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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

De nombreux phénomènes de physique quantique ne peuvent être compris que par l'analyse des systèmes ouverts. Un appareil de mesure, par exemple, est un système macroscopique en contact avec un système quantique. Ainsi, tout modèle d'expérience doit prendre en compte les dynamiques propres aux systèmes ouverts. Ces dynamiques peuvent être complexes : l'interaction du système avec son environnement peut modifier ses propriétés, l'interaction peu créer des effets de mémoire dans l'évolution du système, . . . Ces dynamiques sont particulièrement importantes dans l'étude des expériences d'optique quantique. Nous sommes aujourd'hui capables de manipuler individuellement des particules. Pour cela la compréhension et le contrôle de l'influence de l'environnement est crucial. Dans cette thèse nous étudions d'un point de vue théorique quelques procédures communément utilisées en optique quantique. Avant la présentation de nos résultats, nous introduisons et motivons l'utilisation de la description markovienne des systèmes quantiques ouverts. Nous présentons a la fois les équations maîtresses et le calcul stochastique quantique. Nous introduisons ensuite la notion de trajectoire quantique pour la description des mesures indirectes continues. C'est dans ce contexte que l'on présente les résultats obtenus au cours de cette thèse. Dans un premier temps, nous étudions la convergence des mesures non destructives. Nous montrons qu'elles reproduisent la réduction du paquet d'onde du système mesuré. Nous montrons que cette convergence est exponentielle avec un taux fixe. Nous bornons le temps moyen de convergence. Dans ce cadre, en utilisant les techniques de changement de mesure par martingale, nous obtenons la limite continue des trajectoires quantiques discrètes. Dans un second temps, nous étudions l'influence de l'enregistrement des résultats de mesure sur la préparation d'état par ingénierie de réservoir. Nous montrons que l'enregistrement des résultats de mesure n'a pas d'influence sur la convergence proprement dite. Cependant, nous trouvons que l'enregistrement des résultats de mesure modifie le comportement du système avant la convergence. Nous retrouvons une convergence exponentielle avec un taux équivalent au taux sans enregistrement. Mais nous trouvons aussi un nouveau taux de convergence correspondant a une stabilité asymptotique. Ce dernier taux est interprété comme une mesure non destructive ajoutée. Ainsi l'état du système ne converge qu'après un temps aléatoire. A partir de ce temps la convergence peut être bien plus rapide. Nous obtenons aussi une borne sur le temps moyen de convergence. / Many quantum physics phenomena can only be understood in the context of open system analysis. For example a measurement apparatus is a macroscopic system in contact with a quantum system. Therefore any experiment model needs to take into account open system behaviors. These behaviors can be complex: the interaction of the system with its environment might modify its properties, the interaction may induce memory effects in the system evolution, ... These dynamics are particularly important when studying quantum optic experiments. We are now able to manipulate individual particles. Understanding and controlling the environment influence is therefore crucial. In this thesis we investigate at a theoretical level some commonly used quantum optic procedures. Before the presentation of our results, we introduce and motivate the Markovian approach to open quantum systems. We present both the usual master equation and quantum stochastic calculus. We then introduce the notion of quantum trajectory for the description of continuous indirect measurements. It is in this context that we present the results obtained during this thesis. First, we study the convergence of non demolition measurements. We show that they reproduce the system wave function collapse. We show that this convergence is exponential with a fixed rate. We bound the mean convergence time. In this context, we obtain the continuous time limit of discrete quantum trajectories using martingale change of measure techniques. Second, we investigate the influence of measurement outcome recording on state preparation using reservoir engineering techniques. We show that measurement outcome recording does not influence the convergence itself. Nevertheless, we find that measurement outcome recording modifies the system behavior before the convergence. We recover an exponential convergence with a rate equivalent to the rate without measurement outcome recording. But we also find a new convergence rate corresponding to an asymptotic stability. This last rate is interpreted as an added non demolition measurement. Hence, the system state converges only after a random time. At this time the convergence can be much faster. We also find a bound on the mean convergence time.

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In this thesis we study the Dicke model outside the rotating wave approximation (RWA), by employing phase space techniques and the quantum trajectory theory. We present a review of the basic models of open systems in quantum optics and present an experimental proposition justifying the model to be studied. We use the phase space approach to study, among other subjects, entanglement, squeezing and fluctuations across a quantum phase transition. Three different phase space representations are used and their strengths and weaknesses compared. The quantum trajectory theory is applied to visualise the global quantum fluctuations and to learn how different measurement schemes will affect the creation of entanglement. / The University of Auckland, Department of Physics.

In this thesis we study the Dicke model outside the rotating wave approximation (RWA), by employing phase space techniques and the quantum trajectory theory. We present a review of the basic models of open systems in quantum optics and present an experimental proposition justifying the model to be studied. We use the phase space approach to study, among other subjects, entanglement, squeezing and fluctuations across a quantum phase transition. Three different phase space representations are used and their strengths and weaknesses compared. The quantum trajectory theory is applied to visualise the global quantum fluctuations and to learn how different measurement schemes will affect the creation of entanglement. / The University of Auckland, Department of Physics.

In this thesis we study the Dicke model outside the rotating wave approximation (RWA), by employing phase space techniques and the quantum trajectory theory. We present a review of the basic models of open systems in quantum optics and present an experimental proposition justifying the model to be studied. We use the phase space approach to study, among other subjects, entanglement, squeezing and fluctuations across a quantum phase transition. Three different phase space representations are used and their strengths and weaknesses compared. The quantum trajectory theory is applied to visualise the global quantum fluctuations and to learn how different measurement schemes will affect the creation of entanglement. / The University of Auckland, Department of Physics.

In this thesis we study the Dicke model outside the rotating wave approximation (RWA), by employing phase space techniques and the quantum trajectory theory. We present a review of the basic models of open systems in quantum optics and present an experimental proposition justifying the model to be studied. We use the phase space approach to study, among other subjects, entanglement, squeezing and fluctuations across a quantum phase transition. Three different phase space representations are used and their strengths and weaknesses compared. The quantum trajectory theory is applied to visualise the global quantum fluctuations and to learn how different measurement schemes will affect the creation of entanglement. / The University of Auckland, Department of Physics.

Cette thèse est consacrée à l’étude des trajectoires quantiques issues de la théorie desmesures continues en mécanique quantique non relativiste. On y présente de nouveaux résultatsthéoriques ainsi que des exemples d’applications. Sur le front théorique, on étudie principalementla limite de mesure «forte» dans laquelle on met en évidence l’émergence de sauts quantiques etd’échardes quantiques, deux phénomènes dont on précise la statistique. Hors de la limite forte, onpropose une méthode d’extraction optimale d’information pour un registre de qubits. Sur le frontdes applications, on introduit une méthode originale de contrôle utilisant l’intensité de la mesurecomme unique variable et on explique la transition balistique-diffusif dans les marches aléatoiresquantiques ouvertes; deux sous produits de l’étude théorique préalable des situations de mesureforte. On s’intéresse aussi au problème de la gravité semi-classique et montre que la théorie desmesures continues peut permettre d’en construire un modèle cohérent à la limite newtonienne. Onsuggère enfin quelques extensions possibles de la théorie à l’estimation a posteriori et d’éventuellesgénéralisations des résultats théoriques à des situations de mesures répétées discrètes. Dans laprésentation des résultats, l’accent est mis davantage sur l’explicitation des liens entre les multiplespoints de vue possibles sur les trajectoires quantiques (parallèles avec la théorie classique du filtrageet les modèles de collapse objectif utilisés dans les fondements) que sur la rigueur mathématique. / This thesis is devoted to the study of the quantum trajectories obtained from thetheory of continuous measurement in non relativistic quantum mechanics. New theoretical resultsas well as examples of applications are presented. On the theoretical front, we study mostly thelimit of «strong» measurement where we put forward the emergence of quantum jumps and quantumspikes, two phenomena we characterize in detail. Out of the strong measurement limit, weinvestigate a method to extract information from a register of qubits optimally. On the applicationfront, we introduce an original method to control quantum systems exploiting only the freedomof changing the measurement intensity and we explain the transition between a ballistic and adiffusive behavior in open quantum random walks; two byproduct of the theoretical study of thestrong measurement regime. We further study the problem of semi-classical gravity and show thatcontinuous measurement theory allows to construct a consistent model in the Newtonian regime.We eventually suggest possible extensions of the formalism to a posteriori estimation and hint atgeneralizations of the results for the strong measurement limit in the wider context of discreterepeated measurements. In the course of our presentation, we emphasize the link with other approachesto the theory of continuous measurement (parallels with stochastic filtering and collapsemodels in foundations) rather than aim for mathematical rigor.