No description available.

Nesta dissertação apresentaremos um estudo da adiabaticidade em Ressonância Magnética Nuclear. Para tal, apresentaremos inicialmente uma revisão sobre o conceito de adiabaticidade em sistemas quânticos fechados e abertos. Muitos processos adiabáticos comumente utilizados em Ressonância Magnética apresentam um bom desempenho, embora a aproximação adiabática não seja valida durante todo o processo, então através do formalismo da superadiabaticidade de Berry faremos correções para que o pulso satisfaça a condição adiabática. Nesta abordagem, um Hamiltoniano dependente do tempo e que evolui lentamente é iterativamente transformado em quadros diagonais dependentes do tempo até que a aproximação adiabática mais precisa é obtida. Examinaremos as capacidades das iterações super-adiabáticas para produzir uma sequência de atalhos para a adiabaticidade do sistema e estenderemos o conceito da dinâmica superadiabática para sistemas abertos, cuja evolução é governada por uma equação mestra na forma de Lindblad, fornecendo o quadro geral necessário para determinar a estratégia de controle necessário para alcançar a superadiabaticidade. / We present a study of adiabaticity in Nuclear Magnetic Resonance. First, we present a review on the concept of adiabaticity in closed and open quantum systems. Many commonly used adiabatic processes in Magnetic Resonance perform well even though the adiabatic approximation does not appear to hold throughout the process, then through the use Berry\'s superadiabatic formalism make corrections to the pulse satis_es the adiabatic condition. This approach, a but time-dependent Hamiltonian slowly evolving is iteratively transformed into time-dependent diagonal frames until the most accurate adiabatic approximation is obtained. Examine the capabilities of superadiabatics iterations to produce a shortcut sequence to adiabaticity system and extend the concept of dynamic superadiabatic for open systems, whose evolution is governed by a master equation of Lindblad form, providing the necessary framework to determine the strategy control needed to achieve superadibaticity.

Nesta dissertação apresentaremos um estudo da adiabaticidade em Ressonância Magnética Nuclear. Para tal, apresentaremos inicialmente uma revisão sobre o conceito de adiabaticidade em sistemas quânticos fechados e abertos. Muitos processos adiabáticos comumente utilizados em Ressonância Magnética apresentam um bom desempenho, embora a aproximação adiabática não seja valida durante todo o processo, então através do formalismo da superadiabaticidade de Berry faremos correções para que o pulso satisfaça a condição adiabática. Nesta abordagem, um Hamiltoniano dependente do tempo e que evolui lentamente é iterativamente transformado em quadros diagonais dependentes do tempo até que a aproximação adiabática mais precisa é obtida. Examinaremos as capacidades das iterações super-adiabáticas para produzir uma sequência de atalhos para a adiabaticidade do sistema e estenderemos o conceito da dinâmica superadiabática para sistemas abertos, cuja evolução é governada por uma equação mestra na forma de Lindblad, fornecendo o quadro geral necessário para determinar a estratégia de controle necessário para alcançar a superadiabaticidade. / We present a study of adiabaticity in Nuclear Magnetic Resonance. First, we present a review on the concept of adiabaticity in closed and open quantum systems. Many commonly used adiabatic processes in Magnetic Resonance perform well even though the adiabatic approximation does not appear to hold throughout the process, then through the use Berry\'s superadiabatic formalism make corrections to the pulse satis_es the adiabatic condition. This approach, a but time-dependent Hamiltonian slowly evolving is iteratively transformed into time-dependent diagonal frames until the most accurate adiabatic approximation is obtained. Examine the capabilities of superadiabatics iterations to produce a shortcut sequence to adiabaticity system and extend the concept of dynamic superadiabatic for open systems, whose evolution is governed by a master equation of Lindblad form, providing the necessary framework to determine the strategy control needed to achieve superadibaticity.

In this thesis, current driven domain-wall motion is studied using atomistic simulations with the exchange coupling modeled by the Heisenberg Hamiltonian under the nearest-neighbor approximation. The investigations may be divided into two parts, each concerned with how different aspects of the systems affect the domain-wall motion. The first part deals with domain-wall width dependence of the velocity in a three dimensional geometry with simple cubic crystal structure and uniaxial anisotropy. Results from this part showed that the velocity has a minor domain-wall width dependence. For a fixed current density, the velocity increased with domain-wall width, though only from 61.5 a/ns to 64.5 a/ns as the domain-wall width was increased from 3 to 25 atoms. The second part of the investigations deals with phenomena involving mixed cubic and uniaxial anisotropy, the non-adiabaticity parameter as well as the geometry of the system. The discussion includes an account of how the spin-transfer and cubic anisotropy torques contribute to the motion for different values of the non-adiabaticity parameter. In comparing a one dimensional atomic chain and a three dimensional system with simple cubic crystal structure, but otherwise with the same material properties, results showed a difference in how the two systems responded to currents. This difference is not accounted for by the micromagnetic theory, and its origin was unable to be determined.

Simulating the nonequilibrium behavior of interacting quantum systems is an important way to understand results of experimental quantum simulators, motivate new materials, and refine new quantum algorithms. However, this is a challenging task due to the exponential difficulty of such systems, which motivates dimensional reduction methods, such as semiclassical limits. This work extends semiclassical phase space methods to spin systems with no clear classical limit with the cluster truncated Wigner approximation (cTWA), and improves on Schrieffer-Wolff low energy effective dynamics with variational adiabatic generators. The cTWA was used to compute nonequilibrium dynamics in spin chains, finding behavior such as signatures of many body localization; rapid thermalization and preservation of fluctuations; effective thermodynamic classical behaviors; and signatures of quantum chaos and butterfly velocities, in 1d spin 1/2 chains. Variational Schrieffer-Wolff methods were used to find efficient non-perturbative dressings for the Hubbard model and find effective quasiparticle dynamics and nonthermal states in quantum chaotic spin chains. These methods are potentially effective tools to separate essential quantum behavior from classical behavior, and can be used to diagnose quantum thermalization behavior in interacting quantum systems.

No description available.

A computação quântica adiabática tem sua pedra angular no teorema adiabático, cuja eficiência está relacionada tradicionalmente à proporção da variação temporal do Hamiltoniano que descreve o sistema e o gap mínimo entre o estado fundamental e o primeiro excitado. Normalmente, esse gap tende a diminuir quando aumenta o número de recursos (bit quântico: qubit) de um processador quântico, exigindo dessa maneira variações lentas do Hamiltoniano para assim garantir uma dinâmica adiabática. Entre os candidatos para a sua implementação física, estão os qubits baseados em circuitos supercondutores os quais têm um grande potencial, por causa de seu alto controle e escalabilidade promissora. No entanto, quando esses qubits são implementados, eles têm uma fonte intrínseca de ruído devido a erros de fabricação, que não podem ser desprezados. Por isso, nesta tese nós estudamos como os efeitos causados pelas flutuações dos parâmetros físicos do qubit afetam o comportamento da fidelidade da computação, realizando com esse propósito a simulação da dinâmica de cadeias de spin pequenas desordenadas. A partir do análise exaustivo desse estúdio foi possível propor uma estratégia que permite aumentar a fidelidade considerando um sistema ruidoso. Por outro lado, motivados pelo interesse de obter critérios suficientes e necessários para satisfazer uma computação quântica adiabática e pelo fato que ainda não existe uma condição de adiabaticidade geral apesar de existir inúmeras propostas, nós apresentamos um novo critério que manifesta suficiência para sistemas mais gerais e finalmente apresentamos evidências de que tal condição seria um quantificador consistente. / Adiabatic quantum computation has its cornerstone in the adiabatic theorem, whose efficiency is traditionally related to the ratio of the Hamiltonian temporal variation that describes the system and the minimum gap between the ground state and the first excited state. Usually, this gap tends to decrease when the number of quantum resources (quantum bit: qubit) of a quantum processor increases, thus it requires slow variations of the Hamiltonian to ensure an adiabatic dynamic. Among the candidates for its physical implementation are the qubits superconducting circuit-based which have great potential because of their high control and promising scalability. However, when these qubits are implemented, they have an intrinsic source of noise due to manufacturing errors that can not be despised. Therefore, in this thesis we study how the effects caused by the fluctuations of the physical parameters of the qubit affect the behavior of the fidelity of the computation, accomplishing with this purpose the simulation of the dynamics of small disordered spin chains. From the exhaustive analysis of this studio, it was possible to propose a strategy that allows to increase the fidelity considering a noisy system. On the other hand, motivated by the interest of obtaining sufficient and necessary criteria to satisfy an adiabatic quantum computation and the fact that there is still no general adiabaticity condition despite there being numerous proposals, we present a new criterion that manifests sufficiency for more general systems and we finally presented evidence that such a condition would be a consistent quantifier.

A computação quântica adiabática tem sua pedra angular no teorema adiabático, cuja eficiência está relacionada tradicionalmente à proporção da variação temporal do Hamiltoniano que descreve o sistema e o gap mínimo entre o estado fundamental e o primeiro excitado. Normalmente, esse gap tende a diminuir quando aumenta o número de recursos (bit quântico: qubit) de um processador quântico, exigindo dessa maneira variações lentas do Hamiltoniano para assim garantir uma dinâmica adiabática. Entre os candidatos para a sua implementação física, estão os qubits baseados em circuitos supercondutores os quais têm um grande potencial, por causa de seu alto controle e escalabilidade promissora. No entanto, quando esses qubits são implementados, eles têm uma fonte intrínseca de ruído devido a erros de fabricação, que não podem ser desprezados. Por isso, nesta tese nós estudamos como os efeitos causados pelas flutuações dos parâmetros físicos do qubit afetam o comportamento da fidelidade da computação, realizando com esse propósito a simulação da dinâmica de cadeias de spin pequenas desordenadas. A partir do análise exaustivo desse estúdio foi possível propor uma estratégia que permite aumentar a fidelidade considerando um sistema ruidoso. Por outro lado, motivados pelo interesse de obter critérios suficientes e necessários para satisfazer uma computação quântica adiabática e pelo fato que ainda não existe uma condição de adiabaticidade geral apesar de existir inúmeras propostas, nós apresentamos um novo critério que manifesta suficiência para sistemas mais gerais e finalmente apresentamos evidências de que tal condição seria um quantificador consistente. / Adiabatic quantum computation has its cornerstone in the adiabatic theorem, whose efficiency is traditionally related to the ratio of the Hamiltonian temporal variation that describes the system and the minimum gap between the ground state and the first excited state. Usually, this gap tends to decrease when the number of quantum resources (quantum bit: qubit) of a quantum processor increases, thus it requires slow variations of the Hamiltonian to ensure an adiabatic dynamic. Among the candidates for its physical implementation are the qubits superconducting circuit-based which have great potential because of their high control and promising scalability. However, when these qubits are implemented, they have an intrinsic source of noise due to manufacturing errors that can not be despised. Therefore, in this thesis we study how the effects caused by the fluctuations of the physical parameters of the qubit affect the behavior of the fidelity of the computation, accomplishing with this purpose the simulation of the dynamics of small disordered spin chains. From the exhaustive analysis of this studio, it was possible to propose a strategy that allows to increase the fidelity considering a noisy system. On the other hand, motivated by the interest of obtaining sufficient and necessary criteria to satisfy an adiabatic quantum computation and the fact that there is still no general adiabaticity condition despite there being numerous proposals, we present a new criterion that manifests sufficiency for more general systems and we finally presented evidence that such a condition would be a consistent quantifier.

No description available.

La plupart des systèmes en matière molle sont en contact avec des solutions contenant des espèces chargées. Certains d’entre eux sont bien décrits par des théories de champ moyen, d’autres nécessitent des approches plus fines qui tiennent compte des corrélations entre ions.Dans la première partie de cette thèse, nous analysons la dynamique de relaxation d’un nanocondensateur. Les techniques analytiques et numériques utilisées relèvent du champ moyen (formalisme de Poisson-Nernst-Planck). Nous étudions les temps caractéristiques de relaxation dans les régimes linéaire et non linéaire et caractérisons le comportement du système en fonction de la concentration en sel et du potentiel appliqué. Les géométries planaire et coaxiale sont traitées. Nous nous intéressons ensuite au problème de concevoir un protocole temporel pour le potentiel appliqué, capable de piloter le système d’un état d’équilibre à un autre, que ce soit pour accélérer le processus de formation de la double couche électrique ou celui d’instauration d’un flux électroosmotique.Dans la deuxième partie, nous abordons la physique des systèmes chargés corrélés, avec une attention particulière pour le phénomène d’attraction entre charges du même signe. Nous élaborons une théorie qui décrit les systèmes sans sel à l’équilibre, quelle que soit la valeur du paramètre de couplage électrostatique. Inspirée en partie par le concept du trou de corrélation et en partie par un formalisme à la Poisson-Boltzmann, la théorie satisfait nombre de résultats exacts et elle peut être facilement résolue numériquement.Dans la troisième partie, nous développons la théorie du couplage fort pour les constituants nanoscopiques du ciment. Après avoir présenté l’histoire et l’état de l’art dans notre compréhension de la physique de cet omniprésent matériau, nous analysons des simulations de dynamique moléculaire de l’interface entre plaquettes de C-S-H. Nous montrons que la grande force de cohésion observée est due à une baisse de la permittivité diélectrique sous confinement, ce qui augmente l’importance des corrélations. Nous étudions la statistique du phénomène d’hydratation des ions, et finalement obtenons de façon analytique la pression en fonction de la distance entre plaquettes, en excellent accord avec les simulations. / Most systems in soft matter are immersed in solutions with charged species. Some of them can be described by mean-field techniques, while others require more sophisticated treatments that account for correlations between ions.In the first part of this thesis, we analyze the relaxation dynamics of a nanocapacitor. We use analytical and numerical techniques within mean-field (so-called Poisson-Nernst-Planck formalism). We study characteristic relaxation times in the linear and nonlinear regime and characterize the behavior of the system as a function of salt density and applied voltage. Both the parallel plate and the coaxial geometries are examined. The problem of designing a smart time-dependent applied potential, to drive the system from an initial to a final equilibrium state is also tackled, with regard to both the electric double layer build-up process and the establishment of an electroosmotic flow.In the second part, the physics of correlated charged systems is presented, with particular focus on the like-charge attraction phenomenon. We develop a theory describing salt-free systems, at arbitrary value of the electrostatic coupling parameter. Inspired partly by the correlation-hole concept and partly by the Poisson-Boltzmann formalism, the theory satisfies a number of exact requirements and can be easily solved numerically.In the third part, we develop the theory of strong coupling for the nanoscopic constituents of set cement. After introducing the history and the present understanding of the physics behind this omnipresent material, we analyze molecular dynamics simulations of the interface between C-S-H platelets (Calcium Silicate Hydrate). We show that the strong cohesion force observed is ultimately due to a decrease in the dielectric permittivity under confinement, which enhances correlations. We study the statistics of ion hydration and obtain analytically the pressure as a function of inter-platelet distance, in excellent agreement with simulations.