlation length. This mutual origin of both in correlations is what creates an expectation of a possible link between them. Our work, based on the study of XY dimensional model in a transverse field, brings evidence of multipartite entanglement being favored, in detriment of bipartite in the transition, and thus in the importance of the first in the establishment of long-range correlations. During our journey, we define a class of measures of multipartite entanglement, generalising the Global Entanglement introduced by Meyer and Wallach in 2002. We show that some of these classes provide additional information to the Global Entanglement, as well as being written in a simple way in terms of correlation functions . This simplicity allows the establishment of a formal relationship between those classes and phases transitions marked by non-analycities in the energy. At the end, we studied the role of spontaneous symmetry breaking in the bipartite and multipartite entanglement, demonstrating once again a major role of the last over the first. In a second part, we examine the use of Matrix Product States to approximate ground states of critical systems. This class of states can be seen as the ansatz used in the Density Matrix Renormalization Group (DMRG), when this one is understood as a variational method. Analyzing the power of approximation of these states, now in Ising model, we found that the "dimension" of the ansatz (or number of renormalized degrees of freedom) is a relevant variable in the renormalization group, in a analogous way to the finite size of the system. This enables an analysis of scaling regarding the "size" of Matrix Product States, with a possible acquisition of critical properties at low computation cost / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Orientador: Guillermo Gerardo Cabrera Oyarzun / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Abstract: Quantum spin systems are caracterized by huge spaces of states, whose dimensions grow exponentially with the particles number. If following the preparation of the initial state, the system is kept isolated from external variables, it will develop a unitary time evolution according to Schrödinger equation or to Liouville equation. The system is driven exclusively by quantum uctuations, whose origin is the Uncertainty Principle. The evolution of a quantum state or a physical observable or mathematical nonobservable operator mean values may involve all states of the whole space of states, or big or small fractions of the total number of states. The analysis of the relaxation of a spin system from an arbitrary initial state to the equilibrium has to cope in general with the difficulty of requiring an extraordinarily great number of eigenstates and eigenvalues. In this work the main interest is centered on the evolution of magnetization¿s Fourier components in low dimensional systems of spins 1/2, whose interactions be given by the exchange modeled by Heisenberg Hamiltonians with axial anisotopy, XXZ. Exact solutions, analitic or numeric, are obtained. This is the continuation of work done in our research group which dealt with XY Hamiltonian families. In the analysis of the systems with the Hamiltonian XXZ, it was specially analysed the subspace defined by null total magnetization and the subspace defined by one spin wave, where chains up to 14 and 1200 were treated, respectively. There are emergence of fast and slow relaxation processes, which depend on the interations and on the initial state, and which result from destructive or constructive quantum interferences. Connections between the presence of those processes and the energy spectrum structure is discussed. Finally, the time evolution of some measures of global entanglement from initial states in the subspace of one spin wave are analised: the considered dynamics creates global entanglement until each entanglement measure reaches a saturation / Made available in DSpace on 2018-09-24T18:24:44Z (GMT). No. of bitstreams: 1 Carvalho_JulioGarcia_D.pdf: 5851086 bytes, checksum: fe9467d4e143df319d98e75ddb334401 (MD5) Previous issue date: 2006 / Resumo: Os sistemas quânticos de spin são caracterizados por espaços de estados muito grandes, cujas dimensões crescem exponencialmente com o número de partículas. Se após a preparação do estado inicial, o sistema for mantido isolado de variáveis externas, desenvolve-se uma evolução temporal unitária prescrita pela equação de Schrödinger ou pela equação de Liouville. O sistema é movido exclusivamente por flutuações quânticas, as quais têm sua origem no Princípio da Incerteza. A evolução de um estado quântico ou de valores médios de observáveis físicos ou de operadores matemáticos não observáveis pode envolver todos os estados do espaço de estados, ou frações grandes ou pequenas do número total de estados. A análise da relaxação de um sistema de spins desde um estado inicial arbitrário até o equilíbrio apresenta a dificuldade de requerer em geral um número extraordinariamente grande de auto-estados e autovalores. Neste trabalho o maior interesse está na evolução das componentes de Fourier da magnetização em sistemas de baixa dimensão espacial, com spins 1/2 e cujas interações sejam dadas pela troca modelada por Hamiltonianos de Heisenberg com anisotropia axial, XXZ. Serão obtidas soluções exatas: numéricas ou analíticas. A motivação proveio de trabalhos anteriores realizados no grupo de pesquisa referentes a famílias do Hamiltoniano XY. Ao se considerar o Hamiltoniano XXZ, analisou-se especialmente o subespaço definido por magnetização total nula e o subespa¸ co de uma onda de spin, onde trataram-se cadeias com até 14 e 1200 sítios, respectivamente. Há emergência de processos rápidos e lentos de relaxação, os quais dependem das interações e do estado inicial, e resultam de interferência quântica destrutiva ou construtiva. Serão discutidas conexões entre a presença desses processos e a estrutura do espectro de energia. Finalmente serão analisadas as evoluções temporais de algumas medidas de emaranhamento global, a partir de estados contidos no subespaço de uma onda de spin: a dinâmica considerada cria emaranhamento global até cada medida atingir uma saturação / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Some special features of polaritons, quasi-particles being normal modes of system of excitons interacting with photons in so called strong coupling regime, are theoretically and numerically analyze in low dimensional systems. In Chapter 1 is given a brief overview of 0D, 1D and 2D semiconductor structures with a general introduction to the polariton field. Chapter 2 is devoted to micro / nano wires. The so called whispering gallery modes are studied in the general case of an anisotropic systems as well as polariton formation in ZnO wires. Theoretical model is compared with an experiment. In the Chapter 3 Josephson type dynamics with Bose-Einstein condensates of polaritons is analyzed taking into account pseudospin degree of freedom. Chapter 4 start with an introduction to Aharonov-Bohm effect, as the best known represent of geometrical phases. An another geometrical phase - Berry phase, occurring for a wide class of systems performing adiabatic motion on a closed ring, is main subject of this section. We considered one proposition for an exciton polariton ring interferometer based on Berry phase effect. Chapter 5 concerns one 0D system : strongly coupled quantum dot exciton to cavity photon. We have discussed possibility of obtaining entangled states from a quantum dot embedded in a photonic crystal in polariton regime.

Quantum entanglement is a phenomenon which has shown great potential use in modern technical implementations, but there is still much development needed in the field. One major problem is how to measure the amount of entanglement present in a given entangled state. There are numerous different entanglement measures suggested, all satisfying some conditions being of either operational, or more abstract, mathematical nature. However, in contradiction to what one might expect, the measures show discrepancies in the ordering of entangled states. Concretely this means that with respect to one measure, a state can be more entangled than another state, but the ordering may be opposite for the same states using another measure. In this thesis we take a closer look at some of the most commonly occurring entanglement measures, and find examples of states showing inequivalent entanglement ordering for the different measures. / Kvantmekanisk sammanflätning är ett fenomen som visat stor potential för framtida tekniska tillämpningar, men för att kunna använda oss av detta krävs att vi hittar lämpliga modeller att mäta omfattningen av sammanflätningen hos ett givet tillstånd. Detta har visat sig vara en svår uppgift, då de modeller som finns idag är otillräckliga när det gäller att konsekvent avgöra till vilken grad olika tillstånd är sammanflätade. Exempelvis kan en modell visa att ett tillstånd är mer sammanflätat än ett annat, medan en annan modell kan visa på motsatsen - att det första tillståndet är mindre sammanflätat än det andra. En möljig orsak kan ligga i de olika modellernas deifnition, då vissa utgår från operativa definitioner, medan andra grundas på matematiska, abstrakta villkor. I denna uppsats tittar vi lite närmre på några av de mätmodeller som finns, och hittar exempel på tillstånd som uppvisar olika ordning av sammanflätningsgrad beroende på vilken modell som används.