Theses - Dept. of Physics
http://hdl.handle.net/1721.1/7608
2018-06-21T18:55:13ZMeasurement of helium isotopic composition in cosmic rays with AMS-02
http://hdl.handle.net/1721.1/115695
Measurement of helium isotopic composition in cosmic rays with AMS-02
Behlmann, Matthew Daniel
The isotopic composition of helium in cosmic ray fluxes provides valuable information about cosmic ray propagation through the Galaxy, which is of particular interest to indirect dark matter searches. Helium-3, mainly a secondary cosmic ray species, is primarily produced by spallation of heavier cosmic rays, such as primary helium-4, with interstellar matter. In six years of data taking, AMS has collected the largest available data set on fluxes of cosmic-ray helium. Events are selected to form a clean sample of galactic helium nuclei, for which velocity and rigidity give a measurement of particle mass that allows the measurement of relative isotope abundances. The resolution of measured mass is described in detail by template functions based on the underlying resolutions of the silicon tracker and ring-imaging Cerenkov detector measurements. This thesis presents a measurement of the cosmic ray helium isotope ratio 3 He/ 4He in the range 0.8-10 GeV/nucleon, as obtained through a template fitting approach on AMS data.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 137-145).
2018-01-01T00:00:00ZControlling ultracold fermions under a quantum gas microscope
http://hdl.handle.net/1721.1/115688
Controlling ultracold fermions under a quantum gas microscope
Okan, Melih
This thesis presents the experimental work on building a quantum gas microscope, employing fermionic 40K atoms in an optical lattice, and precision control of the atoms under the microscope. This system works as a natural simulator of the 2D Hubbard model, which describes materials with strongly correlated electrons. After preparing ultracold 40K atoms in an optical lattice and performing Raman sideband cooling, single lattice site resolution was obtained. Metallic, Mott insulating, and band insulating states were observed in situ and local moment was directly accessed as a local observable with the site-resolved imaging. Performing spin-selective imaging also gave access to spin, and spatial correlations of charge and spin was measured with respect to doping. In this measurements, antiferromagnetic correlations were observed in the spin sector. In the charge sector, we observed an anti-bunching behavior at low fillings, as a result of the Pauli exclusion principle and repulsive interactions. We also observed that doublon-hole bunching resulting from the superexchange excitations dominates and causes the charges to bunch. In order to increase the simulation capabilities, we updated the microscope with arbitrary optical potential imprinting ability. Using a digital micromirror device (DMD), a 2D box potential was created with the sharpness of a few lattice sites. A homogenous 2D Hubbard system is created at half-filling in this box potential. Using a magnetic gradient, different spin states were separated within a Mott insulator, being an ideal starting point for performing spin transport measurements. The lowest energy s-wave Feshbach resonance between 19/2, -7/2) and 19/2, -5/2) states of 40K was characterized with an increased precision and established as an interaction varying knob of our quantum simulator. Interaction energy spectrum around this resonance was measured. Confinement induced molecules on the attractive side and deeply bound molecules on the repulsive side are observed in an optical lattice.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 123-130).
2018-01-01T00:00:00ZQuantum electronic transport in atomically layered topological insulators
http://hdl.handle.net/1721.1/115683
Quantum electronic transport in atomically layered topological insulators
Fatemi, Valla
The merger of topology and symmetry established a new foundation for understanding the physics of condensed matter, beginning with the notion of topological insulators (TIs) for electronic systems. For the time-reversal invariant TIs, a key aspect is the "helical" mode at the boundary of the system - that is, the ID edge of a 2D topological insulator or the 2D surface of a 3D topological insulator. These helical modes represent the extreme limit of spin-orbit coupling in that the spin-degenercy has been completely lifted while preserving time-reversal symmetry. This property is crucial for proposals realizing exotic excitations like the Majorana bound state. In this thesis, I present a series of experiments investigating electronic transport through the boundary modes of 3D and 2D topological insulators, specifically Bi1.5 Sb0.5 Te1.7 Se1.3 and monolayer WTe 2 , respectively. For the case of ultra-thin WTe 2 , I also present experiments detailing investigations of the 2D bulk states, finding a semimetallic state for the trilayer and a superconducting phase for the monolayer, both of which are strongly tunable by the electric field effect. The discovery of 2D topological insulator and 2D superconductor phases within the same material, accessible by standard solid state elecrostatic gates, places WTe2 in a unique situation among both TIs and superconductors, potentially enabling gate-configurable topological devices within a homogenous material platform.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 153-180).
2018-01-01T00:00:00ZPractical fault-tolerant quantum computation
http://hdl.handle.net/1721.1/115680
Practical fault-tolerant quantum computation
Yoder, Theodore J
For the past two and a half decades, a subset of the physics community has been focused on building a new type of computer, one that exploits the superposition, interference, and entanglement of quantum states to compute faster than a classical computer on select tasks. Manipulating quantum systems requires great care, however, as they are quite sensitive to many sources of noise. Surpassing the limits of hardware fabrication and control, quantum error-correcting codes can reduce error-rates to arbitrarily low levels, albeit with some overhead. This thesis takes another look at several aspects of stabilizer code quantum error-correction to discover solutions to the practical problems of choosing a code, using it to correct errors, and performing fault-tolerant operations. Our first result looks at limitations on the simplest implementation of fault-tolerant operations, transversality. By defining a new property of stabilizer codes, the disjointness, we find transversal operations on stabilizer codes are limited to the Clifford hierarchy and thus are not universal for computation. Next, we address these limitations by designing non-transversal fault-tolerant operations that can be used to universally compute on some codes. The key idea in our constructions is that error-correction is performed at various points partway through the non-transversal operation (even at points when the code is not-necessarily still a stabilizer code) to catch errors before they spread. Since the operation is thus divided into pieces, we dub this pieceable fault-tolerance. In applying pieceable fault tolerance to the Bacon-Shor family of codes, we find an interesting tradeoff between space and time, where a fault-tolerant controlled-controlled-Z operation takes less time as the code becomes more asymmetric, eventually becoming transversal. Further, with a novel error-correction procedure designed to preserve the coherence of errors, we design a reasonably practical implementation of the controlled-controlled-Z operation on the smallest Bacon-Shor code. Our last contribution is a new family of topological quantum codes, the triangle codes, which operate within the limits of a 2-dimensional plane. These codes can perform all encoded Clifford operations within the plane. Moreover, we describe how to do the same for the popular family of surface codes, by relation to the triangle codes.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 190-201).
2018-01-01T00:00:00Z