- RSS Channel Showcase 2063000
- RSS Channel Showcase 3636955
- RSS Channel Showcase 7910838
- RSS Channel Showcase 3071981
Articles on this Page
- 12/31/17--16:00: _Quantum electronic ...
- 12/31/17--16:00: _Controlling ultraco...
- 12/31/17--16:00: _Measurement of heli...
- 12/31/12--16:00: _Probing and prepari...
- 12/31/68--16:00: _The effects of ligh...
- 12/31/87--16:00: _Trapping sodium wit...
- 12/31/85--16:00: _Transient transport...
- 12/31/75--16:00: _The ballast resisto...
- 12/31/17--16:00: _Neutrinos, neurons ...
- 12/31/17--16:00: _Ultracold ²³Na⁶Li m...
- 12/31/17--16:00: _Aspects of highly-e...
- 12/31/17--16:00: _Quantum enhanced se...
- 12/31/17--16:00: _Classification of b...
- 12/31/17--16:00: Controlling ultracold fermions under a quantum gas microscope
- 12/31/68--16:00: The effects of light propagation in spherically symmetric metrics.
- 12/31/87--16:00: Trapping sodium with light
- 12/31/85--16:00: Transient transport and contact effects in a-As₂Se₃
- 12/31/75--16:00: The ballast resistor : a simple dissipative structure.
- 12/31/17--16:00: Ultracold ²³Na⁶Li molecules in the triplet ground state
- 12/31/17--16:00: Quantum enhanced sensing and communication
- 12/31/17--16:00: Classification of base geometries in F-theory
Quantum electronic transport in atomically layered topological insulators
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).
Controlling ultracold fermions under a quantum gas microscope
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).
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).
Probing and preparing novel states of quantum degenerate rubidium atoms in optical lattices
Ultracold atoms in optical lattices are promising systems to realize and study novel quantum mechanical phases of matter with the control and precision offered by atomic physics. Towards this goal, as important as engineering new states of matter is the need to develop new techniques to probe these systems. I first describe our work on realizing Bragg scattering of infrared light from ultracold atoms in optical lattices. This is a detection technique which probes the spatial ordering of a crystalline system, and has led to our observation of Heisenberg limited wavefunction dynamics. Furthermore, we have observed the superfluid to Mott insulator transition through the matter wave Talbot effect. This technique will be particularly powerful for studying antiferromagnetic phases of matter due to its sensitivity to the crystalline composition. The second major component of this thesis describes a new scheme to realize the Harper Hamiltonian. The Harper Hamiltonian is a model system which effectively describes electrons in a solid immersed in a very high magnetic field. The effective magnetic field manifests itself as a position-dependent phase in the motion of the constituent particles, which can be related to gauge fields and has strong connections to topological properties of materials. We describe how we can engineer the Harper Hamiltonian in a two-dimensional optical lattice with neutral atoms by creating a linear potential tilt and inducing Raman transitions between localized states. In situ measurements provide evidence that we have successfully created the Harper Hamiltonian, but further evidence is needed to confirm the creation of the ground state of this Hamiltonian.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2013.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 137-146).
The effects of light propagation in spherically symmetric metrics.
Massachusetts Institute of Technology. Dept. of Physics. Thesis. 1969. Ph.D.; Vita.; Bibliography: leaf 119.
Trapping sodium with light
Raab, Eric Lowell
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1988.; Bibliography: leaves 177-182.
Transient transport and contact effects in a-As₂Se₃
Gibson, Dwight Deleno
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1986.; MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE; Bibliography: leaves 102-105.
The ballast resistor : a simple dissipative structure.
Ross, Benjamin Ira
Thesis. 1976. Ph.D.--Massachusetts Institute of Technology. Dept of Physics.; Microfiche copy available in Archives and Science.; Includes bibliographical references.
Neutrinos, neurons and neutron stars : applications of new statistical and analysis techniques to particle and astrophysics
Collin, Gabriel L.W.H
The IceCube detector opens a new window into our universe; valuable for both astronomy and particle physics. This thesis spans a wide range of topics that are bound together by a common theme: the development and application of new statistical and computational methods for analysing data from particle and astrophysics experiments. Sterile neutrinos are a hypothetical fourth kind of neutrino, which are motivated by anomalies observed in various short base-line neutrino experiments. These experiments have published results that are not mutually compatible. This thesis presents a global fit to many short base-line datasets with the addition of the recent IceCube sterile neutrino search, constraining the full 3+1 mixing matrix for the first time. The global fit strongly favours the sterile neutrino hypothesis, although significant tension still remains within the datasets. The origin of the observed astrophysical neutrino flux at IceCube remains elusive. Current methods, using a hot-spot model, have seen no significant clustering of events. This thesis presents a new test for point sources of neutrinos, based on the non-Poissonian Template Fitting technique. Constraints on population models for neutrino points sources are shown for the first time. Atmospheric neutrinos form a background for astrophysical analyses on IceCube, but also serve as the signal in particle physics analyses such as the sterile neutrino search. The first comprehensive study of the effect of global atmospheric temperature variations on atmospheric neutrino fluxes is provided. This thesis also presents two studies on using new computational methods for simulation and reconstruction on IceCube. Convolutional neural networks have been used to classify low-level waveform data, with the goal of identifying tau-neutrinos. Metropolis light transport, a rendering technique used in the CGI industry, has been extended to simulate the transport of light inside the IceCube experiment. Both show promising results, exceeding existing algorithms in their test cases.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages -205) and index.
Ultracold ²³Na⁶Li molecules in the triplet ground state
Rvachov, Timur Michael
This thesis describes experiments in spectroscopy and formation of triplet ²³Na⁶Li molecules from an initial mixture of ultracold ²³Na and ⁶Li. The production of quantum degenerate molecules with long-range dipolar interactions is a long-standing goal in low temperature physics. NaLi is a fermionic molecule with an electric dipole moment of 0.2 Debye and a magnetic dipole moment of 2 [mu]B in its triplet ro-vibrational ground state. The formation of an ultracold molecule with both electric and magnetic dipole moments allows for novel opportunities in control of ultracold molecular reactions and studies of quantum many-body systems with dipolar interactions. This experimental work consists of two parts. The first is a thorough spectroscopic study of the excited and ground triplet potentials of NaLi using one- and two-photon photoassociation spectroscopy. We present the spectroscopic positions and strengths of transitions to nearly all vibrational states in the excited c³[sigma]⁺ and ground ³[sigma]⁺ potentials of NaLi. This is the first spectroscopic observation of triplet potentials in NaLi and the first demonstration of photoassociation in the Na-Li system. The second part utilizes our spectroscopic results to coherently form an ultracold gas of NaLi molecules. Starting with an ultracold Na-Li mixture, we use magneto-association to form weakly bound Feshbach molecules. The Feshbach molecules are then transfered to the ro-vibrational triplet ground state using a two-photon stimulated Raman adiabatic passage (STIRAP) technique, forming 3 x 10⁴ molecules at a density of 5 x 10¹⁰ cm⁻³ and temperature of 3 [mu]K. The molecules are long-lived with a measured lifetime of 5 seconds, which highlights their fermionic nature and low universal inelastic loss rate. The utility of the molecule's magnetic moment is demonstrated by performing electron spin resonance spectroscopy to measure the hyperfine structure of the molecule.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 161-173).
Aspects of highly-entangled quantum matter : from exotic phases, to quantum computation, and dynamics
We explore three incarnations of highly-entangled quantum matter: as descriptions of exotic, gapped phases in three spatial dimensions, as resources for fault-tolerant quantum computation, and as the by-product of the unitary evolution of a quantum state, on its approach to equilibrium. In Part 1, we study quantum information processing in platforms hosting Majorana zero modes. We demonstrate that certain highly-entangled states may be engineered in arrays of mesoscopic topological superconducting islands, and used for fault-tolerant quantum computation. We then discuss measurement-based protocols for braiding Majorana zero modes and detecting their non-Abelian statistics in on-going experiments on proximitized, semiconductor nanowires, before proposing new families of error-correcting codes for fermionic qubits, along with concrete realizations. In Part 11, we study gapped, three-dimensional phases of matter with sub-extensive topological degeneracy, and immobile point-like excitations - termed "fractons" - which cannot be moved without nucleating other excitations. We find two broad classes of fracton phases in which (i) composites of fractons form topological excitations with reduced mobility, or (ii) all topological excitations are strictly immobile. We demonstrate a duality between these phases and interacting systems with global symmetries along sub-systems, and use this to find new fracton phases, one of which may also be obtained by coupling an isotropic array of two-dimensional states with Z₂ topological order. We introduce a solvable model in which the fracton excitations are shown to carry a protected internal degeneracy, which provides a generalization of non-Abelian anyons in three spatial dimensions. In Part III, we investigate the dynamics of operator spreading and entanglement growth in quantum circuits composed of random, local unitary operators. We relate quantities averaged over realizations of the circuit, such as the purity of a sub-system and the out-of-time-ordered commutator of spatially-separated operators, to a fictitious, classical Markov process, which yields exact results for the evolution of these quantities in various spatial dimensions. Operator spreading is ballistic, with a front that broadens as a dimension-dependent power-law in time. In this setting, we also map the dynamics of entanglement growth in one dimension to the stochastic growth of an interface and to the Kardar-Parisi-Zhang equation, which leads to a description of entanglement dynamics in terms of an evolving "minimal cut" through the quantum circuit, and provides heuristics for entanglement growth in higher-dimensions. The material presented here is based on Ref. [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. Ref. [11, 12] are not discussed in this thesis, but were completed during my time at MIT.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 307-321).
Quantum enhanced sensing and communication
Quantum phenomena such as entanglement and superposition enable performance beyond what classical physics can provide in tasks of computing, communication and sensing. Quantum sensing aims to enhance the measurement precision in parameter estimation or error probability in hypothesis testing. The first part of this thesis focuses on protocols for entanglement-enhanced sensing. However, various quantum sensing schemes' quantum advantage disappears in presence of decoherence from noise and loss. The quantum illumination protocol, on the other hand, has advantage over classical illumination even in presence of decoherence. This thesis provides the optimum receiver design for quantum illumination, and extends quantum illumination target detection to the realistic scenario with target fading and the Neyman-Pearson decision criterion. Quantum algorithms can solve difficult problems more efficiently than classical algorithms, which makes various classical encryption schemes vulnerable. To remedy this security issue, quantum key distribution enables sharing of secret keys with unconditional protocol security. However, the secret-key-rate of the state-of-art single-mode based quantum key distribution protocols are limited by a fundamental rate-loss trade-off. To enhance the secret-key-rate, this thesis proposes a multi-mode based quantum key distribution protocol. To prove its security, the noisy entanglement assisted classical capacity is developed to enable a security framework for two-way quantum key distribution protocols such as the one proposed here. An essential notion in the entanglement assisted capacity is additivity. This thesis constructs a channel with non-additive classical capacity assisted by limited entanglement assistance, even when the classical capacity of the channel is additive.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018.; Cataloged from PDF version of thesis.; Includes bibliographical references.
Classification of base geometries in F-theory
Wang, Yinan, Ph. D. Massachusetts Institute of Technology
F-theory is a powerful geometric framework to describe strongly coupled type JIB supcrstring theory. After we compactify F-theory on elliptically fibered Calabi-Yau manifolds of various dimensions, we produce a large number of minimal supergravity models in six or four spacetime dimensions. In this thesis, I will describe a current classification program of these elliptic Calabi-Yau manifolds. Specifically, I will be focusing on the part of classifying complex base manifolds of these elliptic fibrations. Besides the usual algebraic geometric description of these base manifolds, F-theory provides a physical language to characterize them as well. One of the most important physical feature of the bases is called the "non-Higgsable gauge groups", which is the minimal gauge group in the low energy supergravity model for any elliptic fibration on a specific base. I will present the general classification program of complex base surfaces and threefolds using algebraic geometry machinery and the language of non- Higgsable gauge groups. While the complex base surfaces can be completely classified in principle, the zoo of generic complex threefolds is not well understood. However, I will present an exploration of the subset of toric threefold bases. I will also describe examples of base manifolds with non-Higgsable U(1)s, which lead to supergravity models in four and six dimensions with a U(1) gauge group but no massless charged matter.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 187-196).