Thèses sur le sujet « Algebraic quantum theory »

La cryptographie à base de réseaux euclidiens repose en grande partie sur l’utilisation du problème Learning With Errors (LWE) comme fondation de sécurité. Ce problème est au moins aussi difficile que les problèmes standards portant sur les réseaux, mais les primitives cryptographiques qui l’utilisent sont inefficaces en termes de consommation en temps et en espace. Les problèmes Polynomial Learning WithErrors (PLWE), dual Ring Learning With Errors (dual-RLWE) et primal Ring Learning With Errors(primal-RLWE) sont trois variantes de LWE qui utilisent des structures algébriques supplémentaires afin de pallier les inconvénients ci-dessus. Le problème PLWE est paramétré par un polynôme f, alors que dual-RLWE et primal-RLWE sont définis à l’aide de l’anneau d’entiers d’un corps de nombres.Ces problèmes, dits algébriques, sont eux-mêmes au moins aussi difficiles que des problèmes standards portant sur les réseaux, mais, dans leur cas, les réseaux impliqués appartiennent à des classes restreintes.Dans cette thèse, nous nous intéressons aux liens entre les variantes algébriques de LWE.Tout d’abord, nous montrons que pour une vaste classe de polynômes de définition, il existe des réductions (non-uniformes) entre dual-RLWE, primal-RLWE et PLWE pour lesquelles l’amplification des paramètres peut être contrôlée. Ces résultats peuvent être interprétés comme une indication forte de l’équivalence calculatoire de ces problèmes.Ensuite, nous introduisons une nouvelle variante algébrique de LWE, Middle-Product Learning WithErrors (MP-LWE). On montre que ce problème est au moins aussi difficile que PLWE pour beaucoup de polynômes de définition f. Par conséquent, un système cryptographique reposant sur MP-LWE reste sûr aussi longtemps qu’une de ces instances de PLWE reste difficile à résoudre.Enfin, nous montrons la pertinence cryptographique de MP-LWE en proposant un protocole de chiffrement asymétrique et une signature digitale dont la sécurité repose sur la difficulté présumée de MP-LWE
Lattice-based cryptography relies in great parts on the use of the Learning With Errors (LWE) problemas hardness foundation. This problem is at least as hard as standard worst-case lattice problems, but the primitives based on it usually have big key sizes and slow algorithms. Polynomial Learning With Errors (PLWE), dual Ring Learning With Errors (dual-RLWE) and primal Ring Learning WithErrors (primal-RLWE) are variants of LWE which make use of extra algebraic structures in order to fix the above drawbacks. The PLWE problem is parameterized by a polynomial f, while dual-RLWE andprimal-RLWE are defined using the ring of integers of a number field. These problems, which we call algebraic, also enjoy reductions from worst-case lattice problems, but in their case, the lattices involved belong to diverse restricted classes. In this thesis, we study relationships between algebraic variants of LWE.We first show that for many defining polynomials, there exist (non-uniform) reductions betweendual-RLWE, primal-RLWE and PLWE that incur limited parameter losses. These results could be interpretedas a strong evidence that these problems are qualitatively equivalent.Then we introduce a new algebraic variant of LWE, Middle-Product Learning With Errors (MP-LWE). We show that this problem is at least as hard as PLWE for many defining polynomials f. As a consequence,any cryptographic system based on MP-LWE remains secure as long as one of these PLWE instances remains hard to solve.Finally, we illustrate the cryptographic relevance of MP-LWE by building a public-key encryption scheme and a digital signature scheme that are proved secure under the MP-LWE hardness assumption

In questa tesi studio in che modi si possono distribuire i sistemi e i processi che quei sistemi eseguono. La nozione centrale per raggiungere l'obiettivo è che, quando un sistema è distribuito, una sua osservazione "da lontano" richiede uno scambio d'informazioni con le diverse parti del sistema. Questo approccio si caratterizza per il fatto che la "sincronizzazione" (o "handshaking") è il modo fondamentale di interazione. I formalismi impiegati sono presi dalla teoria delle reti di Petri. I sistemi elementari e i sistemi di condizioni ed eventi in quella teoria costituiscono le specificazioni di sistemi. Le reti causali e gli insiemi parzialmente ordinati permettono di modellare processi. In questi modelli, lo stato dell'arte offre una nozione di sottoprocesso, cui si può associare una struttura che porta l'informazione su come distribuire il processo. Formalmente, questa struttura è un reticolo ortomodulare. Nella tesi mostro che gli elementi minimali non banali di quel reticolo (sottoprocessi minimali) possono essere ordinati in modo da formare un'astrazione del processo dato. La natura di questa nozione di sottoprocesso consente di mostrare che l'astrazione rappresenta le componenti del processo, cioè le parti che possono operare indipendentemente. Il comportamento dei sistemi elementari e dei sistemi di condizioni e eventi è modellato per mezzo di sistemi di transizioni etichettate. Nella tesi si applica un'interpretazione delle regioni elementari come proprietà localmente osservabili del sistema, motivata dalla sintesi di reti elementari. Secondo questa interpretazione, le regioni elementari offrono una specificazione adeguata dell'infrastruttura su cui si può distribuire un sistema. Era già noto che l'insieme delle regioni di un sistema elementare o di condizioni ed eventi forma un insieme ortomodulare, da cui si può ricavare un sistema di transizioni etichettate canonico, che contiene tutte le regioni dell'insieme ortomodulare dato. Stabilire se il sistema canonico ha più regioni di quelle specificate è un problema aperto. Il sistema canonico è il più "grande" che si può ottenere dall'insieme ortomodulare, nel senso che ogni altro sistema conforme alla specificazione è un suo sottosistema. D'altra parte, non tutti i sottosistemi hanno la stessa struttura regionale. Definisco una condizione sufficiente per avere l'isomorfismo. Il risultato si ottiene dotando di un'opportuna struttura l'insieme degli eventi, o delle etichette, del sistema canonico, così da riflettere la concorrenza. Un insieme ortomodulare si dice stabile quando è isomorfo all'insieme delle regioni del sistema di transizioni canonico derivato. Erano già note condizioni sotto le quali il primo insieme si immerge nel secondo. Si congettura che tutti gli insiemi parzialmente ordinati ottenuti come insiemi di regioni di sistemi elementari (insiemi regionali) sono stabili. Nella tesi si dà una nuova condizione necessaria perché un insieme ortomodulare sia regionale, e si mostra che in quel caso l'immersione è forte. Non tutte le immersioni sono forti, ma tutti gli isomorfismi sono immersioni forti. Dal risultato segue che l'immersione mappa regioni minimali su regioni minimali.
This work studies systems, and the processes they execute, in the way they can be distributed. To this aim, the central notion is that when a system is distributed, a remote observation requires an exchange of information from the different locations of the system. This approach is characterised by the fact that handshaking is the basic mode of interaction. The chosen formalisms are taken in the framework Petri net theory. Elemen- tary net systems, and condition/event net systems provide specifications for the systems. Causal nets and partially ordered sets allow for modelling processes. With these last formalisations, the state of the art provides a notion of subpro- cesses that can be structured so as to carry information on how a process can be distributed. This structure is formalised as an orthomodular lattice. This work shows that the minimal non trivial elements of this lattice, the minimal subprocesses, can be ordered so as to provide an abstraction of the process. The nature of this notion of subprocess permits to show that this abstraction depicts the localities of the process, parts of the process which can run independently from each other. The behaviour of elementary, and condition/event net systems, is modelled with labelled transition systems. This work adheres to an interpretation of the set of elementary regions, as the one of locally observable properties of the sys- tem, motivated by elementary net synthesis. According to this interpretation, elementary regions represent a suitable specification of the available infrastruc- ture on which to distribute a system. The state of the art shows that the set of regions of an elementary, or condition/event system, forms an orthomodular poset, and a way to retrieve a canonical labelled transition system such that all regions of the orthomodular poset are also regions of it. The question of whether this canonical transition system has more regions than the specified ones is an open problem. The canonical transition system is the largest one can obtain from an orthomodular poset, in the sense that systems complying with the specification, can be found as subsystems of it. However, not all its subsystems display the same regional structure. This work presents a sufficient condition for this to happen. This is achieved by providing a structure to the set of events, or labels, of the canonical system, which reflects concurrency. An orthomodular poset is called stable when it is isomorphic to the set of regions of its canonical transition system. The state of the art shows that when the first poset is of a given class, it embeds in the second. It is conjectured that all posets that arise as the set of elementary regions of an elementary system, regional posets, are stable. This work provides a condition necessary for an orthomodular poset to be regional, and shows that when it holds, the embedding is strong. Not every embedding is strong, but all isomorphisms are, in particular, strong embeddings. This result implies that the embedding maps minimal regions to minimal regions.

We discuss a method of classifying 2-dimensional invertible topological quantum field theories (TQFTs) whose domain surface categories allow non-orientable cobordisms. These are known as Klein TQFTs. To this end we study the 1+1 dimensional open-closed unoriented cobordism category K, whose objects are compact 1-manifolds and whose morphisms are compact (not necessarily orientable) cobordisms up to homeomorphism. We are able to compute the fundamental group of its classifying space BK and, by way of this result, derive an infinite loop splitting of BK, a classification of functors K → Z, and a classification of 2-dimensional open-closed invertible Klein TQFTs. Analogous results are obtained for the two subcategories of K whose objects are closed or have boundary respectively, including classifications of both closed and open invertible Klein TQFTs. The results obtained throughout the paper are generalisations of previous results by Tillmann [Til96] and Douglas [Dou00] regarding the 1+1 dimensional closed and open-closed oriented cobordism categories. Finally we consider how our results should be interpreted in terms of the known classification of 2-dimensional TQFTs in terms of Frobenius algebras.

Topological order is a new paradigm for quantum phases of matter developed to explain phase transitions which do not fit the symmetry breaking scheme for classifying phases of matter. They are characterized by patterns of entanglement that lead to topologically depended ground state degeneracy and anyonic excitations. One common approach for studying such phases in two-dimensional systems is through exactly solvable lattice Hamiltonian models such as quantum double models and String-Net models. The former can be understood as the Hamiltonian formulation of lattice gauge theories and, as such, it is defined by a finite gauge group. However, not much is known about topological phases in tridimensional systems. Motivated by this we develop a new class of three-dimensional exactly solvable models which go beyond quantum double models by using finite crossed modules instead of gauge groups. This approach relies on a lattice implementation of 2-gauge theory to obtain models with a richer topological structure. We construct the Hamiltonian model explicitly and provide a rigorous proof that the ground state degeneracy is a topological invariant and that the ground states can only be characterized with nonlocal order parameters.
Ordem topológica é um novo paradigma para fases quânticas da matéria desenvolvido para explicar transições de fase que não se encaixam no esquema de classificação de fases da matéria por quebra de simetria. Estas fases são caracterizadas por padrões de emaranhamento que levam a uma degenerescência de estado fundamental topológica e a excitações anyonicas. Uma abordagem comum para o estudo de tais fases em sistemas bidimensionais é através de modelos Hamiltonianos exatamente solúveis de rede como os modelos duplos quânticos e modelos de String-Nets. O primeiro pode ser entendido como a formulação Hamiltoniana de teorias de gauge na rede e, desta maneira, é definido por um group de gauge finito. Entretanto, pouco é conhecido a respeito de fases topológicas em sistemas tridimensionais. Motivado por isso nós desenvolvemos uma nova classe de modelos tridimensionais exatamente solúveis que vai alem de modelos duplos quânticos pelo uso de módulos cruzados finitos no lugar de grupos de gauge. Esta abordagem se baseia numa implementação em redes de teoria de 2-gauge para obter modelos com uma estrutura topológica mais rica. Nós construímos o modelos Hamiltoniano explicitamente e fornecemos uma demonstração rigorosa de que a degenerescência de estado fundamental é um invariante topológico e que os estados fundamentais só podem ser caracterizados por parâmetros de ordem não locais.

L'objectif de cette thèse est de développer une théorie de types pour le λ-calcul linéaire-algébrique, une extension du λ-calcul motivé par l'informatique quantique. Cette extension algébrique comprend tous les termes du λ-calcul plus leurs combinaisons linéaires, donc si t et r sont des termes, α.t+β.r est aussi un terme, avec α et β des scalaires pris dans un anneau. L'idée principale et le défi de cette thèse était d'introduire un système de types où les types, de la même façon que les termes, constituent un espace vectoriel, permettant la mise en évidence de la structure de la forme normale d'un terme. Cette thèse présente le système Lineal , ainsi que trois systèmes intermédiaires, également intéressants en eux-même : Scalar, Additive et λCA, chacun avec leurs preuves de préservation de type et de normalisation forte
The objective of this thesis is to develop a type theory for the linear-algebraic λ-calculus, an extension of λ-calculus motivated by quantum computing. This algebraic extension encompass all the terms of λ-calculus together with their linear combinations, so if t and r are two terms, so is α.t + β.r, with α and β being scalars from a given ring. The key idea and challenge of this thesis was to introduce a type system where the types, in the same way as the terms, form a vectorial space, providing the information about the structure of the normal form of the terms. This thesis presents the system Lineal, and also three intermediate systems, however interesting by themselves: Scalar, Additive and λCA, all of them with their subject reduction and strong normalisation proofs

Resumo Neste trabalho apresentamos, em detalhes, a construção de uma teoria quântica dos campos topológica (TQCT). Podemos definir uma TQCT como um funtor simétrico monoidal da categoria dos cobordismos para a categoria dos espaços vetoriais. Em duas dimensões podemos encontrar uma descrição completa da categoria dos cobordismos e classificar todas as TQCT\'s. Em três dimensões é possível estender alguns invariantes para 3-variedades e construir uma TQCT 3D. Nossa construção é baseada no invariante para 3-variedades de Kuperberg, o qual envolve diagramas de Heegaard e álgebras de Hopf. Começamos com a apresentação do invariante de Kuperberg definido para toda variedade 3D compacta, orientável e sem bordo. Para cada álgebra de Hopf de dimensão finita constrói-se um invariante. Por fim, apresentamos a TQCT associada com o invariante de Kuperberg. Isto é feito usando-se o fato de que o invariante de Kuperberg é definido como uma soma de pesos locais tal qual uma função de partição. A TQCT decorre dos operadores advindos de variedades com bordo.
Abstract In this work we present in detail a construction of a topological quantum field theory (TQFT). We can define a TQFT as a symmetric monoidal functor from cobordism categories to category of vector spaces. In two dimension, we can give a complete description of cobordism categories and classify all TQFT\'s. In three dimension it is possible to extend some specific 3-manifold invariants and to construct a TQFT 3D. Our construction is based on the Kuperberg 3-manifold invariant which involves Heegaard diagrams and Hopf algebras. We start with the presentation of the Kuperberg invariant defined for every orientable compact 3-manifold without boundary. For each finite-dimensional Hopf algebra we can construct a invariant. Finally we presente the TQFT associated with the Kuperberg invariant. This is made using the fact that the Kuperberg invariant is defined like a sum of local weights in the same way as a partition function. The TQFT is constructed from the operators given by manifolds with boundary.

Je souhaite réaliser un modèle théorique optimal pour les protocoles de partage de secret quantique basé sur l'utilisation des états graphes. Le paramètre représentatif d'un partage de secret à seuil est, entre autres la taille du plus grand ensemble de joueurs qui ne peut pas accéder au secret. Je souhaite donc trouver un famille de protocoles pour laquelle ce paramètre est le plus petit possible. J'étudie également les liens entre les protocoles de partage de secret quantique et des familles de courbes en géométrie algébrique
I want to realize an optimal theoretical model for quantum secret sharing protocols based on graph states. The main parameter of a threshold quantum secret sharing scheme is the size of the largest set of players that can not access the secret. Thus, my goal is to find a collection of protocols for which the value of this parameter is the smallest possible. I also study the links between quantum secret sharing protocols and families of curves in algebraic geometry

Wir geben eine rigorose Konstruktion von analytisch-regularisierten Feynman-Integralen im D-dimensionalen Minkowski-Raum als meromorphe Distributionen in den externen Impulsen, sowohl in der Impuls- als auch in der parametrischen Darstellung. Wir zeigen, dass ihre Pole durch die üblichen Power-counting Formeln gegeben sind, und dass ihr singulärer Träger in mikrolokalen Verallgemeinerungen der (+alpha)-Landauflächen enthalten ist. Als weitere Anwendungen geben wir eine Konstruktion von dimensional regularisierten Integralen im Minkowski-Raum und beweisen Diskontinuitätsformeln für parametrische Amplituden.
We give a rigorous construction of analytically regularized Feynman integrals in D-dimensional Minkowski space as meromorphic distributions in the external momenta, both in the momentum and parametric representation. We show that their pole structure is given by the usual power-counting formula and that their singular support is contained in a microlocal generalization of the alpha-Landau surfaces. As further applications, we give a construction of dimensionally regularized integrals in Minkowski space and prove discontinuity formula for parametric amplitudes.

In this work the extension of the criterion for local thermal equilibrium by Buchholz, Ojima and Roos to curved spacetime as introduced by Schlemmer is investigated. Several problems are identified and especially the instability under time evolution which was already observed by Schlemmer is inspected. An alternative approach to local thermal equilibrium in quantum field theories on curved spacetimes is presented and discussed. In the following the dynamic system of the linear field and matter perturbations in the generic model of inflation is studied in the view of ambiguity of quantisation. In the last part the compatibility of the temperature fluctuations of the cosmic microwave background radiation with local thermal equilibrium is investigated
In dieser Arbeit wird die von Schlemmer eingeführte Erweiterung des Kriteriums für lokales thermisches Gleichgewicht in Quantenfeldtheorien von Buchholz, Ojima und Roos auf gekrümmte Raumzeiten untersucht. Dabei werden verschiedene Probleme identifiziert und insbesondere die bereits von Schlemmer gezeigte Instabilität unter Zeitentwicklung untersucht. Es wird eine alternative Herangehensweise an lokales thermisches Gleichgewicht in Quantenfeldtheorien auf gekrümmten Raumzeiten vorgestellt und deren Probleme diskutiert. Es wird dann eine Untersuchung des dynamischen Systems der linearen Feld- und Metrikstörungen im üblichen Inflationsmodell mit Blick auf Uneindeutigkeit der Quantisierung durchgeführt. Zuletzt werden die Temperaturfluktuationen der kosmischen Hintergrundstrahlung auf Kompatibilität mit lokalem thermalem Gleichgewicht überprüft

We investigate an alternative formulation of quantum field theory that elevates the Wilson- Zimmermann operator product expansion (OPE) to an axiom of the theory. We observe that the information contained in the OPE coefficients may be straightforwardly repackaged into "vertex operators". This way of formulating quantum field theory has quite obvious similarities to the theory of vertex algebras. As examples of this framework, we discuss the free massless boson in D dimensions and the massless Thirring model. We set up perturbation theory for vertex algebras. We discuss a general theory of perturbations of vertex algebras, which is similar to the Hochschild cohomology describing the deformation theory of ordinary algebras. We pass on to a more explicit discussion by looking at perturbations of the free massless boson in D dimensions. The perturbations we consider correspond to some interaction Lagrangian P(