Submitted by Bruna Rodrigues (bruna92rodrigues@yahoo.com.br) on 2016-10-10T14:16:22Z No. of bitstreams: 1 TeseRFIG.pdf: 4866078 bytes, checksum: fad89e3fc2a4023db453218c47475574 (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-10-21T13:36:40Z (GMT) No. of bitstreams: 1 TeseRFIG.pdf: 4866078 bytes, checksum: fad89e3fc2a4023db453218c47475574 (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-10-21T13:36:50Z (GMT) No. of bitstreams: 1 TeseRFIG.pdf: 4866078 bytes, checksum: fad89e3fc2a4023db453218c47475574 (MD5) / Made available in DSpace on 2016-10-21T13:36:59Z (GMT). No. of bitstreams: 1 TeseRFIG.pdf: 4866078 bytes, checksum: fad89e3fc2a4023db453218c47475574 (MD5) Previous issue date: 2016-04-01 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / The main focus of this work is to explore the feasibility of conducting the teleportation protocol in scenarios where the presence of an external environment reduces the purity of the input state and the transmission channel. To investigate the behavior of these protocols under realistic conditions, we selected the main decoherence maps reported in the literature concerning the description of this phenomenon, including the representation of mixed states through the Kraus operators and systems in thermal equilibrium whose Hamiltonian is described by the Heisenberg model. In this context, we proved that there are cases in which the in uence of external factors (such as increasing temperature and decoherence rates) and the immersion of those qubits in distinct environments help increasing the similarity between the properties of the original information and the teleported state. In addition, we studied the probabilistic teleportation protocol under the in uence of the same noise conditions analysed for the deterministic protocol, and the results here obtained show that the rst process can outperform the expected value of the average delity of the teleported state. / O foco do presente trabalho consiste em explorar a viabilidade de se realizar o teletransporte de estados quânticos arbitrários em cenários onde a presença de um ambiente externo reduz a pureza do estado de entrada e do canal de transmissão de informação. Para investigar o comportamento destes protocolos em condições realistas, selecionamos os principais mapas de de coerência registrados na literatura relativos à descrição deste fenômeno, incluindo a representação de estados mistos através dos operadores de Kraus e de sistemas em equilíbrio térmico cujo Hamiltoniano é descrito pelo modelo de Heisenberg. Neste contexto, provamos que existem casos nos quais a influência de fatores externos (como o aumento da temperatura e das taxas de de coerência) e a imersão de qubits em ambientes distintos contribui para aumentar a semelhança entre as propriedades da informação original e as do estado teletransportado. Além disso, estudamos o método de teletransporte probabilístico sob a influência dos mesmos tipos de ruídos analisados para o protocolo determinístico, e os resultados aqui apresentados demonstram que o primeiro processo é capaz de aumentar ainda mais o valor esperado da fidelidade média do estado teletransportado.

A existência de Correlações Quânticas de Caráter Geral (CQCGs) - discórdia e coerência quântica, baseadas em argumentos entrópicos e geométricos, tem sido extensamente investigada e caracterizada nas últimas décadas. Estudar o efeito dinâmico da decoerência sobre estas medidas é um ponto chave em teoria de informação quântica, já que fenômenos de mudança súbita (sudden-change) e congelamento (freezing) na dinâmica das CQCGs podem ser úteis na exploração destes recursos. Do ponto de vista experimental, spins nucleares se apresentam como excelentes candidatos para testar fenômenos relacionados às CQCGs, tendo em vista a sua fácil manipulação através do método de Ressonância Magnética Nuclear (RMN). Além disso, neste contexto, os spin nucleares estão naturalmente sujeitos a ambientes bem caracterizados pelos canais de atenuação de fase (PD) e atenuação de amplitude generalizada (GAD). A proposta deste trabalho consiste em, a partir de uma revisão teórica dos desenvolvimentos na área de caracterização da decoerência das CQCGs, fornecer provas experimentais destas previsões empregando-se o método de RMN. O foco do trabalho se baseia em estados Bell diagonais (sistemas de 2 q-bits), ou, para casos mais gerais, em estados MN3 . Primeiramente, investigamos o fenômeno de duplo sudden-change em dois sistemas distintos de 2 q-bits, mas cada um sujeito ao efeito de um dos canais de atenuação: PD ou GAD. Estes sistemas permitiram a observação do aparecimento da base ponteiro quando o canal PD atua sobre o sistema. Em segundo lugar, as condições para o aparecimento do fenômeno de freezing são investigadas para sistemas contendo 2, 3 e 4 q-bits, todos associados à spins nucleares distintos e sujeitos a canais de PD independentes. O fenômeno de freezing foi observado nos casos de sistemas com número par de q-bits. Para medidas de dicórdia quântica, o freezing permanece constante por um período de tempo determinado pelas características do estado inicial, já para medidas de coerência, o freezing se mantém por tempo indeterminado. Todos estes resultados estão de acordo com as previsões teóricas encontradas na literatura. Com o objetivo de generalizar estes resultados, demonstramos ainda que estados Bell diagonais fornecem um limite inferior para a coerência presente em estados mais gerais. / Recently, the different formulations of General Quantum Correlations (GQCs) - quantum discord and quantum coherence, based on entropic and geometric arguments, have been extensively investigated and characterized. Following their behavior under decoherence is one of the remarkable points in quantum information theory, as the presence of surprisingly phenomena like sudden-change and freezing can be useful to exploit those resources. From the experimental side, nuclear spins appear as excellent candidates to test benching GQCs related phenomena, since they are easily manipulated by the Nuclear Magnetic Resonance (NMR) method and, in this context, are naturally subject to well characterized environments equivalent to Phase Damping (PD) and Generalized Amplitude Damping (GAD) Channels. Our proposal here is, after an overview of theoretical developments in GQCs decoherence characterization, provide experimental proofs through NMR. All the characterization is made for Bell diagonal states (2 qubit systems), or, in more general cases, for MN3 states. First of all, the double sudden-change phenomenon is investigated in two different 2 qubit systems, each one affected by PD and GAD separately. This allowed the observation of the pointer basis emergence in the PD case, as theoretically predicted. Second, the freezing phenomenon appearence conditions are investigated in 2, 3 and 4 qubit systems, where heteronuclear molecules were employed as setup and independent PD channels act on. The freezing phenomenon is experimentally observed in systems with an even number of qubits. For discord-like measures, the freezing remains constant an amount of time determined by the initial state, however in the coherence-like case it endures forever. All those results agree with the theoretical predictions. To generalize those results, it was also proved that Bell diagonal states provide a lower bond of coherence to general states that are characterized for the same correlation triple.

Num primeiro momento, abordamos neste trabalho as correlações Eistein-Podolsky-Rosen quando simuladas no âmbito da teoria clássica da radiação. Pretendemos com isso investigar o fenômeno da polarização da luz em um e outro domínios da sua descrição. Continuando em teoria da medida, através de uma implementação no modelo de colapso da função de onda proposto por Zurek, onde se observa a reversibilidade da coerência de fase, apresentamos expressões para os tempos de decoerência e recorrência associados. Outro tópico considerado diz respeito ao hamiltoniano quadrático dependente do tempo. Procedendo-se a uma transformação unitária associada ao método dos invariantes desenvolvido por Lewis e Riesenfeld, solucionamos a equação de Schrödinger e apresentamos o operador de evolução. Uma análise dos estados \"squeezed\" da radiação é apresentada segundo trajetórias no espaço de fase. A consideração do método empregado, segundo algumas aproximações, possibilita a abordagem do processo de dissipação/flutuação junto ao hamiltoniano original, numa análise mais realista do que compreende um amplo espectro de modelos físicos, tais como o oscilador ou partícula carregada em presença de um campo eletromagnético não homogêneo e as armadilhas de Paul. / At the beging, we tackle in this work the Einstein-Podolsky-Rosen correlations when simulated in the radiation classic theory. We attempt in this way to study the light polarization phenomenon in both aproaches of its descriptions. Still in measurement theory, through a wave function Zurek colapse model implementation, where there is observed a phase coherence reversibility, we introduce expression for the associated decoherence and recurrence tymes. Another considered topic deals with the time-dependent quadratic hamiltonian. Carrying out a unitary transformation associated with the invariants method developed by Lewis and Riesenfeld, we solved the Schrödinger equation and we introduced the evolution operator. An squeezed states of radiation analysis is also included in the phases space trajectories. The considerations of the attempted method according to same approximations makes it possible to deal with the processes of dissipation/flutuation in the original hamiltonian, in a more realistic analysis of those physical models, such as the oscillator or charged particle in a non-homogeneous electromagnetic field and in the Paul trap.

Apresentamos um estudo sobre o fenômeno da decoerência durante a evolução temporal de um estado atômico ao interagir com o campo eletromagnético de uma cavidade não ideal. Apresentamos um modelo em que o campo da cavidade esta acoplado a um banho externo e mostramos os efeitos dissipativos que este acoplamento pode gerar na decoerência para um sistema átomo campo. Discutimos as grandezas relevantes para analise da decoerência através de modelos analíticos e numéricos, principalmente entre os acoplamentos átomo campo e campo banho. Sugerimos que o retardamento observado no processo de decoerência seja uma característica geral em sistemas analisáveis como constituídos de três subsistemas acoplados sequencialmente, com propriedades espectrais e acoplamentos suficientemente assimétricos. / We present a study of the phenomenon of decoherence in the time evolution of an atomic state with the electromagnetic field of a non-ideal cavity. We present a model where the cavity field is coupled to an external bath and show the dissipative effects this coupling can generate in the decoherence of the atom field system. We discuss the relevant variables for the analysis of decoherence in terms of analytic and numerical models, especially the atom-field and field-bath couplings. We suggest that the observed hindrance of the decoherence process may be a general property in systems which can be considered as formed by three subsystems coupled sequentially, with spectral properties and sufficiently asymmetric coupling.

Une théorie de gravitation quantique propose de décrire l'interaction gravitationnelle à toutes les échelles de distance et d'énergie. Cependant, comprendre l'émergence de notre espace-temps classique reste un problème toujours ouvert. Cette thèse s'y attaque en gravité quantique à boucles à partir d'outils de l'information quantique.Ceci est fait en plusieurs étapes. La gravité quantique à boucles étant toujours une théorie en cours de développement, un point de vue pragmatique est adopté en étudiant une classe d'état physique du champ gravitationnel, motivée à la fois par des intuitions simples et les résultats de la physique à N corps. Une analyse de la reconstruction de la géométrie à partir des corrélations peut être faite et des leçons peuvent être tirées sur la forme de la dynamique fondamentale. Dans un second temps, la physique des sous-systèmes est analysée en commençant d'abord par évaluer l'entropie d'intrication entre l'intérieur et l'extérieur de la région, permettant ainsi de retrouver la loi holographique de l'entropie des trous noirs et donnant une forme possible des états holographiques de la théorie. Plusieurs dynamiques de la frontière, vu comme un système isolé ou ouvert, sont ensuite analysées, éclairant de nouveau la forme de la dynamique fondamentale. Enfin, la dernière étape de ces recherches étudie la dynamique de la frontière en interaction avec un environnement formé des degrés de liberté (de matière ou gravitationnels) formant le reste de l'Univers et la décohérence sur la frontière qu'il induit. Ceci permet de discuter la transition quantique/classique et de mettre en lumière, dans un modèle donné, les états pointeurs de la géométrie. / A quantum theory of gravitation aims at describing the gravitational interaction at every scales of energy and distance. However, understanding the emergence of our classical spacetime is still an open issue in many proposals. This thesis analyzes this problem in loop quantum gravity with tools borrowed from quantum information theory.This is done in several steps. Since loop quantum gravity is still under construction, a pragmatic point of view is advocated and an ansazt for physical states of the gravitational field is studied at first, motivated from condensed matter physics and simple intuitions. We analyze the proposal of reconstructing geometry from correlations. Lessons on the quantum dynamics and the Hamiltonian constraint are extracted. The second aspect of this work focuses on the physics of sub-systems and especially the physics of their boundary. We begin by calculating the entanglement entropy between the interior and the exterior of the region, recovering the holographic law known from classical black hole physics. Then different boundary dynamics are studied, both in the isolated and open cases, which shed lights again on the fundamental dynamics. Finally, the last aspect of this research studies the dynamics of the boundary interacting with an environment whose degrees of freedom (gravitational or matter) forming the rest of the Universe and especially the decoherence it induces. This allows to discuss the quantum to classical transition and understand, in a given model, the pointer states of geometry.

Hybridization of quantum emitters and plasmonic nanostructures has attracted much attention over the last years, due to their potential use as plasmon-based nanolasersor to achieve long-range quantum bit entanglement. Recent theoretical studies suggest that the plasmonic field can induce efficient cross-talking between emitters and lead to the formation of collective superradiant states. In this thesis, we developed a theoretical modelable to analyse collective effects in large ensemble of dipoles coupled by an electromagnetic nanoresonator. We experimentally investigated the plasmon-mediated superradiance of organic emitters grafted at a well-controlled distance from a metal nanosphere at room temperature. We report on the measured decay rates of these hybrid structures at the ensemble and single object levels. We find that the decay rate increases i) with the number ofemitters and ii) as the spacing between the emitters and the metal core decreases, a direct and clear evidence of plasmonic superradiance. This trend was observed for two types of hybrid structures, differing both by the size of the metal core and the type of organic dye used as emitter. The observation of plasmonic superradiance at room temperature opens questions about the robustness of these collective states against decoherence mechanisms.This robustness is of major interest for potential applications of quantum systems at room temperature. / Placer des nanostructures plasmoniques à proximité d’émetteurs quantiques est une approche prometteuse pour concevoir des nanolasers plasmoniques ou réaliser l’intrication de bits quantiques à longue distance. Des études théoriques récentes suggèrent que le champ plasmonique peut induire un couplage efficace entre émetteurs et mener à la formationd’états collectifs superradiants. Dans ce travail de thèse, nous avons développé un modèle théorique afin d’analyser les effets collectifs pour un ensemble de dipoles couplés à un nanorésonateur électromagnétique. Nous avons étudié expérimentalement la superradiance plasmonique d’émetteurs organiques greffés à une distance contrôlée d’une nanosphère metallique,à température ambiante. Nous avons mesuré le taux de relaxation de ces structures hybrides, en ensemble et à l’échelle de l’objet unique. Nous observons que le taux de relaxation augmente i) avec le nombre d’émetteurs et ii) lorsque la distance entre les émetteurs et le coeur métallique diminue, une preuve directe et claire de la superradiance plasmonique.Cette tendance a été observée pour deux types de structure hybride, différentes par la taille du coeur métallique et par le type de molécule utilisée comme émetteur. L’observation de la superradiance plasmonique à température ambiante ouvre des questions sur la robustesse d’un état superradiant contre des mécanismes de décohérence. Cette robustesse présente un intérêt majeur pour des applications potentielles de systèmes quantiques à température ambiante.

This thesis addresses the problem of a form of anomalous decoherence that sheds light into the spectroscopy of blinking quantum dots. The system studied is a two-state system, interacting with an external environment that has the effect of establishing an interaction between the two states, via a coherence generating coupling, called inphasing. The collisions with the environment produce also decoherence, named dephasing. Decoherence is interpreted as the entanglement of the coherent superposition of these two states with the environment. The joint action of inphasing and dephasing generates a Markov master equation statistically equivalent to a random walker jumping from one state to the other. This model can be used to describe intermittent fluorescence, as a sequence of "light on" and "light off" states. The experiments on blinking quantum dots indicate that the sojourn times are distributed with an inverse power law. Thus, a proposal to turn the model for Poisson fluorescence intermittency into a model for non-Poisson fluorescence intermittency is made. The collision-like interaction of the two-state system with the environment is assumed to takes place at random times rather than at regular times. The time distance between one collision and the next is given by a distribution, called the subordination distribution. If the subordination distribution is exponential, a sequence of collisions yielding no persistence is turned into a sequence of "light on" and "light off" states with significant persistence. If the subordination function is an inverse power law the sequel of "light on" and "light off" states becomes equivalent to the experimental sequences. Different conditions are considered, ranging from predominant inphasing to predominant dephasing. When dephasing is predominant the sequel of "light on" and "light off" states in the time asymptotic limit becomes an inverse power law. If the predominant dephasing involves a time scale much larger than the minimum time scale accessible to the experimental observation, thereby generating persistence, the resulting distribution becomes a Mittag-Leffler function. If dephasing is predominant, in addition to the inverse power law distribution of "light off" and "light on" time duration, a strong correlation between "light on" and "light off" state is predicted.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22245号 / 理博第4559号 / 新制||理||1655(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 田中 貴浩, 教授 川合 光, 教授 向山 信治 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

This thesis intends to familiarise the reader with quantum error correction, and also show some relations to the well known concept of information - and the lesser known quantum information. Quantum information describes how information can be carried by quantum states, and how interaction with other systems give rise to a full set of quantum phenomena, many of which have no correspondence in classical information theory. These phenomena include decoherence, as a consequence of entanglement. Decoherence can also be understood as "information leakage", i.e., knowledge of an event is transferred to the reservoir - an effect that in general destroys superpositions of pure states. It is possible to protect quantum states (e.g., qubits) from interaction with the environment - but not by amplification or duplication, due to the "no-cloning" theorem. Instead, this is done using coding, non-demolition measurements, and recovery operations. In a typical scenario, however, not all types of destructive events are likely to occur, but only those allowed by the information carrier, the type of interaction with the environment, and how the environment "picks up" information of the error events. These characteristics can be incorporated into a code, i.e., a channel-adapted quantum error-correcting code. Often, it is assumed that the environment's ability to distinguish between error events is small, and I will denote such environments "memory-less".  This assumption is not always valid, since the ability to distinguish error events is related to the \emph{temperature} of the environment, and in the particular case of information coded onto photons, <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?k_%7B%5Ctext%7BB%7D%7DT_%7B%5Ctext%7BR%7D%7D%5Cll%5Chbar%5Comega" /> typically holds, and one must then assume that the environment has a "memory". In this thesis, I describe a short quantum error-correcting code (QECC), adapted for photons interacting with a cold environment, i.e., this code protects from an environment that continuously records which error occurred in the coded quantum state. Also, it is of interest to compare the performance of different QECCs - But which yardstick should one use? We compare two such figures of merit, namely the quantum mutual information and the quantum fidelity, and show that they can not, in general, be simultaneously maximised in an error correcting procedure. To show this, we have used a five-qubit perfect code, but assumed a channel that only cause bit-flip errors. It appears that quantum mutual information is the better suited yardstick of the two, however more tedious to calculate than quantum fidelity - which is more commonly used. / Denna avhandling är en introduktion till kvantfelrättning, där jag undersöker släktskapet med teorin om klassisk information - men också det mindre välkända området kvantinformation. Kvantinformation beskriver hur information kan bäras av kvanttillstånd, och hur växelverkan med andra system ger upphov till åtskilliga typer av fel och effekter, varav många saknar motsvarighet i den klassiska informationsteorin. Bland dessa effekter återfinns dekoherens - en konsekvens av s.k. sammanflätning. Dekoherens kan också förstås som "informationsläckage", det vill säga att kunskap om en händelse överförs till omgivningen - en effekt som i allmänhet förstör superpositioner i rena kvanttillstånd.  Det är möjligt att med hjälp av kvantfelrättning skydda kvanttillstånd (t.ex. qubitar) från omgivningens påverkan, dock kan sådana tillstånd aldrig förstärkas eller dupliceras, p.g.a icke-kloningsteoremet. Tillstånden skyddas genom att införa redundans, varpå tillstånden interagerar med omgivningen. Felen identifieras m.h.a. icke-förstörande mätningar och återställs med unitära grindar och ancilla-tillstånd.Men i realiteten kommer inte alla tänkbara fel att inträffa, utan dessa begränsas av vilken informationsbärare som används, vilken interaktion som uppstår med omgivningen, samt hur omgivningen "fångar upp" information om felhändelserna. Med kunskap om sådan karakteristik kan man bygga koder, s.k. kanalanpassade kvantfelrättande koder. Vanligtvis antas att omgivningens förmåga att särskilja felhändelser är liten, och man kan då tala om en minneslös omgivning. Antagandet gäller inte alltid, då denna förmåga bestäms av reservoirens temperatur, och i det speciella fall då fotoner används som informationsbärare gäller typiskt <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?k_%7B%5Ctext%7BB%7D%7DT_%7B%5Ctext%7BR%7D%7D%5Cll%5Chbar%5Comega" />, och vi måste anta att reservoiren faktiskt har ett "minne". I avhandlingen beskrivs en kort, kvantfelrättande kod som är anpassad för fotoner i växelverkan med en "kall" omgivning, d.v.s. denna kod skyddar mot en omgivning som kontinuerligt registrerar vilket fel som uppstått i det kodade tillståndet.  Det är också av stort intresse att kunna jämföra prestanda hos kvantfelrättande koder, utifrån någon slags "måttstock" - men vilken? Jag jämför två sådana mått, nämligen ömsesidig kvantinformation, samt kvantfidelitet, och visar att dessa i allmänhet inte kan maximeras samtidigt i en felrättningsprocedur. För att visa detta har en 5-qubitarskod använts i en tänkt kanal där bara bitflip-fel uppstår, och utrymme därför finns att detektera fel. Ömsesidig kvantinformation framstår som det bättre måttet, dock är detta mått betydligt mer arbetskrävande att beräkna, än kvantfidelitet - som är det mest förekommande måttet. / <p>QC 20121206</p>

This dissertation studies quantum decoherence in anharmonic oscillator systems to monitor and understand the way the systems evolve. It also explores methods to control the systems' evolution, and the effects of decoherence when applicable. We primarily do this by finding the time evolution of the systems using their Lie algebraic structures. We solve for a generalized Caldirola-Kanai Hamiltonian, and propose a general way to produce a desired evolution of the system. We apply the analysis to the effects of Dirac delta fluctuations in mass and frequency, both separately and simultaneously. We also numerically demonstrate control of the generalized Caldirola-Kanai system for the case of timed Gaussian fluctuations in the mass term. This is done in a way that can be applied to any system that is made up of a Lie algebra. We also explore the evolution of an optomechanical coupled mirror-laser system while maintaining a second order coupling. This system creates anharmonic effects that can produce cat states which can be used for quantum computing. We find that the decoherence in this system causes a rotational smearing effect in the Husimi function which, with the second order term added, causes rotational smearing after a squeezing effect. Finally, we also address the dynamic evolution and decoherence of an anharmonic oscillator with infinite coupling using the Born-Markov master equation. This is done by using the Lie algebraic structure of the Born-Markov master equation's superoperators when applying a strategic mean field approximation to maintain dynamic flexibility. The system is compared to the Born-Markov master equation for the harmonic oscillator, the regular anharmonic oscillator, and the dynamic double anharmonic oscillator. Throughout, Husimi plots are provided to visualize the dynamic decoherence of these systems.