Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1997. / Includes bibliographical references (p. 73-78). / by Jianchun Wang. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 129-133). / Measuring galaxy cluster total masses and the amount of dark matter substructure within galaxy cluster haloes is a fundamental probe of the ACDM model of structure formation, as well as the interactions between baryonic and non-baryonic matter. In this thesis I approach the topic of cluster mass structure in two ways. With a combination of optical imaging, spectroscopy, and X-ray observations I determine that the cluster RCS043938-2904.7, while apparently anomalous initially due to its high optical richness and low X-ray surface brightness, is in fact an association of structures along the line of sight. Accounting for this structure brings the observed cluster properties into agreement with known scaling relations. I also present a novel method for measuring weak gravitational lensing flexion to inform mass measurements on small scales. While previously published methods for measuring flexion focus on measuring derived properties of the lensed images, such as shapelet coefficients or surface brightness moments, my method fits a fully mass-sheet-invariant parametrized Analytic Image Model (AIM) to the each galaxy image. This simple parametric model traces the distortion of lensed image isophotes. I tested the AIM method using simulated data images with realistic noise and a variety of input image properties, and I show that it successfully reproduces the input lensing fields. I also apply the AIM method for flexion measurement to Hubble Space Telescope observations of Abell 1689, and detect mass structure in that cluster using only flexion measured with the AIM method. / by Benjamin Martin Cain. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Physics, 1997. / Includes bibliographical references (p. 79). / by Hongbin Yao. / M.S.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 89-92). / Understanding mechanisms of evolution under in real biological systems is a fundamental problem. Natural selection is one of the mechanisms that drive evolution. Due to the natural selection, phenotypes with higher fitness than the rest of the population increase in frequency and eventually dominate the populations. In real biological systems due to interactions between individuals within a population, it is not always obvious how natural selection manifests itself. Here we consider two types of interactions cooperative antibiotic break down and spatially expanding populations. In each of the cases predicting which phenotype is the most fit and the patterns corresponding to selection of this phenotype could be not straightforward. (1) Bacterial antibiotic resistance is typically quantified by the minimum inhibitory concentration (MIC), which is defined as the minimal concentration of antibiotic that inhibits bacterial growth starting from a standard cell density. However, when antibiotic resistance is mediated by degradation, the collective inactivation of antibiotic by the bacterial population causes the measured MIC to depend strongly on the initial cell density. Given this dependence, the relationship between MIC and bacterial fitness in such cases is not well-defined. Here we demonstrate that the resistance of a single, isolated cell-which we call the single cell MIC (scMIC)-provides a superior metric for quantifying antibiotic resistance. Unlike the MIC, we find that the scMIC predicts the direction of selection and also specifies the antibiotic concentration at which selection begins to favor new mutants. Understanding the cooperative nature of bacterial growth in antibiotics is therefore essential in predicting the evolution of antibiotic resistance. (2) During the expansions of natural populations, new phenotype can emerge. If it is fitter than the rest of the population, it will take over. However, the exact spatial patterns of this process are unknown. Here we show that for a wide class of models the fraction of the fit mutant should increase exponentially. We also observe this pattern experimentally by observing bacterial populations expanding in soft agar, as well as connection between the steepness of the exponent to the fitness difference. / by Tatiana Artemova. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2013. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (p. 201-210). / Usually when sufficient heating power is injected, tokamak plasma will make an abrupt transition into a state with improved confinement, known as the high-confinement mode, or H-mode. Given the greatly enhanced fusion yield, H-mode is foreseen as the baseline scenario for the future plasma operation of the International Thermonuclear Experimental Reactor (ITER). Many research efforts have been given to understand the criteria for H-mode access. To further contribute to this research, a primary focus of this thesis is characterizing the H-mode access conditions in the Alcator C-Mod tokamak, across a broad range of plasma density, magnetic field, and plasma current. In addition, dedicated experiments were designed and executed on C-Mod, to explore the effects of divertor geometry, ICRF resonance location, and main ion species on H-mode access conditions. Results from these experiments will be included in this thesis. The underlying physics of H-mode access is very complex, and the critical mechanisms remain largely unresolved. To promote our understanding, some models proposed for the H-mode transition are tested, using well documented local plasma conditions, obtained in C-Mod experiments. In particular, this thesis pioneers the test of a recently developed model for H-mode threshold power predictions. / by Yunxing Ma. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 2008. / Includes bibliographical references (p. 51-53). / Organic materials, with their superior photoluminescence and absorbance properties have revolutionized the technologies for displays and solar energy conversion. Due to the large transition dipoles, the localization of excited states or excitons in organic materials necessitates optical models that extend beyond classical far field methods. In this thesis we propose an extended near field calculation method using dyadic Green's functions and demonstrate the applications of both our extended model and traditional far field models for different types of devices such as surface plasmon detectors, cavity organic light emitting devices and organic photovoltaics with external antennas. / by Kemal Celebi. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2008. / Includes bibliographical references (p. 133-144). / The quantum circuit model is the most widely used model of quantum computation. It provides both a framework for formulating quantum algorithms and an architecture for the physical construction of quantum computers. However, several other models of quantum computation exist which provide useful alternative frameworks for both discovering new quantum algorithms and devising new physical implementations of quantum computers. In this thesis, I first present necessary background material for a general physics audience and discuss existing models of quantum computation. Then, I present three new results relating to various models of quantum computation: a scheme for improving the intrinsic fault tolerance of adiabatic quantum computers using quantum error detecting codes, a proof that a certain problem of estimating Jones polynomials is complete for the one clean qubit complexity class, and a generalization of perturbative gadgets which allows k-body interactions to be directly simulated using 2-body interactions. Lastly, I discuss general principles regarding quantum computation that I learned in the course of my research, and using these principles I propose directions for future research. / by Stephen Paul Jordan. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2001. / Includes bibliographical references (leaves 93-96). / In this thesis, we study the effects of interparticle interactions on the optical spectrum of cold gases. We first consider homogenous gas in the weak excitation regime and find that the optical spectrum of a system of Bosons is highly sensitive to interactions. We find that optical excitations, at temperatures low enough for the thermal de Broglie wavelength to be larger than the scattering length, become collective modes. We study collective affects in the optical spectrum both above and below Bose-Einstein condensation, and show that the spectrum acquires a doublet structure when the condensate forms. We present a detailed theory of spectral shift and an estimate of some of the broadening effects. We derive a sum rule for the average frequency shift of an optical spectrum and investigate the basic conservation laws and symmetries of the system lying at the basis of this sum rule. We also compare the sum rule for the optical spectrum with the f-sum rule for the density-density correlation function. Finally we derive a transport equation for the optical modes in a dilute Bose system, which allows us to study the non-linear response to the excitation field. We map the problem onto the dynamics of two interacting anisotropic spins, and calculate the precession frequencies exactly both below and above Bose condensation. We demonstrate a relation between Rabi oscillations and internal Josephson oscillations, and find that an analogue of the internal Josephson effect exists in a non-condensed system. / (cont.) We also derive the transport equation for a dilute Fermi system and find that the dependence of the precession frequencies on interparticle interactions is very weak for fermions. / by Mehmet Özgür Oktel. / Ph.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 69-71). / In this thesis, I present a beamline system designed and built at the Harvard-Smithsonian Center for Astrophysics (CfA) to measure the reflectivity and resolution of small diameter multilayer coated Wolter optics. These optics, used for imaging of x-rays and neutrons, have numerous applications in areas such as medicine, fusion research and planetary science. The beamline consists of a divergent x-ray source, an energy sensitive detector, and a set of precise, computer controlled motorized stages for alignment. A dedicated software package was developed to interface with the detector and stages using the Python programming language. The beamline was used to measure the reflectivity and spatial resolution of two x-ray optics recently fabricated at the CfA. These results are presented and compared with theoretical models for reflectivity from a multilayer surface. / by Andrew O. Ames. / S.B.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Physics, 1995. / Vita. / Includes bibliographical references (p. 87-90). / by Matthew C. Osborn. / M.S.