Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, February 2005. / Includes bibliographical references (leaves 131-142). / Bose-Einstein condensates (BEC) provide a novel tool for the study of macroscopic quantum phenomena and condensed matter systems. Two of the recent frontiers, quantized vortices and ultracold molecules, are the subject of this thesis. The formation of highly-ordered vortex lattices in a Bose-condensed gas has been observed. These triangular lattices contain more than 150 vortices with lifetimes of several seconds. The vortices were generated by rotating the condensate with a scanning blue-detuned laser beam. Depending on the stirrer size, vortices were either nucleated at discrete surface-mode resonances (large beams) or over a broad range of stirring frequencies (small beams). Additionally, the dynamics of the lattices have been studied at finite temperature by varying the condensed fraction of atoms in the system. The decay of angular momentum is observed to be strongly temperature-dependant, while the crystallization of the lattice appears to be insensitive to temperature change. Recently, the field of BEC has been extended to include cold molecules. Here ultra-cold sodium molecules were produced from an atomic BEC by ramping an applied magnetic field across a Feshbach resonance. These molecules were used to demonstrate coherent molecular optics. In particular, we have extended Kapitza-Dirac and Bragg diffraction to cold molecules. By measuring the Bragg spectrum of the molecules immediately after their creation, the conversion from atoms to molecules was shown to be coherent - the matter wave analog to frequency doubling in optics. In addition, the more general process of sum-frequency generation was demonstrated. / (cont.) Atoms prepared in two momentum states, prior to creating molecules, were observed to cross-pair, generating a third momentum state. Finally, molecular matter-wave interference was realized using an autocorrelation technique. / by Jamil R. Abo-Shaeer. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 1999. / Includes bibliographical references (leaf 34). / by Yoko Hammond. / S.M.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2014. / 120 / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 175-185). / In this thesis, I describe the search for a Higgs boson through its decay to a pair of tan leptons with the tau-pair subsequently decaying to ail electron, a muon, and neutrinos. The search is performed using data collected from proton-proton collisions by the Compact Muon Solenoid experiment at the Large Hadron Collider, corresponding to 5.0 fb-1 of integrated luminosity recorded at a center-of-mass energy of 7 TeV and 19.7 fb-1 at 8 TeV. The expected significance for a Standard Model Higgs boson signal with a mass of 125 GeV is at the level of 1.2 standard deviations for the electron muon tau-pair decay mode. A mild excess of events is seen above the SM background expectation in this decay mode, consistent with a SM Higgs boson of mass 125 GeV. In combination with results using other tau-pair decay modes, an excess of events above the background expectation is seen at the level of 3.4 standard deviations. This constitutes the first evidence for a Higgs boson to decay to leptons. This thesis also describes an analysis of the data in the context of physics beyond the Standard Model, particularly in the framework of its Minimal Supersymnnetric extension. / by Valentina Dutta. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2004. / Includes bibliographical references (leaves 103-112). / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Proteins are the functional machinery in living cells. Proteins interact with each other and bind to DNA to form so-called gene regulatory networks and in this way regulate the level, location and timing of expression of other proteins. Cells implement feedback loops to create a memory of their gene expression states. In this way, every differentiated cell in a multicellular organism remembers its expression profile throughout its life. On the other hand, biochemical reactions that take place during gene expression involve small numbers of molecules, and are therefore dominated by large concentration fluctuations. This intrinsic noise has the potential to corrupt memory storage and might result in random transitions between different gene expression states. In the first part of my thesis, I will discuss how the fluctuations in gene expression levels are regulated. The results provided the first experimental evidence that cells can regulate noise in their gene expression by tuning their genetic parameters. In the second half of my thesis, I will discuss how cells create memory by experimentally studying a gene regulatory network that implements a positive feedback loop. A positive feedback loop with nonlinear interactions creates two distinct stable gene expression states. A phase diagram, coupled with a mathematical model of the network, was used to quantitatively investigate the biochemical processes in this network. The response of the network depends on its previous history (hysteresis). Despite the fluctuations in the gene expression, the memory of the gene expression state is preserved for a long time for a broad range of system parameters. / (cont.) On the other hand, for some of the parameters, noise causes random transitions of the cells between different gene expression states and results in a bimodal response. Finally, the hysteretic response of the natural system is experimentally converted to an ultrasensitive graded response as predicted by our model. / by Ertugrul M. Ozbudak. / Ph.D.

Massachusetts Institute of Technology. Dept. of Physics. Thesis. 1972. Ph.D. / MICROFICHE COPY ALSO AVAILABLE IN SCIENCE LIBRARY. / Vita. / Bibliography: leaves 231-234. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1997. / Vita. / Includes bibliographical references (p. 363-366). / by Larry Ahle. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Includes bibliographical references (leaves 105-113). / Rapidly and reliably recognizing an object (is that a cat or a tiger?) is obviously an important skill for survival. However, it is a difficult computational problem, because the same object may appear differently under various conditions, while different objects may share similar features. A robust recognition system must have a capacity to distinguish between similar-looking objects, while being invariant to the appearance-altering transformation of an object. The fundamental challenge for any recognition system lies within this simultaneous requirement for both specificity and invariance. An emerging picture from decades of neuroscience research is that the cortex overcomes this challenge by gradually building up specificity and invariance with a hierarchical architecture. In this thesis, I present a computational model of object recognition with a feedforward and hierarchical architecture. The model quantitatively describes the anatomy, physiology, and the first few hundred milliseconds of visual information processing in the ventral pathway of the primate visual cortex. There are three major contributions. First, the two main operations in the model (Gaussian and maximum) have been cast into a more biologically plausible form, using monotonic nonlinearities and divisive normalization, and a possible canonical neural circuitry has been proposed. Second, shape tuning properties of visual area V4 have been explored using the corresponding layers in the model. It is demonstrated that the observed V4 selectivity for the shapes of intermediate complexity (gratings and contour features) can be explained by the combinations of orientation-selective inputs. Third, shape tuning properties in the higher visual area, inferior temporal (IT) cortex, have also been explored. It is demonstrated that the selectivity and invariance properties of IT neurons can be generated by the feedforward and hierarchical combinations of Gaussian-like and max-like operations, and their responses can support robust object recognition. Furthermore, experimentally-observed clutter effects and trade-off between selectivity and invariance in IT can also be observed and understood in this computational framework. / (cont.) These studies show that the model is in good agreements with a number of physiological data and provides insights, at multiple levels, for understanding object recognition process in the cortex. / by Minjoon Kouh. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 117-124). / Ultracold atoms enable the precise study of novel systems where the correlations between particles are strong. These systems can be simple to describe yet impossible to efficiently simulate on a classical computer; understanding their behavior addresses fundamental questions in condensed-matter physics. The first part of this thesis describes measurements of spin and density fluctuations in degenerate Fermi gases. We begin by presenting a proof-of-principle experiment that demonstrates how information about atomic fluctuations can be extracted from experimental images and used to measure the temperature of a noninteracting system. We then describe a new technique for measuring spin fluctuations that employs an effect analogous to optical speckle, using it to characterize the pair correlations in a strongly attractive Fermi gas. Finally, we use the methods we have developed to characterize the magnetic correlations of a Fermi gas with strong repulsive interactions on the upper branch of a Feshbach resonance, and show that, contrary to earlier experimental and theoretical predictions, this system does not undergo a ferromagnetic phase transition. The second part describes the development of an apparatus for performing experiments with sodium and lithium in optical lattices. We describe progress towards the implementation of synthetic magnetic fields in systems of lattice fermions, which would enable the study of new topological phases. This includes the development of general precursors such a Bose-Einstein condensate and a stable Mott insulator of bosons, as well as more specific studies of heating and dynamical instabilities in tilted and shaken lattices. / by Edward Su. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 249-263). / Ultracold atoms in optical lattices are among the most developed platforms of interest for building quantum devices suitable for quantum simulation and quantum computation. Ultracold trapped atoms are advantageous because they are fundamentally indistinguishable qubits that can be prepared with high fidelity in well-defined states and read-out with similarly high fidelities. However, an outstanding challenge for ultracold atoms in optical lattices is to engineer interesting interactions and control the effects of heating that couple the system to states that lie outside the Hilbert space we wish to engineer. In this thesis, I describe a series of experiments and theoretical proposals that address several critical issues facing ultracold atoms in optical lattices. First, I describe experiments where the tunneling behavior of atoms in the lattice is modified to make our fundamentally neutral particles behave as though they are charged particles in a magnetic field. We show how engineering this interaction creates intrinsic degeneracy in the single particle spectrum of the many-body system and how to introduce strong interactions in the system with the goal of producing exotic many-body states such as a bosonic fractional quantum Hall states. Then, I discuss how this technique can be easily generalized to include spin and higher spatial dimensions in order to access a rich variety of new physics phenomena. Next, I report on the realization of a spin-1 Heisenberg Hamiltonian which emerges as the low energy effective theory describing spin ordering in the doubly-occupied Mott insulator of two spin components. This integer spin Heisenberg model is qualitatively different from the half-integer spin model because it contains a gapped, spin-insulating ground state for small inter-spin interaction energies which we call the spin Mott. Using a spin-dependent lattice to control the inter-spin interactions, we demonstrate high-fidelity, reversible loading of the spin-Mott phase and develop a probe of local spin correlations in order to demonstrate a spin entropy below 0.2 kB per spin. Progress on adiabatically driving the quantum phase transition from the spin Mott to the xy-ferromagnetic is discussed along with the progress towards the creation of a quantum gas microscope for single atom detection and manipulation.. / by Colin Kennedy. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2006. / Includes bibliographical references (p. 143-150). / The physics of blackbodies has been an ongoing source of fascination and scientific research for over a hundred years. Kirchhoff's law states that emissivity and absorptivity are equal for an object in thermal equilibrium. Coupled with the Second Law of Thermodynamics, one can show that no object can emit more than a blackbody at any given frequency, direction or polarization. While this provides a theoretical maximum to the intensity of thermal emission from an object, few come even close to the level of emission exhibited by a blackbody. Thus, there is much room for research into enhancing thermal radiation from many different types of materials. The ability to modify or tailor the thermal emission profile of an object is of great importance and interest in many areas of applied physics and engineering. It turns out that thermal emission spectra can be changed by altering the geometry of the system or the materials used. For instance, nanoscale patterning can enhance emission at certain frequencies, while point defects can localize light at microcavities. General periodic electromagnetic structures, also known as photonic crystals, are therefore a natural medium in which to carry out such investigations, since they are metallodielectric systems that lend themselves relatively easily to sub-wavelength scale patterning and design. / (cont.) This research program aims to study, through both theoretical and computational means, physical phenomena that drive thermal emission in photonic crystals and the design of point defects in the presence of fabrication constraints. First, we explore point defect geometries in inverted opal photonic crystals that can be fabricated by colloidal self-assembly. We identify and study substitutional point defects that introduce a usable defect band into the photonic band gap. It is found that a silica sphere of radius between 0.33a and 0.35a (where a is the lattice constant) introduces a triply degenerate state into the band gap. Reflectance and local density of states calculations are performed to verify the existence and frequency of this defect. Such a defect can be used as a microcavity for localizing light at a point, with a quality factor Q that is limited primarily by the proximity of the defect to the surface of the photonic crystal and other such defects. Second, we present a useful framework within which we can understand some of the physical phenomena that drive thermal emission in one- and two-dimensionally periodic metallic photonic crystals, emphasizing phenomenology and physical intuition. We find that polarization and periodicity play key roles in determining the types of physical phenomena that can be excited in these systems. / (cont.) Promising structures in both 1D and 2D systems are identified as good candidates for thermal design. We discuss how the emissive properties of these systems can be tailored to our needs. Third, we establish that the significant enhancement of thermal emission via Q-matching, which has been possible in 1D systems only, can be extended to 2D systems by means of Fano resonances in the 2D system. We demonstrate through detailed numerical and analytical studies that the Fano resonances characteristic of 2D-periodic photonic crystal slabs can be understood in terms of a 1D-model, thereby showing the existence of essentially 1D behavior in a 2D system. Moreover, we show how properties of these spectra can be controlled by changing the geometrical parameters of the photonic crystals. This work provides a path to the creation of graybodies that have tailored thermal emission spectra, with highly anomalous behavior. Fourth, we perform direct thermal emission calculations for 2D- and 3D-periodic photonic crystal slabs using stochastic electrodynamics following the Langevin approach, implemented via an FDTD algorithm. / (cont.) We demonstrate that emissivity and absorptivity are equal, and thereby numerically verify Kirchhoff's law, by showing that such photonic crystal systems emit as much radiation as they absorb, for every frequency, up to statistical fluctuations. This has been an issue of great controversy because of experimental work indicating the violation of Kirchhoff's law. We also study the effect of surface termination on absorption and emission spectra from these systems. / by David Lik Chin Chan. / Ph.D.