The work at hand presents a novel approach to transport in closed quantum systems. To this end a method is introduced which is essentially based on projection operator techniques, in particular on the time-convolutionless (TCL) technique. The projection onto local densities of quantities such as energy, magnetization, particles, etc. yields the reduced dynamics of the respective quantities in terms of a systematic perturbation expansion. Especially, the lowest order contribution of this expansion is used as a strategy for the analysis of transport in "modular" quantum systems. The term modular basically corresponds to (quasi-) one-dimensional structures consisting of identical or at least similar many-level subunits. Modular quantum systems are demonstrated to represent many physical situations and several examples are given. In the context of these quantum systems lowest order TCL is shown as an efficient tool which also allows to investigate the dependence of transport on the considered length scale. In addition an estimation for the validity range of lowest order TCL is derived. As a first application a "design" model is considered for which a complete characterization of all available transport types as well as the transitions to each other is possible. For this model the relationship to quantum chaos and the validity of the Kubo formula is further discussed. As an example for a "real" system the Anderson model is finally analyzed. The results are partially verified by the numerical solution of the full time-dependent Schroedinger equation which is obtained by exact diagonalization or approximative integrators.

Classical partial transport barriers govern both classical and quantum dynamics of generic Hamiltonian systems. Chaotic eigenstates of quantum systems are known to localize on either side of a partial barrier if the flux connecting the two sides is not resolved by means of Heisenberg's uncertainty. Surprisingly, in open systems, in which orbits can escape, chaotic resonance states exhibit such a localization even if the flux across the partial barrier is quantum mechanically resolved. We explain this using the concept of conditionally invariant measures by introducing a new quantum mechanically relevant class of such fractal measures. We numerically find quantum-to-classical correspondence for localization transitions depending on the openness of the system and on the decay rate of resonance states. Moreover, we show that the number of long-lived chaotic resonance states that localize on one particular side of the partial barrier is described by an individual fractal Weyl law. For a generic phase space, this implies a hierarchy of fractal Weyl laws, one for each region of the hierarchical decomposition of phase space.

Gegenstand dieser Arbeit sind Wellenphänomene in mesoskopischen Ringstrukturen. In Teil I der Arbeit befassen wir uns mit spinabhängigem Transport von Elektronen in effektiv eindimensionalen Ringen in Gegenwart inhomogener Magnetfelder. Wir benutzen die exakten Lösungen der Schrödinger-Gleichung im allgemeinen nicht-adiabatischen Fall in einem Transfer-Matrix-Formalismus und untersuchen Auswirkungen von geometrischen Phasen auf den Magnetwiderstand. Für den Spezialfall eines Magnetfeldes in der Ringebene sagen wir einen interessanten Spin-Flip-Effekt vorher, der die Steuerung der Polarisationsrichtung von Elektronen über einen externen Aharonov-Bohm-Fluß erlaubt. Optische mesoskopische Systeme sind Thema von Teil II dieser Arbeit. Wir betrachten zweidimensionale annulare Strukturen, charakterisiert durch unterschiedliche Brechungsindizes, sowohl im klassischen Bild der geometrischen Optik als auch mit Wellenmethoden auf der Grundlage der Maxwellschen Gleichungen. Insbesondere diskutieren wir erstmals eine Streumatrixbeschreibung optischer Mikroresonatoren und wenden sie auf das dielektrische annulare Billard an. Ein Vergleich der Ergebnisse des Wellen- und Strahlenbildes liefert eine gute Übereinstimmung, jedoch sind im Grenzfall großer Wellenlängen von der Ordnung der Systemabmessungen Korrekturen zum Strahlenbild nötig. Wir zeigen am Beispiel von Fresnel-Gesetzen für gekrümmte Oberflächen erstmals, daß der Goos-Hänchen-Effekt diese Korrekturen quantitativ erfaßt. Ausgehend von der Wellenbeschreibung leiten wir neue analytische Formeln für verallgemeinerte Fresnel-Gesetze für beide möglichen Polarisationsrichtungen ab. Die Anwendung des Strahlenbildes erlaubt eine schlüssige Interpretation eines Experiments mit einer quadrupolaren Glasfaser, außerdem schlagen wir Strahlenkonzepte als Grundlage der Konstruktion von Mikrolasern mit maßgeschneiderten Charakteristika vor. / In this work we investigate wave phenomena in mesoscopic systems using different theoretical approaches. In Part I, we focus on effectively one-dimensional electronic ring structures and address the phenomenon of geometric phases in spin-dependent electronic transport in the presence of non-uniform magnetic fields. In the general non-adiabatic case, exact solutions of the Schrödinger equation are used in a transfer matrix formalism to compute the transmission probability through the ring. In the magneto-conductance we identify clear signatures of interference effects due to geometric phases, for example in rings where the non-uniform field is created by a central micromagnet. For the special case of an in-plane magnetic field we predict an interesting spin-flip effect that allows one to control the spin polarization of electrons by applying an external Aharonov-Bohm flux. Optical mesoscopic systems are the subject of Part II. We consider two-dimensional annular structures characterized by different refractive indices, and apply classical methods from geometric optics as well as wave concepts based on Maxwell's equations. For the first time, an S-matrix approach is successfully employed in the description of resonances in optical microresonators; in particular we propose the dielectric annular billiard as an attractive model system. Comparing ray and wave pictures, we find general agreement, except for large wavelengths of the order of the system size, where corrections to the ray model are necessary. The Goos-Hänchen effect as an extension of the ray picture is shown to quantitatively account for wave modifications of Fresnel's laws due to curved interfaces. We derive novel analytical expressions for the corrected Fresnel formulas for both polarizations of light. Motivated by the successful ray description, we give a conclusive interpretation of a recent filter experiment on a quadrupolar glass fibre, and suggest novel concepts for microresonator-based lasers.

No description available.

Classical partial transport barriers govern both classical and quantum dynamics of generic Hamiltonian systems. Chaotic eigenstates of quantum systems are known to localize on either side of a partial barrier if the flux connecting the two sides is not resolved by means of Heisenberg's uncertainty. Surprisingly, in open systems, in which orbits can escape, chaotic resonance states exhibit such a localization even if the flux across the partial barrier is quantum mechanically resolved. We explain this using the concept of conditionally invariant measures by introducing a new quantum mechanically relevant class of such fractal measures. We numerically find quantum-to-classical correspondence for localization transitions depending on the openness of the system and on the decay rate of resonance states. Moreover, we show that the number of long-lived chaotic resonance states that localize on one particular side of the partial barrier is described by an individual fractal Weyl law. For a generic phase space, this implies a hierarchy of fractal Weyl laws, one for each region of the hierarchical decomposition of phase space.

Für das Verständnis einer Vielzahl von Problemen von der Himmelsmechanik bis hin zur Beschreibung von Molekülen spielen Systeme mit mehr als zwei Freiheitsgraden eine entscheidende Rolle. Aufgrund der Dimensionalität gestaltet sich ein Verständnis dieser Systeme jedoch deutlich schwieriger als bei Systemen mit zwei oder weniger Freiheitsgraden. Die vorliegende Arbeit soll zum besseren Verständnis der klassischen und quantenmechanischen Eigenschaften getriebener Systeme mit zwei Freiheitsgraden beitragen. Hierzu werden dreidimensionale Schnitte durch den Phasenraum von 4D Abbildungen betrachtet. Anhand dreier Beispiele, deren Phasenräume zunehmend kompliziert sind, werden diese 3D Schnitte vorgestellt und untersucht. In einer sich anschließenden quantenmechanischen Untersuchung gehen wir auf zwei wichtige Aspekte ein. Zum einen untersuchen wir die quantenmechanischen Signaturen des klassischen "Arnold Webs". Es wird darauf eingegangen, wie die Quantenmechanik dieses Netz im semiklassischen Limes auflösen kann. Darüberhinaus widmen wir uns dem wichtigen Aspekt quantenmechanischer Kopplungen klassisch getrennter Phasenraumgebiete anhand der Untersuchung dynamischer Tunnelraten. Für diese wenden wir sowohl den in der Literatur bekannten "fictitious integrable system approach" als auch die Theorie des resonanz-unterstützen Tunnelns auf 4D Abbildungen an.:Contents ..... v 1 Introduction ..... 1 2 2D mappings ..... 5 2.1 Hamiltonian systems with 1.5 degrees of freedom ..... 5 2.2 The 2D standard map ..... 6 3 Classical dynamics of higher dimensional systems ..... 11 3.1 Coupled standard maps as paradigmatic example ..... 12 Stability of fixed points in 4D maps ..... 13 Center manifolds of elliptic degrees of freedom ..... 13 3.2 Near-integrable systems ..... 15 3.2.1 Analytical description of multidimensional, near-integrable systems ..... 15 Resonance structures in 4D maps ..... 16 3.2.2 Pendulum approximation ..... 18 3.2.3 Normal forms ..... 24 3.2.4 Arnold diffusion and Arnold web ..... 24 3.3 Numerical tools for the analysis of regular and chaotic motion ..... 26 3.3.1 Frequency analysis ..... 26 Aim of the frequency analysis ..... 26 Realizations of the frequency analysis ..... 27 Wavelet transforms ..... 30 3.3.2 Fast Lyapunov indicator ..... 31 3.3.3 Phase-space sections ..... 33 Skew phase-space sections containing invariant eigenspaces ..... 34 3.4 Systems with regular dynamics and a large chaotic sea ..... 35 3.4.1 Designed maps: Map with linear regular region, P_llu ..... 36 Phase space of the designed map with linear regular region ..... 38 FLI values ..... 41 Estimating the size of the regular region ..... 43 3.4.2 Designed maps: Islands with resonances, P_nnc ..... 46 Frequency analysis ..... 46 FLI values and volume of the regular and stochastic region ..... 50 Frequency analysis for rank-2 resonance ..... 52 Phase-space sections at different positions p_1 and p_2 ..... 53 Using color to provide the 4-th coordinate ..... 53 Skew phase-space sections containing invariant eigenspaces ..... 57 Arnold diffusion ..... 58 3.4.3 Generic maps: Coupled standard maps, P_csm ..... 63 FLI values and volume of the regular and stochastic region ..... 63 Analysis of fundamental frequencies ..... 66 Skew phase-space sections containing invariant eigenspaces ..... 69 4 Quantum Mechanics ..... 75 4.1 Quantization of Classical Maps ..... 77 4.2 Eigenstates of the time evolution operator U ..... 79 4.2.1 Eigenstates of P_llu ..... 80 4.2.2 Eigenstates of P_nnc ..... 84 4.2.3 Eigenstates of P_csm ..... 87 4.3 Quantum signatures of the stochastic layer ..... 89 4.3.1 Eigenstates resolving the stochastic layer ..... 90 4.3.2 Wave-packet dynamics into the stochastic layer ..... 94 4.4 Dynamical tunneling rates ..... 98 4.4.1 Numerical calculation of dynamical tunneling rates ..... 99 4.4.2 Direct regular-to-chaotic tunneling rates gamma^d of P_llu ..... 101 4.4.3 Prediction of gamma^d using the fictitious integrable system approach ..... 103 4.4.4 Dynamical tunneling rates of P_nnc ..... 105 4.4.5 Interlude: Theory of resonance assisted tunneling (RAT) ..... 106 4.4.6 Prediction of tunneling rates for P_nnc, RAT ..... 111 Selection rules from nonlinear resonances ..... 111 Energy denominators ..... 114 Estimating the parameters of the pendulum approximation from phase-space properties ..... 116 Prediction ..... 118 4.4.7 Dynamical tunneling rates of P_csm ..... 120 5 Summary and outlook ..... 123 Appendix ..... 125 A Potential of the designed map ..... 125 B Quantum-number assignment-algorithm ..... 128 C Alternate paths due to alternate resonances in the description of RAT ..... 131 D Alternate resonances in the description of RAT leading to different tunneling rates ..... 133 E Tunneling rates of map with nonlinear resonances but uncoupled regular region ..... 133 F Interpolation of quasienergies ..... 135 G 2D Poincar'e map for the pendulum approximation ..... 137 H RAT prediction broken down to single paths ..... 139 I Linearization of the pendulum approximation ..... 140 J Iterative diagonalization schemes for the semiclassical limit ..... 143 Inverse iteration ..... 143 Arnoldi method ..... 144 Lanczos algorithm ..... 144 List of figures ..... 148 Bibliography ..... 163 / Systems with more than two degrees of freedom are of fundamental importance for the understanding of problems ranging from celestial mechanics to molecules. Due to the dimensionality the classical phase-space structure of such systems is more difficult to understand than for systems with two or fewer degrees of freedom. This thesis aims for a better insight into the classical as well as the quantum mechanics of 4D mappings representing driven systems with two degrees of freedom. In order to analyze such systems, we introduce 3D sections through the 4D phase space which reveal the regular and chaotic structures. We introduce these concepts by means of three example mappings of increasing complexity. After a classical analysis the systems are investigated quantum mechanically. We focus especially on two important aspects: First, we address quantum mechanical consequences of the classical Arnold web and demonstrate how quantum mechanics can resolve this web in the semiclassical limit. Second, we investigate the quantum mechanical tunneling couplings between regular and chaotic regions in phase space. We determine regular-to-chaotic tunneling rates numerically and extend the fictitious integrable system approach to higher dimensions for their prediction. Finally, we study resonance-assisted tunneling in 4D maps.:Contents ..... v 1 Introduction ..... 1 2 2D mappings ..... 5 2.1 Hamiltonian systems with 1.5 degrees of freedom ..... 5 2.2 The 2D standard map ..... 6 3 Classical dynamics of higher dimensional systems ..... 11 3.1 Coupled standard maps as paradigmatic example ..... 12 Stability of fixed points in 4D maps ..... 13 Center manifolds of elliptic degrees of freedom ..... 13 3.2 Near-integrable systems ..... 15 3.2.1 Analytical description of multidimensional, near-integrable systems ..... 15 Resonance structures in 4D maps ..... 16 3.2.2 Pendulum approximation ..... 18 3.2.3 Normal forms ..... 24 3.2.4 Arnold diffusion and Arnold web ..... 24 3.3 Numerical tools for the analysis of regular and chaotic motion ..... 26 3.3.1 Frequency analysis ..... 26 Aim of the frequency analysis ..... 26 Realizations of the frequency analysis ..... 27 Wavelet transforms ..... 30 3.3.2 Fast Lyapunov indicator ..... 31 3.3.3 Phase-space sections ..... 33 Skew phase-space sections containing invariant eigenspaces ..... 34 3.4 Systems with regular dynamics and a large chaotic sea ..... 35 3.4.1 Designed maps: Map with linear regular region, P_llu ..... 36 Phase space of the designed map with linear regular region ..... 38 FLI values ..... 41 Estimating the size of the regular region ..... 43 3.4.2 Designed maps: Islands with resonances, P_nnc ..... 46 Frequency analysis ..... 46 FLI values and volume of the regular and stochastic region ..... 50 Frequency analysis for rank-2 resonance ..... 52 Phase-space sections at different positions p_1 and p_2 ..... 53 Using color to provide the 4-th coordinate ..... 53 Skew phase-space sections containing invariant eigenspaces ..... 57 Arnold diffusion ..... 58 3.4.3 Generic maps: Coupled standard maps, P_csm ..... 63 FLI values and volume of the regular and stochastic region ..... 63 Analysis of fundamental frequencies ..... 66 Skew phase-space sections containing invariant eigenspaces ..... 69 4 Quantum Mechanics ..... 75 4.1 Quantization of Classical Maps ..... 77 4.2 Eigenstates of the time evolution operator U ..... 79 4.2.1 Eigenstates of P_llu ..... 80 4.2.2 Eigenstates of P_nnc ..... 84 4.2.3 Eigenstates of P_csm ..... 87 4.3 Quantum signatures of the stochastic layer ..... 89 4.3.1 Eigenstates resolving the stochastic layer ..... 90 4.3.2 Wave-packet dynamics into the stochastic layer ..... 94 4.4 Dynamical tunneling rates ..... 98 4.4.1 Numerical calculation of dynamical tunneling rates ..... 99 4.4.2 Direct regular-to-chaotic tunneling rates gamma^d of P_llu ..... 101 4.4.3 Prediction of gamma^d using the fictitious integrable system approach ..... 103 4.4.4 Dynamical tunneling rates of P_nnc ..... 105 4.4.5 Interlude: Theory of resonance assisted tunneling (RAT) ..... 106 4.4.6 Prediction of tunneling rates for P_nnc, RAT ..... 111 Selection rules from nonlinear resonances ..... 111 Energy denominators ..... 114 Estimating the parameters of the pendulum approximation from phase-space properties ..... 116 Prediction ..... 118 4.4.7 Dynamical tunneling rates of P_csm ..... 120 5 Summary and outlook ..... 123 Appendix ..... 125 A Potential of the designed map ..... 125 B Quantum-number assignment-algorithm ..... 128 C Alternate paths due to alternate resonances in the description of RAT ..... 131 D Alternate resonances in the description of RAT leading to different tunneling rates ..... 133 E Tunneling rates of map with nonlinear resonances but uncoupled regular region ..... 133 F Interpolation of quasienergies ..... 135 G 2D Poincar'e map for the pendulum approximation ..... 137 H RAT prediction broken down to single paths ..... 139 I Linearization of the pendulum approximation ..... 140 J Iterative diagonalization schemes for the semiclassical limit ..... 143 Inverse iteration ..... 143 Arnoldi method ..... 144 Lanczos algorithm ..... 144 List of figures ..... 148 Bibliography ..... 163

Mon travail de thèse est consacré à l’étude de quelques aspects de la physique quantique des systèmes quantiques à N corps en interaction. Il est orienté vers l’étude des chaînes de spins quantiques. Je me suis intéressé à plusieurs questions relatives aux chaînes de spins quantiques, du point de vue numérique et analytique à la fois. J'aborde en particulier les questions relatives à la structure des fonctions d'onde, la forme de la densité d'états et les propriétés spectrales des Hamiltoniens de chaînes de spins. Dans un premier temps, je présenterais très rapidement les techniques numériques de base pour le calcul des vecteurs et valeurs propres des Hamiltonien de chaînes de spins. Les densités d’états des modèles quantiques constituent des quantités importantes et très simples qui permettent de caractériser les propriétés spectrales des systèmes avec un grand nombre de degrés de liberté. Alors que dans la limite thermodynamique, les densités d'états de la plupart des modèles intégrables sont bien décrites par une loi gaussienne, dans certaines limites de couplage de la chaîne de spins au champ magnétique et pour un nombre de spins N fini sur la chaîne, on observe l’apparition de pics dans la densité d’états. Je montrerais que la connaissance des deux premiers moments du Hamiltonien dans le sous-espace dégénéré associé à chaque pics donne une bonne approximation de la densité d’états. Dans un deuxième temps je m'intéresserais aux propriétés spectrales des Hamiltoniens de chaînes de spins quantiques. L’un des principal résultats sur la statistique spectrale des systèmes quantiques concerne le comportement universel des fluctuations des mesures telles que l’espacement entre valeurs propres consécutives. Ces fluctuations sont bien décrites par la théorie des matrices aléatoires mais la comparaison avec les prédictions de cette théorie nécessite généralement une opération sur le spectre du Hamiltonien appelée unfolding. Dans les problèmes quantiques de N corps, la taille de l’espace de Hilbert croît généralement exponentiellement avec le nombre de particules, entraînant un manque de données pour pouvoir faire une statistique. Ces limitations ont amené l’introduction d’une nouvelle mesure se passant de la procédure d’unfolding basée sur le rapport d’espacements successifs plutôt que les espacements. En suivant l’idée du “surmise” de Wigner pour le calcul de la distribution de l’espacement, je montre comment calculer une approximation de la distribution du rapport d’espacements dans les trois ensembles gaussiens invariants en faisant le calcul pour des matrices 3x3. Les résultats obtenus pour les différents ensembles de matrices aléatoires se sont révélés être en excellent accord avec les résultats numériques. Enfin je m’intéresserais à la structure des fonctions d’ondes fondamentales des modèles de chaînes de spins quantiques. Les fonctions d’onde constituent, avec le spectre en énergie, les objets fondamentaux des systèmes quantiques : leur structure est assez compliquée et n’est pas très bien comprise pour la plupart des systèmes à N corps. En raison de la croissance exponentielle de la taille de l’espace de Hilbert avec le nombre de particules, l’étude des vecteurs propres est une tâche très difficile, non seulement du point de vue analytique mais aussi du point de vue numérique. Je démontrerais en particulier que l’état fondamental de tous les modèles que nous avons étudiés est multifractal avec en général une dimension fractale non triviale. / My thesis is devoted to the study of some aspects of many body quantum interacting systems. In particular we focus on quantum spin chains. I have studied several aspects of quantum spin chains, from both numerical and analytical perspectives. I addressed especially questions related to the structure of eigenfunctions, the level densities and the spectral properties of spin chain Hamiltonians. In this thesis, I first present the basic numerical techniques used for the computation of eigenvalues and eigenvectors of spin chain Hamiltonians. Level densities of quantum models are important and simple quantities that allow to characterize spectral properties of systems with large number of degrees of freedom. It is well known that the level densities of most integrable models tend to the Gaussian in the thermodynamic limit. However, it appears that in certain limits of coupling of the spin chain to the magnetic field and for finite number of spins on the chain, one observes peaks in the level density. I will show that the knowledge of the first two moments of the Hamiltonian in the degenerate subspace associated with each peak give a good approximation to the level density. Next, I study the statistical properties of the eigenvalues of spin chain Hamiltonians. One of the main achievements in the study of the spectral statistics of quantum complex systems concerns the universal behaviour of the fluctuation of measure such as the distribution of spacing between two consecutive eigenvalues. These fluctuations are very well described by the theory of random matrices but the comparison with the theoretical prediction generally requires a transformation of the spectrum of the Hamiltonian called the unfolding procedure. For many-body quantum systems, the size of the Hilbert space generally grows exponentially with the number of particles leading to a lack of data to make a proper statistical study. These constraints have led to the introduction of a new measure free of the unfolding procedure and based on the ratio of consecutive level spacings rather than the spacings themselves. This measure is independant of the local level density. By following the Wigner surmise for the computation of the level spacing distribution, I obtained approximation for the distribution of the ratio of consecutive level spacings by analyzing random 3x3 matrices for the three canonical ensembles. The prediction are compared with numerical results showing excellent agreement. Finally, I investigate eigenfunction statistics of some canonical spin-chain Hamiltonians. Eigenfunctions together with the energy spectrum are the fundamental objects of quantum systems: their structure is quite complicated and not well understood. Due to the exponential growth of the size of the Hilbert space, the study of eigenfunctions is a very difficult task from both analytical and numerical points of view. I demonstrate that the groundstate eigenfunctions of all canonical models of spin chain are multifractal, by computing numerically the Rényi entropy and extrapolating it to obtain the multifractal dimensions.

科学研究費補助金 研究種目:一般研究(C) 課題番号:01540396 研究代表者:高塚 和夫 研究期間:1989-1990年度

Η Κβαντική Πληροφορία είναι μια ιδιότητα των κβαντικών συστημάτων που σχετίζεται με την κβαντομηχανική επαλληλία και την συσχέτιση των συστημάτων σε Ενδιαπλοκή. Λόγω της αλληλεπίδρασης με το κλασικό περιβάλλον η Ενδιαπλοκή χάνεται ταχύτατα με συνέπεια να περιορίζεται δραματικά η πρακτική της χρησιμότητα. Πραγματοποιήθηκε διερεύνηση διαφόρων διατάξεων κβαντικών συστημάτων είτε σε αλληλεπίδραση με άλλα συστήματα είτε μεμονωμένων, αλλά με την συνθήκη το αντίστοιχο κλασικό μη γραμμικό σύστημα να είναι χαοτικό ή ολοκληρώσιμο. Ερευνητικά αποτελέσματα: 1. Ο ρυθμός μεταβολής της Ενδιαπλοκής με την μεταβολή μιας παραμέτρου σύζευξης φράσσεται από την καμπυλότητα των ενεργειακών επιπέδων. Το αποτέλεσμα έχει γενική ισχύ, διότι βασίζεται σε γενικές ιδιότητες των τυχαίων πινάκων που κωδικοποιούν την χαοτική ή την κανονική συμπεριφορά των συστημάτων,2. Ο ρόλος της λεπτομερούς δομής του κλασικού χώρου των φάσεων. Ερευνήθηκε η εξάρτηση της Ενδιαπλοκής από την τιμή παραμέτρου σύζευξης σε σχέση με το διάγραμμα διακλαδώσεων του Βηματικού Στρόβου, καθώς και σε σχέση με την ύπαρξη Κβαντικών Ουλών, 3. Η μη γραμμικότητα συστημάτων σε συνδυασμό με την παρουσία εξωτερικών πεδίων μπορεί να οδηγήσει σε βέλτιστες τιμές ορισμένων παραμέτρων που ευνοούν την δημιουργία Ενδιαπλοκής θερμικά. Μελετήθηκε κατάλληλο μοντέλο, 4. Κβαντικός δίαυλος βρίσκεται σε αλληλεπίδραση με το τοπικό περιβάλλον. Οι ιδιότητες του τοπικού περιβάλλοντος (χάος ή κανονικότητα) μπορούν να επηρεάσουν τις δυνατότητες του διαύλου, 5. Ένα ανοικτό κβαντικό σύστημα αλληλεπιδρά με τοπικό κβαντικό περιβάλλον λίγων βαθμών ελευθερίας, καθώς και με ένα ολικό Μαρκοβιανό περιβάλλον και τελικά προκαλείται απώλεια κβαντικής συνάφειας. Διερευνήθηκε, αριθμητικά, ο τρόπος με τον οποίο, οι κλασικές ιδιότητες του τοπικού περιβάλλοντος επηρεάζουν τον ρυθμό απώλειας συνάφειας του συστήματος. / Quantum Information is a particular property of quantum systems which is associated with the quantum mechanical superposition principle and the correlation of the systems in the entangled states. Due to the interaction with the classical environment, this basic property of Entanglement is lost very quickly, with the result, its practical usefulness to be dramatically reduced. The present Thesis is concerned with the study of various arrangements of quantum systems either in interaction with other systems, or isolated, but with the condition that the corresponding classical non linear system is chaotic or integrable. Results presented in the thesis: 1. Τhe rate of change of entanglement of a quantum system with respect to the change of an interaction parameter is bounded by the curvature of the energy levels. This result has a general validity, because it is based on general properties of random matrices, which may encode the regular or chaotic behavior of physical systems, 2. The role of the detailed structure of the classical phase space. The dependence of entanglement on the position of a parameter of interaction in connection to the bifurcation diagram in the model of quantum kicked top was studied. An analysis was carried out for a possible impact of the existence of scars on the behavior of entanglement, 3. The non linearity of systems combined with the presence of external fields may lead to optimal values of certain parameters which favor the thermal creation of entanglement. A particular model was studied in which this behavior is observed, 4. A quantum channel is in interaction with its local environment. The question posed, is whether the properties of the local environment (chaos or integrability) may influence the capabilities of the channel, 5. An open quantum system interacts with a quantum local environment, which in general has few degrees of freedom, and a global infinite one. It was numerically investigated how the classical properties of the local environment Influence the decoherence rate of the system.

No description available.