As últimas décadas testemunharam intensa atividade de pesquisa teórica e experimental visando compreender o conceito do tempo na mecânica quântica. Este tema desencadeou significante progresso na busca por dispositivos mais rápidos e eficientes no processamento de informação e implementação de tecnologias de comunicação. Motivados pela pergunta quão rápido um sistema quântico evolui sob uma dada dinâmica?, tais avanços levaram a formulação do chamado limite quântico de velocidade ou quantum speed limit, (QSL), i.e., um limite inferior definindo o tempo mínimo de evolução entre estados quânticos distintos. Diversos resultados reportaram QSLs obtidos via tratamentos diferentes e aparentemente desconexos, muitas vezes sob configurações específicas, que deixaram uma lacuna fundamental à resposta da questão geral colocada anteriormente. Neste projeto investigamos como a não-unicidade de uma medida de distinguibilidade de operadores densidade definida no espaço de estados quânticos influencia o QSL e pode ser explorada no intuito de obter limites inferiores mais robustos no tempo de evolução de estados arbitrários. Em particular, baseando-nos no formalismo da geometria da informação, estabelecemos uma família infinita de QSLs válidos para evoluções unitárias e não-unitárias. Este trabalho se propõe unificar e generalizar resultados existentes sobre QSLs na literatura, além de fornecer exemplos de limites mais precisos do que aqueles baseados na informação de Fisher convencional. Em termos físicos, esta investigação é a primeira a destacar o papel das populações e coerências quânticas no cálculo e saturação dos QSLs. Nossos resultados podem encontrar aplicações na otimização de protocolos em computação quântica e metrologia, além de fornecer novos pontos de vista em investigações fundamentais da termodinâmica quântica. / The last decades witnessed intense theoretical and experimental research activity in order to understand the concept of time in quantum mechanics. This subject triggered significant progress in the search for faster and efficient schemes in the implementation of quantum information and communication technologies. Starting from the puzzle How fast can a quantum state evolve under a given dynamics?, such advances have led to the establishment of quantum speed limits (QSLs), i.e., a lower bound setting the minimum time evolution between two distinct quantum states. Past results have included different, apparently unrelated approaches to quantum speed limits, and sometimes tailored to specific settings, which therefore left a fundamental gap in obtaining a satisfactory answer to the general question posed above. In this work we provide a breakthrough for the study and applications of quantum speed limits. We approach the problem from a general information theoretic point of view and we adopt an elegant geometric formalism to construct an infinite family of quantum speed limits valid for closed and open system evolutions. Our description is based on the geometrization of the quantum state space by introducing an information metric which defines a non-unique measure of distinguishability on the state space. We show in particular how our approach incorporates and unifies the previous specialized results, interpreting them under a new comprehensive framework, and allowing us to reach significantly beyond. From the physical point of view, our investigation is the first to highlight the role of populations versus quantum coherences in the determination and saturation of the speed limits. Our results can find applications in the optimization of quantum protocols in quantum computation and metrology, and might provide new insights in fundamental investigations of quantum thermodynamics.

Orientador: Marcos César de Oliveira / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin. / Made available in DSpace on 2018-08-13T03:07:21Z (GMT). No. of bitstreams: 1 Steinhoff_FrankEduardodaSilva_M.pdf: 633661 bytes, checksum: 4c5305fba85a1d80713ed5dfe1f90f32 (MD5) Previous issue date: 2009 / Resumo: Propomos um método para obtenção de propriedades de sistemas quânticos compostos utilizando apenas medições estritamente locais e comunicação clássica. Isso difere dos esquemas usualmente utilizados em protocolos de informação quântica, que se utilizam em sua maioria de operações globais e/ou operações locais conjuntas. Nosso tratamento consiste em analisar o efeito de medições em um subsistema sobre as submatrizes da matriz densidade do sistema composto. Analisamos então como outros subsistemas podem ser expressos em termos dessas submatrizes, obtendo assim o efeito das medições feitas em um subsistema sobre os outros. Aplicamos esse resultado - válido para sistemas com espaços de Hilbert discretos de dimensão arbitrária - a dois problemas centrais em protocolos de informação quântica: a reconstrução de estados de sistemas compostos e a caracterização de emaranhamento. Para a tarefa de reconstrução, mostramos como determinar um estado de um sistema composto de dimensão finita arbitrária utilizando apenas medições locais e comunicação clássica de uma via. A vantagem de nossa proposta está em eliminar a necessidade de medições conjuntas, o que se traduz, no contexto de ótica linear, em eliminar medições de coincidência. Analisamos a caracterização de emaranhamento considerando classes especiais de estados. Para estados com alta simetria - estados isotrópicos, de Werner e de simetria rotacional constituídos por um qubit e um qudit - mostramos uma relação entre o grau de emaranhamento e a diferença de população medida por um subsistema condicionada a medições de paridade do outro subsistema, relação essa já evidenciada em estados gaussianos simétricos. Além disso, propomos uma nova família de estados cujo emaranhamento pode ser caracterizado com muito menos recursos do que os utilizados para a reconstrução, sendo esses recursos estritamente locais. / Abstract: We propose a method to obtain properties of composite quantum systems using strictly local measurements and classical communication only. This differs from schemes usually employed in quantum information protocols where global and/or joint local operations are commonly used. Our treatment consists of analysing the effect of measurements of a system over submatrices of the density matrix of the compound system. We analyse then how copies of a subsystem can be expressed in terms of these submatrices, obtaining thus the effect of the measurements done in a subsystem upon the others. We apply this result - valid for discrete Hilbert spaces of arbitrary dimension - to two central problems in quantum information protocols: the state reconstruction of composite systems and entanglement characterization. For the reconstruction task, we show how to determine a state of a arbitrary finite dimension composite system using local measurements and one-way classical communnication only. The advantage of our proposal lies in elimnating the need for joint measurements, which translates as eliminating coincidence measurements in linear optics context. We analyse entanglement characterization considering special classes of states. For high symmetry states -isotropic states, Werner states and rotationally symmetric states composed of a qubit and a qudit - we show a relation between entanglement degree and population difference measured by one subsystem conditioned to parity measurements of the other subsystem, this relation already present in gaussian symmetric states. Moreover, we propose a new family os states whose entanglement can be characterized with much fewer resources than that used for reconstruction, this resources being strictly local. / Mestrado / Física / Mestre em Física

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21531号 / 理博第4438号 / 新制||理||1638(附属図書館) / 京都大学大学院理学研究科数学・数理解析専攻 / (主査)准教授 COLLINS Benoit Vincent Pierre, 教授 堤 誉志雄, 教授 泉 正己 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21567号 / 理博第4474号 / 新制||理||1642(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 高柳 匡, 教授 青木 慎也, 教授 田中 貴浩 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Cette thèse suit un plan à deux parties. Le point de départ en est la notion de résistance à la localisation, qui est importante en calcul distribué quantique.Dans la première partie, qui est plutôt théorique, nous retraçons l’historique de certaines notions et résultats en information quantique et en calcul distribué, plus précisément le phénomène d’intrication et la condition non-signalling en information quantique et le modèle LOCAL et le problème de coloration en calcul distribué. Ensuite, nous évoquons le modèle φ-LOCAL, développé en 2009 comme adaptation de la condition non-signalling au modèle LOCAL dans le but d’étudier l’existence d’algorithmes distribués quantiques. Finalement, nous soulignons quelques limites du modèle φ-LOCAL à l’aide des notions de consistance globale et de consistance locale, et nous présentons une version plus adéquate de ce modèle.La deuxième partie comporte les principaux résultats techniques obtenus au cours de cette thèse dans le domaine de la théorie des probabilités. Nous introduisons la notion de k-localisabilité qui est une traduction probabiliste du modèle φ-LOCAL. Nous montrons en quoi cette notion est proche, mais plus faible, que la notion de k-dépendance, largement étudiée dans la littérature probabiliste. Nous évoquons des résultats récents autour de la coloration 1-dépendante du chemin qui permettent de conclure au sujet de la coloration 1-localisable du chemin : elle est possible dès qu’il y a plus de quatre couleurs. Dans la suite, nous traitons la question de la possibilité de la coloration 1-localisable du chemin à l’aide de trois couleurs : nous verrons qu’elle n’est pas possible. Pour répondre à cette question, nous avons eu recours à la programmation linéaire et à la combinatoire : en particulier, nous démontrons un théorème qui donne la solution explicite d’un programme linéaire ayant une forme particulière, ainsi qu’une formule pour les nombres de Catalan. / This thesis is divided in two parts. Its starting point is the concept of resistance to localisation, an important concept in distributed quantum computing.In the first, theoretical part of this thesis, we go over the history of certain concepts and results in quantum information theory and distributed computing, such as the phenomenon of entanglement and the non-signalling condition in the first domain, and the LOCAL model and the colouring problem in the second domain. We then focus on the φ-LOCAL model, whose goal is to study the possibility of quantum distributed algorithms, and which was developedin 2009 by adapting the non-signalling condition to the LOCAL model. We introduce the concepts of global and local consistency in order to emphasise some shortcomings of this model. Finally, we present a more adequate version ofthe φ-LOCAL model.The second part of this thesis contains our major technical results in probability theory. We define the concept of k-localisability which is a probabilistic translation of the φ-LOCAL model. We show that this concept is close to but weaker than the concept of k-dependence which is well-studied in the probabilistic literature. We mention recent results concerning 1-dependent colouring of the path graph and the conclusion they allow us to reach with regards to 1-localisable colouring of the path graph : that it is possible with four or more colours. The rest of this part is dedicated to answering the question of the possibility of 1-localisable colouring of the path graph using three colours which we will show to be impossible. In answering this question we have made use of methods in linear programming and combinatorics. In particular, we prove a theorem on the explicit solution of a linear programming problem having a certain form, and a formula for the Catalan numbers.

We introduce the notion of an abstract projection in an operator system and when a finite number of positive contractions in an operator system are all simultaneously abstract projections in that operator system. We extend this notion to Archimedean order unit spaces where we prove when a positive contraction is an abstract projection in some operator system, and furthermore when a finite number of positive contractions in an Archimedean order unit space are all simultaneously abstract projections in a single operator system. These methods are then used to provide new characterizations of both nonsignalling and quantum commuting correlations. In particular, we construct a universal Archimedean order unit space such that every quantum commuting correlation may be realized as the image of a unital linear positive map acting on the generators of that Archimedean order unit space. We also construct an Archimedean order unit space which is universal (in the same way) to nonsignalling correlations. We conclude with results concerning weak dual matrix ordered *-vector spaces and the operator systems they induce.

Quantum holonomies are non-Abelian Geometric Phases predominantly observed in adiabatic, non-adiabatic, or measurement-based quantum evolutions. Their significance lies in their potential utility within quantum computing due to their robustness against noise throughout the parameter path. In this report, we detail the foundational methods necessary for constructing holonomic non-Abelian gates specifically designed for tripartite states <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%7CW%3E" data-classname="equation" data-title="" />and <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%7C%5Cbar%7BW%7D%3E" data-classname="equation" data-title="" />, which serve as the logical qubits in our project. Given that the existence of a universal set of gates has already been demonstrated for each of these evolution types, our project delves into the advantages of applying these basis states across the three evolution categories. We have reformulated the Nuclear Quadrupole Resonance (NQR) Hamiltonian to be exclusively composed of two-body terms, thus rendering it more experimentally feasible. Furthermore, we have connected the W states with the remaining tripartite states to construct a four-level model system and generalized gates within this framework. Lastly, we introduce a measurement-based method that maintains its non-Abelian attributes even in the Zeno limit, where the process of projective measurement gradually approaches the adiabatic model. / Icke-Abelska geometriska faser, så kallade kvantholonomier, observeras huvudsakligen i adiabatiska, icke-adiabatiska eller mätningsbaserade manipulationer av kvanttillstånd. De har stor potential till användning inom kvantdatorberäkningar på grund av deras robusthet mot olika typer av brus. I den här rapporten beskriver vi de grundläggande metoderna som är nödvändiga för att konstruera holonoma kvantgrindar som är speciellt utformade för trekroppstillstånden <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%7CW%3E" data-classname="equation" /> och <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%7C%5Cbar%7BW%7D%3E" data-classname="equation" data-title="" />, som fungerar som de logiska kvantbitarna i projektet, givet att det är redan bevisat att alla dessa modeller kan klara kraven för universalitet. Den här rapporten fokuserar på fördelarna med att tillämpa dessa logiska kvantbitar för tre olika evolutionskategorier. Vi har omformulerat kärnkvadrupolresonans Hamiltonianen så att den uteslutande består av tvåkroppstermer, vilket gör den mer experimentellt genomförbar för att realisera adiabatiska holonoma kvantgrindar. Vidare har vi kopplat <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?W" data-classname="equation" data-title="" />-tillstånden med andra trekroppstillstånden för att konstruera ett så kallat sammanflätat <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5CLambda" data-classname="equation" data-title="" />-system och icke-adiabatiska holonoma kvantgrindar inom detta ramverk. Slutligen introducerar vi en mätningsbaserad metod som, till skillnad från tidigare resultat, bibehåller sina icke-Abelska attribut även i Zeno-gränsen, där processen med projektiv mätning gradvis närmar sig den adiabatiska kärnkvadrupolresonans-modellen.

The aim of this work is to lay down a formalism for parametric models that encapsulates both Classical and Quantum Information Geometry. This will be done introducing parametric models on spaces of normal positive linear functionals on W*-algebras and providing a way of defining a Riemannian structure on this models that comes from the Jordan product of the W*-algebra. This Riemannian structure will have some features that are appealing from the viewpoint of Information Geometry. After introducing this W*-algebraic framework, we will move to Estimation Theory. We will see how and to what extent it is possible to formulate in this framework two well-known statistical bounds: the Cramér-Rao bound and the Helstrom bound. Finally, we will explicitly construct some examples that show how it is possible to reduce this general framework to obtain well-known structures in Classical and Quantum Information Geometry.

Atoms in optical lattices are promising candidates to implement quantum information processing. Their behaviour is well understood on a microscopic level, they exhibit excellent coherence properties, and they can be easily manipulated using external fields. In very deep optical lattices, each atom is restricted to a single lattice site and can be used as a qubit. If the lattice is shallow enough such that the atoms can move, their properties can be used to simulate certain condensed matter phenomena such as superconductivity. In this thesis, we show how technical problems of optical lattices such as restricted decoherence times, or fundamental shortcomings such as the lack of phonons or strong spin interactions, can be overcome by using current or near-future experimental techniques. We introduce a scheme that makes it possible to simulate model Hamiltonians known from high-temperature superconductivity. For this purpose, previous simulation schemes to realise the spin interaction terms are extended. We especially overcome the condition of a filling factor of exactly one, which otherwise would restrict the phase of the simulated system to a Mott-insulator. This scheme makes a large range of parameters accessible, which is difficult to cover with a condensed matter setup. We also investigate the properties of optical lattices submerged into a Bose-Einstein condensate (BEC). A weak-coupling expansion in the BEC-impurity interaction strength is used to derive a model that describes the lattice atoms in terms of polarons, i.e.~atoms dressed by Bogoliubov phonons. This is analogous to the description of electrons in solids, and we observe similar effects such as a crossover from coherent to incoherent transport for increasing temperatures. Moreover, the condensate mediates an attractive off-site interaction, which leads to macroscopic clusters at experimentally realistic parameters. Since the atoms in the lattice can also be used as a quantum register with the BEC mediating a two-qubit gate, we derive a quantum master equation to examine the coherence properties of the atomic qubits. We show that the system exhibits sub- and superdecoherence and that a fast implementation of the two-qubit gate competes with dephasing. Finally, we show how to realise the encoding of qubits in a decoherence-free subspace (DFS) using optical lattices. We develop methods for implementing robust gate operations on qubits encoded in a DFS exploiting collisional interactions between the atoms. We also give a detailed analysis of the performance and stability of the gate operations and show that a robust implementation of quantum repeaters can be achieved using our setup. We compare the robust repeater scheme to one that makes use of conventional qubits only, and show the conditions under which one outperforms the other.

One of the main goals of the theory of open quantum systems is to devise methods which help preserve the quantum properties of a system interacting with its environment. One possible pathway to achieve this goal is to use non-Markovian reservoirs, characterized by information backflows and revivals of certain quantum properties. These reservoirs usually require advanced engineering techniques, which may turn their implementation impractical. In this dissertation we propose an alternative technique: the injection of a classical colored noise, which induces the desired quantum non-Markovianity. In order to do that, we investigate the dynamics of a quantum system interacting with its surrounding environment and under the injection of a classical stochastic colored noise. A time-local master equation for the system is derived by using the stochastic wave function formalism and functional calculus. Afterwards, the non-Markovianity of the evolution is detected by using the Andersson, Cresser, Hall and Li measure, which is based on the decay rates of the master equation in canonical Lindblad-like form. Finally, we evaluate the measure for three different colored noises and study the interplay between environment and noise pump necessary to induce quantum non-Markovianity, as well as the energy balance of the system. / Um dos objetivos principais da teoria de sistemas quânticos abertos é desenvolver métodos que ajudem a preservar as propriedades quânticas de um sistema interagindo com o ambiente. Um possível caminho para alcançar essa meta é usar reservatórios não-Markovianos, caracterizados por refluxos de informação e renascimento de certas propriedades quânticas. Esses reservatóris geralmente requerem o uso de técnicas avançadas de engenharia, o que pode tornar sua implementação impraticável. Nessa dissertação nós propomos uma técnica alternativa: a injeção de um ruído colorido clássico, o qual induz a desejada não-Markovianidade quântica. De modo a fazer isso, nós investigamos a dinâmica de um sistema quântico interagindo com o ambiente e sob a injeção de um ruído colorido clássico estocástico. Uma equação mestra local no tempo é derivada usando-se do formalismo da função de onda estocástica e de técnicas de cálculo funcional. Após isso, a não-Markovianidade da evolução é detectada através da medida de Andersson, Cresser, Hall e Li, a qual é baseada nos coeficientes da equação mestra na forma de Lindblad-like canônica. Finalmente, nós calculamos a medida para três diferentes ruídos coloridos e estudamos a relação entre o ambiente e o bombeio estocástico necessária para induzir não-Markovianidade quântica, assim como o balanço de energia do sistema.