Physics - Ph.D. / Sc.D.
http://hdl.handle.net/1721.1/7866
Thu, 19 Jan 2017 15:11:02 GMT2017-01-19T15:11:02ZLessons on interacting quantum field theories from string theory
http://hdl.handle.net/1721.1/106452
Lessons on interacting quantum field theories from string theory
Wang, Yifan, Ph. D. Massachusetts Institute of Technology
In this thesis, we use string theory constructions and dualities to explore various features of interacting quantum field theories. We begin with an overview in Chapter 1, of past and recent developments in quantum field theories, explaining the advantages of string theoretic techniques over traditional approaches in answering a range of questions about interacting dynamics. In Chapter 2 we study the holographic duality between the 6d (1, 1) Ak-1 little string theory (LST) and type II string theory in the double scaled limit. By identifying the low energy states, which are Cartan gluons in the 6d maximal super-Yang-Mills (SYM) that describes the massless sector of the LST, we compute the four-point amplitudes from both sides of the duality and demonstrate matching results. Since the two computations concern different regimes in the parameter space, their amazing agreement implies the presence of certain nonrenormalization theorems in the 6d SYM. In Chapter 3, motivated by the AdS/CFT duality, we develop a systematic procedure to derive an off-shell action for hydrodynamics from classical Einstein gravity. We first identity the boundary fluid degrees of freedom in the hydrodynamic regime, in terms of gapless modes of the metric in the bulk gravity. This allows us to derive an off-shell action, for relativistic fluids that have gravity duals, at leading order in derivative expansion, by explicitly integrating out gapped degrees of freedom in the bulk. We also explain the strategy to incorporate dissipation and higher order effects. In Chapter 4, we discuss 4d N = 2 superconformal field theories (SCFT) of the Argyres-Douglas (AD) type, which can be constructed in string/M theory by either wrapping M5 branes on punctured Riemann surface or probing 3-fold singularity by IIB string. We classify the punctures (irregular defects in Hitchin system) on the Riemann surface in the former construction, that will give rise to N = 2 SCFTs and demonstrate how to extract exact information about the Coulomb branch spectrum and central charges. We further identify these AD theories constructed from M5 branes with a special class of theories from IIB probing compound Du Val (cDV) singularities, thereby establishing a mathematical connection between singular Hitchin systems and cDV singularities through N = 2 SCFTs. We end with a short summary and outlook for future directions in Chapter 5.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 185-194).
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/1721.1/1064522016-01-01T00:00:00Z21 cm cosmology with optimized instrumentation and algorithms
http://hdl.handle.net/1721.1/104536
21 cm cosmology with optimized instrumentation and algorithms
Zheng, Haoxuan, Ph. D. Massachusetts Institute of Technology
Precision cosmology has made tremendous progress in the past two decades thanks to a large amount of high quality data from the Cosmic Microwave Background (CMB), galaxy surveys and other cosmological probes. However, most of our universe's volume, corresponding to the period between the CMB and when the first stars formed, remains unexplored. Since there were no luminous objects during that period, it is called the cosmic "dark ages". 21 cm cosmology is the study of the high redshift universe using the hyperfine transition of neutral hydrogen, and it has the potential to probe that unchartered volume of our universe and the ensuing cosmic dawn, placing unprecedented constraints on our cosmic history as well as on fundamental physics. My Ph.D. thesis work tackles the most pressing observational challenges we face in the field of 21 cm cosmology: precision calibration and foreground characterization. I lead the design, deployment and data analysis of the MIT Epoch of Reionization (MITEoR) radio telescope, an interferometric array of 64-dual polarization antennas whose goal was to test technology and algorithms for incorporation into the Hydrogen Epoch of Reionization Array (HERA). In four papers, I develop, test and improve many algorithms in low frequency radio interferometry that are optimized for 21 cm cosmology. These include a set of calibration algorithms forming redundant calibration pipeline which I created and demonstrated to be the most precise and robust calibration method currently available. By applying this redundant calibration to high quality data collected by the Precision Array for Probing the Epoch of Reionization (PAPER), we have produced the tightest upper bound of the redshifted 21 cm signals to date. I have also created new imaging algorithms specifically tailored to the latest generation of radio interferometers, allowing them to make Galactic foreground maps that are not accessible through traditional radio interferometry. Lastly, I have improved on the algorithm that synthesizes foreground maps into the Global Sky Model (GSM), and used it to create an improved model of diffuse sky emission from 10 MHz through 5 THz.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 213-236).
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/1721.1/1045362016-01-01T00:00:00ZNovel angular and frequency manipulation of light in nano-scaled dielectric photonic systems
http://hdl.handle.net/1721.1/104534
Novel angular and frequency manipulation of light in nano-scaled dielectric photonic systems
Shen, Yichen, Ph. D. Massachusetts Institute of Technology
Humankind has long endeavored to control light. In modern society, with the rapid development of nanotechnology, the control of light is moving toward devices at micrometer and even nanometer scales. At such scales, traditional devices based on geometrical optics reach their fundamental diffraction limits and cease to work. Nano-photonics, on the other hand, has attracted wide attention from researchers, especially in the last decade, due to its ability to manipulate light at the nanoscale. In this thesis, we explore novel control of light created by nanophotonic structures, with a common theme on light interference in nanoscaled dielectric photonic systems. The first part of the thesis focuses on broadband angular selective nanophotonic systems. We survey the literatures and the current state of the art focused on enabling optical broadband angular selectivity. We also present a novel way of achieving broadband angular selectivity using Brewster mode in nanophotonic systems. We propose two categories of potential applications for broadband angularly selective systems. The first category aims at enhancing the efficiency of solar energy harvesting, through photovoltaic process or solar thermal process. The second category aims at enhancing light extracting efficiency and detection sensitivity. Finally, we discuss the most prominent challenges in broadband angular selectivity and some prospects on how to solve these challenges. The second part of the thesis focuses on spectrum control of light using all-dielectric surface resonator. We proposes a new structural color generation mechanism that produces colors by the Fano resonance effect on thin photonic crystal slab. We experimentally realize the proposed idea by fabricating the samples that show resonance-induced colors with weak dependence on the viewing angle. We also show that the colors can be dynamically tuned by stretching the photonic crystal slab fabricated on an elastic substrate. In a follow up work, we address how to overcome the challenge of mode leaking on dielectric substrate. We present a class of low-index zigzag surface structure that supports resonance modes even without index contrast with the substrate. In the third part, we investigate neuromorphic computation using the interference of light in on-chip dielectric photonic waveguide network. We first mathematically prove that conventional neural networks architecture can be equivalently represented by nanoscaled optical systems. We then experimentally demonstrate that our optical neural networks are able to give equivalent accuracy on a standard training datasets. In the last part, we show that in principle optical neural nets are at least 3 orders of magnitude faster and power efficient in forward propagation than conventional neural nets.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 93-114).
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/1721.1/1045342016-01-01T00:00:00ZSemiclassical studies of decoherence produced by scattering
http://hdl.handle.net/1721.1/104533
Semiclassical studies of decoherence produced by scattering
Schram, Matthew Christopher
The conventional notion of coherent atom-surface scattering originates from the existence of Bragg peaks in elastic scattering. The helium atom acts as a quantum mechanical matter wave that is coherent with itself; the well-defined phase relationship of the particle beam at the different spatial positions at surface impact implies the possibility of different non-specular outgoing beams thanks to the constructive interference of the emitted waves from each surface atom. Moreover, we still observe diffraction peaks when scattering off a lattice at finite temperature, although the peaks are here diminished by the Debye-Waller factor. However, in the case of inelastic scattering, the surface particles are displaced by the scattering atom itself and may then emit or absorb one or more phonons to the scatterer. Acoustic phonons produced by this process are gapless excitations; hence, extremely long-wavelength phonons will contribute vanishingly small shifts in energy and momentum. The difficulty in observing this is exacerbated due to the roughly 1eV resolution of high energy helium scattering experiments. So through phonon excitation the surface has "measured" the particle's presence which acts to destroy quantum coherence, though we still observe diffraction spots which imply coherent scattering. How do we reconcile these disparate viewpoints? We propose a new way of looking at the question of coherence in atom-surface scattering. Instead of considering a single beam of helium particles, we instead use semiclassical techniques to simulate an initially coherent superposition of helium particles with equal probabilities of interacting with the surface or not interacting with the surface. We then evolve the classical mechanical trajectories, and recombine the atoms after scattering to observe the resulting interference pattern. The degree to which phonons are excited in the lattice by the scattering process dictates the fringe contrast of the interference pattern of the resulting beams. We show that for a wide range of conditions, despite the massive change in the momentum perpendicular to the surface, we can still expect to have coherent (in the superposition sense) scattering.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 146-152).
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/1721.1/1045332016-01-01T00:00:00Z