Methods of controlling molecular rotations using linearly polarized femtosecond and picosecond pulses are considered and analyzed theoretically. These laser pulses, typically in the infrared, are highly non-resonant with respect to the electronic degrees of freedom of the molecules and have intensities of &sim; 10^13 to 10^14 W/cm&sup2;. It is shown how these laser pulses can force small linear molecules to align with the direction of the electric field vector of the laser both in the presence of the laser field as well as after the application of a short laser pulse. Recent experiments on laser-induced molecular alignment are modeled and excellent agreement between experiment and theory is found. Additional methods of controlling molecular rotational dynamics are outlined. The first method considers the forced rotational acceleration of diatomic molecules, called the <i>optical centrifuge</i>. Here, the direction of polarization of a linearly polarized laser field is made to smoothly rotate faster and faster. The molecules, which tend to align with the polarization vector of the laser field, follow the rotation of the laser polarization and are accelerated to high angular momentum. The second method considers the control of field-free rotational dynamics by applying phase shifts to the molecular wave function at select times called <i>fractional revivals</i>. At these select moments, an initially localized wave function splits into several copies of the initial state. Adding phase shifts to the copies then induces interference effects which can be used to control the subsequent evolution of the rotational wave function. This same control scheme has a close link to quantum information and this connection is outlined. Finally, a recently proposed method of controlling the quantum dynamics of the classically chaotic kicked rotor system [J. Gong and P. Brumer, Phys. Rev. Lett. 86, 1741 (2001)] is analyzed from a phase space perspective. It is shown that the proposed quantum control can be linked to small islands of stability in the classical phase space. An experimentally feasible variant of this control scenario using wave packets of molecular alignment is proposed. Two applications of molecular alignment are discussed. The first application uses field-free aligned molecules as a non-linear medium for compression of a laser pulse to the 1 fs regime at optical wavelengths. At such durations, these laser pulses contain nearly a single oscillation of the electric field and represent the shortest laser pulses physically achievable for such frequencies. The second application uses molecular alignment to create a sort of gas phase "molecular crystal" which forms a basis for laser-induced electron diffraction and imaging of the aligned molecules. Here, a first laser pulse aligns the molecules in space. A second laser pulse is then used to ionize outer-shell electrons, accelerate them in the laser field, and steer them back to collide with the parent ion creating a diffraction image with sub-femtosecond and sub-Angstrom resolution.

Quantum information processing has been the subject of countless discoveries since the early 1990's. It is believed to be the way of the future for computation: using quantum systems permits one to perform computation exponentially faster than on a regular classical computer. Unfortunately, quantum systems that not isolated do not behave well. They tend to lose their quantum nature due to the presence of the environment. If key information is known about the noise present in the system, methods such as quantum error correction have been developed in order to reduce the errors introduced by the environment during a given quantum computation. In order to harness the quantum world and implement the theoretical ideas of quantum information processing and quantum error correction, it is imperative to understand and quantify the noise present in the quantum processor and benchmark the quality of the control over the qubits. Usual techniques to estimate the noise or the control are based on quantum process tomography (QPT), which, unfortunately, demands an exponential amount of resources. This thesis presents work towards the characterization of noisy processes in an efficient manner. The protocols are developed from a purely abstract setting with no system-dependent variables. To circumvent the exponential nature of quantum process tomography, three different efficient protocols are proposed and experimentally verified. The first protocol uses the idea of quantum error correction to extract relevant parameters about a given noise model, namely the correlation between the dephasing of two qubits. Following that is a protocol using randomization and symmetrization to extract the probability that a given number of qubits are simultaneously corrupted in a quantum memory, regardless of the specifics of the error and which qubits are affected. Finally, a last protocol, still using randomization ideas, is developed to estimate the average fidelity per computational gates for single and multi qubit systems. Even though liquid state NMR is argued to be unsuitable for scalable quantum information processing, it remains the best test-bed system to experimentally implement, verify and develop protocols aimed at increasing the control over general quantum information processors. For this reason, all the protocols described in this thesis have been implemented in liquid state NMR, which then led to further development of control and analysis techniques.

As technological advances allow us to peer into and beyond microscopic phenomena, new theoretical developments are necessary to facilitate this exploration. In particular, the potential afforded by utilizing quantum resources promises to dramatically affect scientific research, communications, computation, and material science. Control theory is the field dedicated to the manipulation of systems, and quantum control theory pertains to the manoeuvring of quantum systems. Due to the inherent sensitivity of quantum ensembles to their environment, time-optimal solutions should be found in order to minimize exposure to external sources. Furthermore, the complexity of the Schr\"odinger equation in describing the evolution of a unitary operator makes the analytical discovery of time-optimal solutions rare, motivating the development of numerical algorithms. The seminal result of classical control is the Pontryagin Maximum Principle, which implies that a restriction to bounded control amplitudes admits two classifications of trajectories: bang-bang and singular. Extensions of this result to generalized control problems yield the same classification and hence arise in the study of quantum dynamics. While singular trajectories are often avoided in both classical and quantum literature, a full optimal synthesis requires that we analyze the possibility of their existence. With this in mind, this treatise will examine the issue of time-optimal quantum control. In particular, we examine the results of existing numerical algorithms, including Gradient Ascent Pulse Engineering and the Kaya-Huneault method. We elaborate on the ideas of the Kaya-Huneault algorithm and present an alternative algorithm that we title the Real-Embedding algorithm. These methods are then compared when used to simulate unitary evolution. This is followed by a brief examination on the conditions for the existence of singular controls, as well possible ideas and developments in creating geometry based numerical algorithms.

As technological advances allow us to peer into and beyond microscopic phenomena, new theoretical developments are necessary to facilitate this exploration. In particular, the potential afforded by utilizing quantum resources promises to dramatically affect scientific research, communications, computation, and material science. Control theory is the field dedicated to the manipulation of systems, and quantum control theory pertains to the manoeuvring of quantum systems. Due to the inherent sensitivity of quantum ensembles to their environment, time-optimal solutions should be found in order to minimize exposure to external sources. Furthermore, the complexity of the Schr\"odinger equation in describing the evolution of a unitary operator makes the analytical discovery of time-optimal solutions rare, motivating the development of numerical algorithms. The seminal result of classical control is the Pontryagin Maximum Principle, which implies that a restriction to bounded control amplitudes admits two classifications of trajectories: bang-bang and singular. Extensions of this result to generalized control problems yield the same classification and hence arise in the study of quantum dynamics. While singular trajectories are often avoided in both classical and quantum literature, a full optimal synthesis requires that we analyze the possibility of their existence. With this in mind, this treatise will examine the issue of time-optimal quantum control. In particular, we examine the results of existing numerical algorithms, including Gradient Ascent Pulse Engineering and the Kaya-Huneault method. We elaborate on the ideas of the Kaya-Huneault algorithm and present an alternative algorithm that we title the Real-Embedding algorithm. These methods are then compared when used to simulate unitary evolution. This is followed by a brief examination on the conditions for the existence of singular controls, as well possible ideas and developments in creating geometry based numerical algorithms.

La première partie de la thèse est dédiée à la contrôlabilité exacte globale de l'équation de Schrödinger bilinéaire (BSE).Nous montrons comment construire un voisinage de toute fonction propre du Laplacien Dirichlet où la contrôlabilité exacte locale est satisfaite à un temps explicit. Ensuite, pour tout couple de telles fonctions propres, nous étudions comment construire des contrôles et des temps tels que le flot de (BSE) envoie la première sur un voisinage de la seconde arbitrairement petit. Finalement, en regroupant les deux résultats précédents, nous définissons une dynamique entre états propres et nous fournissons un temps explicite requis pour atteindre l'état propre ciblé.Dans la deuxième partie, nous étudions la contrôlabilité exacte globale en projection d'une infinité d'équation de type (BSE) et nous prouvons la contrôlabilité exacte locale en projection à des termes dephases près pour tout temps positif. Dans la démonstration, nous adoptons différentes techniques provenant de la méthode du retour de Coron habituellement utilisée pour ces types de résultats. La principale nouveauté de ce travail est le fait que nous fournissons un ensemble de conditions en le champ de contrôle, impliquant la validité du résultat. Pour un champs de contrôle donné, nous pouvons vérifier si ces hypothèses sont satisfaites.La troisième partie du travail traite de la contrôlabilité de l'équation de Schrödinger bilinéaire (BSE) sur des graphiques compactes. Considérer (BSE) sur un telle structure est utile quand nous devons étudier la dynamique des paquets d'ondes sur un modèle de type graphes. Nous étudions les hypothèses sur le graphe et le champ de contrôle implique que (BSE) soit bien posée dans des espaces appropriés que nous caractérisons en utilisant les méthodes d'interpolation. Ensuite, nous fournissons la contrôlabilité exacte globale dans ces espaces en étudiant comment la structure du graphe et des conditions de bords affectent le résultat. Nous donnons également des exemples de graphes et de champ de contrôle, tels que les hypothèses spectrales de la contrôlabilité exacte globale soient vérifiées, par exemple les graphes en étoile, graphe dit « têtard » et graphe à double anneau. Enfin, quand nos hypothèses de la contrôlabilité exacte globale ne sont pas vérifiées, nous définissons une notion plus faible de contrôlabilité appelée « contrôlabilité énergétique » qui assure l'existence d'un ensemble d'états liés pour lesquels la contrôlabilité exacte est vérifiée. En d'autres termes, nous prouvons l'existence de niveaux d'énergie pour lesquelles il est possible de changer l'état du système. Cette technique permet de traiter un grand nombre de problèmes intéressants. En effet, pour des graphes complexes, il n'est pas possible de vérifier les hypothèses spectrales donnant la contrôlabilité exacte globale. Cependant, la contrôlabilité énergétique permet d'obtenir des résultats intéressants en regardant seulement des sous-graphes particuliers. / The first part of the research is dedicated to the global exact controllability of the bilinear Schrödinger equation (BSE).We show how to construct a neighborhood of some eigenfunctions of the Dirichlet Laplacian where the local exact controllability is satisfied in a specific time. Then, for any couple of those eigenfunctions, we study how to construct controls and times such that the relative dynamics of (BSE) drives the first close to the second as much desired. Third, by gathering the two previous results, we define a dynamics steering eigenstates in eigenstates and we provide an explicit time required to reach the target.In the second part, we study the simultaneous global exact controllability in projection of infinitely many (BSE) and we prove the simultaneous local exact controllability in projection up to phases for any positive time. In the proof, we use different techniques from the Coron's return method usually adopted for those types of results. The main novelty of the work is the fact that it provides a set of conditions implying the validity of the result. Given any control field, one can verify if those assumptions are satisfied.The third part of the work treats the controllability of the bilinear Schrödinger equation (BSE) on compact graph. Considering (BSE) on such a complex structure is useful when one has to study the dynamics of wave packets on graph type model. We investigate assumptions on the graph and on the control field implying the well-posedness of (BSE) in suitable spaces that we characterize by providing peculiar interpolation features.Then, we provide the global exact controllability in those spaces by studying how the structure of the graph and the boundary conditions affect the result. We also provide examples of graphs and control fields so that the spectral assumptions of the global exact controllability are satisfied, e.g. star graphs, tadpole graphs and double-ring graphs.Afterwards, when the hypothesis for the global exact controllability fail, we define a weaker notion of controllability, the so-called “energetic controllability" which ensures the existence of a set of bounded states for which the exact controllability is verified. In other words, we prove the existence of energy levels in which it is possible to change the energy of the system.This technique allows to treat a large number of interesting problems. Indeed, for complex graphs, it is not possible to verify the spectral hypothesis of the global exact controllability. However, the energetic controllability allows to obtain interesting results only by looking for particular substructure contained in the graph.

<p dir="ltr">This thesis explores the intersection of quantum interference, entanglement, and quantum algorithms in the context of chemical reactions. The initial exploration delves into the constructive quantum interference in the photoassociation reaction of a 87Rb Bose Einstein condensate (BEC), where a coherent superposition of multiple bare spin states is achieved and it’s impact on photo-association (PA) was studied. Employing a quantum processor, the study illustrates that interferences can function as a resource for coherent control in photochemical reactions, presenting a universally applicable framework relevant to a spectrum of ultracold chemical reactions. The subsequent inquiry scrutinizes the entanglement dynamics between the spin and momentum degrees of freedom in an optically confined BEC of 87Rb atoms, induced by Raman and RF fields. Significantly, this study unveils substantial spin momentum entanglement under specific experimental conditions, indicating potential applications in the realm of quantum information processing. Finally, the third study advances a quantum algorithm for the computation of scattering matrix elements in chemical reactions, adeptly navigating the complexities of quantum interactions. This algorithm, rooted in the time-dependent method and Möller operator formulation, is applied to scenarios such as 1D semi-infinite square well potentials and co-linear hydrogen exchange reactions, showcasing its potential to enhance our comprehension of intricate quantum interactions within chemical systems.</p>

Many-Body Perturbation Theory is put to test as a method for reliable calculations of the electron-electron interaction in two-dimensional quantum dots. We show that second order correlation gives qualitative agreement with experiments on a level which was not found within the Hartree-Fock description. For weaker confinements, the second order correction is shown to be insufficient and higher order contributions must be taken into account. We demonstrate that all order Many-Body Perturbation Theory in the form of the Coupled Cluster Singles and Doubles method yields very reliable results for confinements close to those estimated from experimental data. The possibility to use very large basis sets is shown to be a major advantage compared to Full Configuration Interaction approaches, especially for more than five confined electrons. Also, the possibility to utilize two-electron correlation in combination with tailor made potentials to achieve useful properties is explored. In the case of a two-dimensional quantum dot molecule we vary the interdot distance, and in the case of a two-dimensional quantum ring we vary the ring radius, in order to alter the spectra. In the latter case we demonstrate that correlation in combination with electromagnetic pulses can be used for the realization of quantum logical gates. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 5: Manuscript.

The establishment of robust laser-cooling techniques for the formation of ultracold atoms has provided a test-bed for low-temperature science, with scattering events changing character from incoherent thermal interactions to coherent quantum mechanical events. A natural extension is the pursuit of ultracold molecules in prescribed low-energy internal states. Atomic cooling techniques, however, do not generalize to the molecular regime due to the complex energy-level structure afforded by its extra degrees of motion. An indirect approach to ultracold molecule formation - photoassociation using ultrafast laser pulses - is the focus of this thesis. A broadband field associates atom pairs into a localized molecular wavepacket that evolves within the attractive excited-state potential. A suitably timed dump pulse may thus be applied to stabilize population into deeply bound ground vibrational states. This strategy may be generalized to any species whose spectroscopy matches the pulse spectrum, and offers a coherent population transfer scheme that does not require precise knowledge of the system. This thesis presents experiments using high-energy photoassociation pulses applied to ultracold rubidium atoms. The pulses quench the background ground-state molecular population but form bound dimers within the excited state. A pump-probe experiment was designed to chart the excited-state dynamics; however, the oscillations predicted by theoretical calculations were not evident in the molecular signal. The nature of the dynamics is expected to be strongly dependent on the initial state of the atom pairs addressed by the ultrafast pulse: a bound molecular population provides an additional candidate to free atoms. A spectroscopic measurement characterizes these bound molecules and identifies their formation mechanism. A subsequent experiment provides evidence that the predominant contributor to the pump-probe signal is the unbound initial population. The consequences with regard to both the observation of excited-state dynamics and the subsequent application of a dump pulse are discussed.

This PhD work took place in the framework of theoretical research aimed at implementation of quantum computing schemes and algorithms in solid state devices. The electron and nuclear spins of dopant atoms implanted in silicon crystals, that already lie at the core of commercial diodes and the photovoltaic industry, are able to store quantum information longer than anything else in the solid state. Controlled manipulations of silicon qubits depend on the ability to tune the nanoscopic donor electron state: we provide a complete theoretical picture that includes, within the insightful and analytic framework of effective mass theory, the effects of the non-trivial silicon conduction band and the different lattice distortions caused by the implantation of the donor species. Calibration of the multi-valley bulk theory to account for binding energies and electron-nuclear hyperfine couplings allows improved estimates of the exchange splittings between two neighbouring donors, that provide the simplest handle for tuning two-qubit operations. Further refinements to our approach lead to exceptional agreement with experimental measurements of Stark effects, where an external electric field is used to enable local single qubit manipulations within global driving fields: we set reliable thresholds on such gating speeds across all group V donors. Finally, we propose a scalable scheme for silicon quantum computing that relies on the coherent transfer of information from Si:Bi donors, that are established as excellent memory qubits, to surface quantum dots that are easier to manipulate, within a topological surface code which enables outstanding tolerance to errors. Analysis of the optimal working regimes and inclusion of the leading sources of decoherence allow us to set out a robust design of the basic building block of future realizations.

Inicialmente, os conceitos fundamentais e a problemática básica subjacentes ao Controle de Sistemas Quânticos são apresentados, destacando-se, por exemplo, as questões físicas e dinâmicas envolvidas, os principais tipos e metodologias de controle no contexto quântico, bem como aplicações existentes e potenciais de Controle Quântico, muitas das quais situando-se na vanguarda da Ciência e da Tecnologia. Segue-se uma exposição do arcabouço teórico básico e do formalismo padrão da Mecânica Quântica, tendo em vista prover os elementos necessários à compreensão de sistemas quânticos, sua dinâmica e seu controle. O conceito de Controlabilidade é, então, apresentado no contexto de Sistemas Quânticos. Em seqüência, os fundamentos do Controle Quântico Ótimo são desenvolvidos como uma extensão da Teoria Clássica de Controle Ótimo, apresentando-se exemplos de aplicações. Ao problema da transferência de estados quânticos para um estado-alvo em tempo mínimo é devotada especial atenção, dada sua grande relevância em aplicações tecnológicas de ponta, como em Computação Quântica e Processamento de Informação Quântica. A partir de limitações físicas que são inerentes a qualquer sistema quântico, no tocante ao tempo mínimo necessário para que ocorra uma transição de estados, propõem-se Fatores de Mérito para quantificar a eficiência dos controles quânticos ótimos que minimizam o tempo de transferência de estados. Exemplos de aplicação, estudos teóricos e estudos de casos são levados a cabo para a definição dos Fatores de Mérito associados. Este trabalho termina com estudos relativos a uma possível formulação da Teoria de Controle Quântico Ótimo em termos de Integrais de Trajetória para o tratamento de sistemas quânticos contínuos, em especial, o controle espaço-temporal de partículas quânticas. Um possível emprego do Efeito Aharonov-Bohm é também discutido como estratégia de Controle Quântico. / Firstly, the fundamental concepts and the basic issues concerning the Control of Quantum Systems are presented, highlighting, for example, related physical and dynamical questions, the main control types and methodologies in the quantum context, as well as current and potential applications of Quantum Control, many of them situated on the avant-garde of Science and Technology. Then follows an exposition of the basic theoretical framework and the standard formalism of Quantum Mechanics, whose aim is to provide the necessary elements for understanding quantum systems, quantum dynamics and control. The concept of Controlability is then presented in the context of Quantum Systems. Subsequently, the fundamental concepts of Quantum Optimal Control are developed as an extension of the Classical Optimal Control Theory, featuring some examples of application. To the problem of transfering quantum states to a certain target state at minimal time a special attention is devoted, having in mind its great relevance in state-of-art technological applications, e.g., Quantum Computation and Quantum Information Processing. From physical limitations that are inherent to any quantum systems, regarding the minimal time necessary to perform a state transition, one proposes Figures of Merit in order to quantify the efficiency of optimal quantum controls which minimize the state transfer time. Examples of applications, theoretical studies and case studies are carried out in order to define the associated Figures of Merit. This work ends with studies concerning a possible formulation of Optimal Quantum Control Theory in terms of Path Integrals for handling continuous quantum systems, particularly, the space-time control of quantum particles. A possible use of the Aharonov-Bohm Effect is also discussed as a Quantum Control strategy.