Microscopic Theory of Semiconductor Laser Material Systems

This Thesis provides an overview on microscopic theories for the description of semiconductor laser material systems. Therefore, it gives an overview about three theoretical models used for the description of different properties of semiconductors. First, an extension to the original Jaynes-Cummings model (JCM) is introduced. It is later used for the investigation of quantum dots hosting multiple electronic levels placed inside a microcavity. Advancing to a different approach, second, the semiconductor Bloch equations (SBEs) are discussed together with the system Hamiltonian and the resulting measurable macroscopic quantities, i.e. absorption and refractive index change. As third model, the semiconductor luminescence equations (SLEs) are presented to calculate photoluminescence (PL) spectra where the quantized properties of the light are taken into account. Last, the evaluation of photomodulated reflectance (PR) spectroscopy based on the SBEs is presented. Additionally, it reviews and extends all investigations made in the context of type-II band-aligned "W"-systems. Besides the content presented in these publications, it starts with a general introduction of type-II and especially "W"-aligned multiple quantum-well heterostructures (MQWHs). They are compared to traditional type-I systems in terms of temperature and charge carrier density dependence. The differences are studied based on the SBEs. Subsequently, as part of the closed-loop process, an experiment--theory comparison for PL measurements of epitaxially grown "W"-MQWHs is presented. Based on the nominal parameters, i.e. quantum-well thickness and concentration, the material gain of this structure is computed. Excitonic transitions and their spatial recombination path are investigated to identify their type-II character. Subsequently, a systematic analysis of the "W"-VECSEL sample is carried out. Here, charge carrier dependent reflection spectra are presented to confirm the experimentally determined lasing wavelength. The investigation of the VECSEL concludes with the determination of detuning and modal gain of the sample. In addition, optimization capabilities are discussed by the means of the carrier confinement due to graded interfaces and different barrier materials. As a last point, material compositions suitable to increase the emission wavelength to 1300 nm are suggested based on calculations. Unexpected oscillations in the emission of optically pumped semiconductor quantum-dot microcavities are discussed and analyzed. The usual linear slope of the I/O characteristics of this setup is modified. To figure out the origin of the nonlinearities, a systematic theoretical investigation is applied which identifies them as genuine quantum-memory effect. They are found to be directly addressable by utilizing the quantum-optical fluctuations of the exciting light field

Coherent Effects in Dispersive Quantum Dynamics

This doctoral dissertation is concerned with the study of quantum dynamics where finite dimensional systems (typically two-level `qubits') interact with or through a set of bosonic modes, in various different configurations. Our main focus is on identifying and investigating signatures of quantum coherence emerging between the qubits in such dynamical situations. We first present a toy model where two qubits are encoded in the single-excitation subspace of the global system and study the average fidelity of a controlled-Z (CZ) quantum gate mediated by the bosonic modes. Next, we turn to analytically intractable spin-boson like models, by adopting the Multi-configurational Ehrenfest (MCE) method. We apply MCE to the study of the Choi fidelity of a CZ gate between two distant qubits, mediated by sets of bosonic modes (including sets which represent discretization of bath's continua) under different coupling Hamiltonians. The testing of the MCE method is then pushed further by a comparative analysis with full variational approaches and adiabatic path integral techniques in a case of super-Ohmic spin-boson model. Finally, we determine a general error bound applicable to most approximated treatments of unitary quantum evolutions, and suitable to compare MCE with other numerical techniques for the study of spin-boson dynamics

On some nonlinear partial differential equations for classical and quantum many body systems

This thesis deals with problems arising in the study of nonlinear partial differential equations arising from many-body problems. It is divided into two parts: The first part concerns the derivation of a nonlinear diffusion equation from a microscopic stochastic process. We give a new method to show that in the hydrodynamic limit, the particle densities of a one-dimensional zero range process on a periodic lattice converge to the solution of a nonlinear diffusion equation. This method allows for the first time an explicit uniform-in-time bound on the rate of convergence in the hydrodynamic limit. We also discuss how to extend this method to the multi-dimensional case. Furthermore we present an argument, which seems to be new in the context of hydrodynamic limits, how to deduce the convergence of the microscopic entropy and Fisher information towards the corresponding macroscopic quantities from the validity of the hydrodynamic limit and the initial convergence of the entropy. The second part deals with problems arising in the analysis of nonlinear Schrödinger equations of Gross-Pitaevskii type. First, we consider the Cauchy problem for (energy-subcritical) nonlinear Schrödinger equations with sub-quadratic external potentials and an additional angular momentum rotation term. This equation is a well-known model for superfluid quantum gases in rotating traps. We prove global existence (in the energy space) for defocusing nonlinearities without any restriction on the rotation frequency, generalizing earlier results given in the literature. Moreover, we find that the rotation term has a considerable influence in proving finite time blow-up in the focusing case. Finally, a mathematical framework for optimal bilinear control of nonlinear Schrödinger equations arising in the description of Bose-Einstein condensates is presented. The obtained results generalize earlier efforts found in the literature in several aspects. In particular, the cost induced by the physical work load over the control process is taken into account rather then often used L^2- or H^1-norms for the cost of the control action. We prove well-posedness of the problem and existence of an optimal control. In addition, the first order optimality system is rigorously derived. Also a numerical solution method is proposed, which is based on a Newton type iteration, and used to solve several coherent quantum control problems

Theoretical study of weakly bound vibrational states of the sodium trimer : numerical methods; prospects for the formation of Na3 in an ultracold gas

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Coherent Manipulation of Rydberg Polaritons

This thesis contains a statistical analysis of the resonant transmission of photons through an ensemble of cold Rubidium 87 atoms \textit{in-vacuo}, where the resonant excited state is coupled to one or two highly-excited Rydberg states via optical and microwave fields. Transient emission with decay rates far below the excited state decay rate $\Gamma_e$ are observed. Analysis of the second-order auto-correlation statistic reveals Rydberg-mediated anti-bunching of transient photons, a signature of Rydberg blockade. The application of resonant microwave fields creates strong resonant interactions between Rydberg atoms. This presents a new, transient regime for the study of interaction-induced dephasing and blockade physics in cold atomic ensembles. A demonstration of a collective Rydberg qubit is presented. Quantum information is encoded into a superposition of Rydberg polariton states with a direct photonic interface suitable for applications in quantum networking. The coherence of Rydberg qubits is demonstrated through Ramsey interferometry. Sensitivity to AC and DC electric fields through differential Stark shifts of the qubit states is confirmed through a study of interferometric fringe shifts and dephasing. Controlled removal of atoms from the collective qubit under the action of a resonant scattering beam is shown to diminish readout fidelity but have little effect upon coherence due to the collective nature of the encoding. Theoretical models of the effect of photon scattering and electrical noise on the Rydberg qubit are confirmed experimentally. Ramsey fringe visibility is observed to scale with the fourth power of an applied noise field, matching a theoretical model

Modeling the Interactions of Anticancer Compounds with DNA and Lipid Membranes

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química. Fecha de Lectura: 15-07-202

The Second International Workshop on Squeezed States and Uncertainty Relations

This conference publication contains the proceedings of the Second International Workshop on Squeezed States and Uncertainty Relations held in Moscow, Russia, on 25-29 May 1992. The purpose of this workshop was to study possible applications of squeezed states of light. The Workshop brought together many active researchers in squeezed states of light and those who may find the concept of squeezed states useful in their research, particularly in understanding the uncertainty relations. It was found at this workshop that the squeezed state has a much broader implication than the two-photon coherent states in quantum optics, since the squeeze transformation is one of the most fundamental transformations in physics