UMD CMNS Physics S1 Color

Thomas Rensink - January 20, 2017

Dissertation Title: An Investigation of Nonlocal Atomic Potentials for Modeling Non-Relativistic Quantum Laser-Atom Interactions

Date and Time: Friday, January 20, 3:00 pm

Location: PSC 3150

Committee Chair: Prof. Thomas Antonsen

Committee: 

Dr. Phillip Sprangle
Dr. Mohammad Hafezi
Dr. Rajarshi Roy
Dr. Ki-Yong Kim

Abstract:

Atom-field interactions in the ionization regime give rise to a wide range of physical phenomena, and their study continues to be an active field of research. Many process are may be explained by modeling the time-dependent Schrodinger equation; however, simulation in the strong field regime is computationally expensive and time-consuming. Here, a nonlocal model potential replaces the Coulomb potential in the time dependent Schrodinger equation, and examined for suitability of modeling strong field-atom dynamics while significantly reducing computation time.

Nonlocal potentials have been used to model many quantum mechanical systems, from multi-electron molecular configurations to semiconductor theory. Despite their relative success, nonlocal potentials are largely unexplored for modeling high field laser-gas interactions in the ionizing regime. This work examines theory and numerical results of a gaussian nonlocal model atom in intense, femtosecond laser pulses, with main findings: Nonlocal potentials are useful for obtaining the photoionization rate in the tunnel and multiphoton regimes, and qualitatively characterize electron wavefunction dynamics of irradiated atoms. The model is used to explore the two-color technique for producing Terahertz (THz) frequency radiation.

Richard Knoche - December 9, 2016

Dissertation Title: Corrections and Calibrations in the LUX Dark Matter Detector

Date and Time: Friday, December 9, 3:00 pm

Location: PSC 3150

Committee Chair: Prof. Carter Hall

Committee:

Dr. Elizabeth Beise
Dr. Alberto Belloni
Dr. Luis Orozco
Dr. Massimo Ricotti

Abstract:

The Large Underground Xenon (LUX) Detector has recently finished a 332 day exposure and placed world-leading limits on the spin-independent WIMP-nucleon scattering cross-section. In this work, I discuss the basic techniques to produce signal corrections, energy scale calibrations, and recoil band calibrations in a dark matter detector. I discuss the nonuniform electric field that was present during LUX's 332 day exposure, and detail how such a field complicates these calibration techniques. Finally, I present novel techniques that account for all of the complications introduced by the nonuniform electric field, and allow a WIMP-nucleon cross scattering limit to be produced from the data.

Jeffery Demers - December 9, 2016

Dissertation Title: Stochastic Processes in Physics: Deterministic Origins and Control

Date and Time: Friday, December 9, 1:00 pm

Location: IPST 1116

Committee Chair: Prof. Christopher Jarzynski

Committee:

Dr. Dionisios Margetis
Dr. Edward Ott
Dr. Charles Levermore
Dr. Perinkulam Krishnaprasad

Abstract:

Stochastic processes are ubiquitous in the physical sciences and engineering. While often used to model imperfections and experimental uncertainties in the macroscopic world, stochastic processes can attain deeper physical significance when used to model the seemingly random and chaotic nature of the underlying microscopic world. Nowhere more prevalent is this notion than in the field of stochastic thermodynamics - a modern systematic framework used describe mesoscale systems in strongly fluctuating thermal environments which has revolutionized our understanding of, for example, molecular motors, DNA replication, far-from equilibrium systems, and the laws of macroscopic thermodynamics as they apply to the mesoscopic world. With progress, however, come further challenges and deeper questions, most notably in the thermodynamics of information processing and feedback control. Here it is becoming increasingly apparent that, due to divergences and subtleties of interpretation, the deterministic foundations of the stochastic processes themselves must be explored and understood.

This thesis presents a survey of stochastic processes in physical systems, the deterministic origins of their emergence, and the subtleties associated with controlling them. First, we study time-dependent billiards in the quivering limit - a limit where a billiard system is indistinguishable from a stochastic system, and where the simplified stochastic system allows us to view issues associated with deterministic time-dependent billiards in a new light and address some long-standing problems. Then, we embark on an exploration of the deterministic microscopic Hamiltonian foundations of non-equilibrium thermodynamics, and we find that important results from mesoscopic stochastic thermodynamics have simple microscopic origins which would not be apparent without the benefit of both the micro and meso perspectives. Finally, we study the problem of stabilizing a stochastic Brownian particle with feedback control, and we find that in order to avoid paradoxes involving the first law of thermodynamics, we need a model for the fine details of the thermal driving noise. The underlying theme of this thesis is the argument that the deterministic microscopic perspective and stochastic mesoscopic perspective are both important and useful, and when used together, we can more deeply and satisfyingly understand the physics occurring over either scale.

Xu Jiang - December 2, 2016

Dissertation Title: QUANTITATIVE STUDY OF LONGITUDINAL RELAXATION (T1) CONTRAST MECHANISMS IN BRAIN MRI

Date and Time: Friday, December 2, 3:00 pm

Location: PSC 3150 

Committee Chair: Prof. Steven M. Anlage

Committee:

Dr. Wolfgang Losert
Dr. Rajarshi Roy
Dr. Peter van Gelderen
Dr. Jeff H. Duyn
Dr. Yang Tao

Abstract:

Longitudinal relaxation (T1) contrast in MRI is important for studying brain morphology and is widely used in clinical applications. Although MRI only detects signal from water hydrogen (1H) protons (WPs), T1 contrast is known to be influenced by other species of 1H protons, including those in macromolecules (MPs), such as lipids and proteins, through magnetization transfer (MT) between WPs and MPs. This complicates the use and quantification of T1 contrast to study the underlying tissue composition and the physiology of brain.

MT contributes to T1 contrast to an extent that is generally dependent on MT kinetics, as well as the concentration and NMR spectral properties of MPs. However, the MP spectral properties and MT kinetics are both difficult to measure directly, as the signal from MPs is generally invisible to MRI. Therefore, to investigate MT kinetics and further quantify T1 contrast, we first developed a reliable way to indirectly measure the MP fraction and its exchange rate with WPs, with minimal dependence on the spectral properties of MPs. For this purpose, we used brief, high power radiofrequency (RF) NMR excitation pulses to almost completely saturate the magnetization of MPs. Based on this, both MT kinetics and the contribution of MPs to T1 contrast through MT were studied. The thus obtained knowledge allowed us to subsequently infer the spectral properties of MPs by applying low power, frequency-selective off-resonance RF pulses and measuring the offset-frequency dependent effect of MPs on the WP MRI signal. A two pool exchange model was used in both cases to account for direct effects of the RF pulse on WP magnetization.

Consistent with earlier work using MRI and post-mortem analysis of brain tissue, our novel measurement approach found that MPs constitute an up to 27% fraction of the total 1H protons in human brain white matter, and their spectrum follows a super-Lorentzian line with a T2 of 9.6±0.6 μs and a resonance frequency centered at -2.58±0.05 ppm. T1 contrast was found to be dominated by MP fraction, but also negatively correlated with iron concentration in iron rich regions of brain.

Ismail Volkan Inlek - November 7, 2016

Dissertation Title: Multi-Species Trapped Atomic Ion Modules for Quantum Networks

Date and Time: Monday, November 7, 3:00 pm

Location: PSC 2136

Committee Chair: Prof. Christopher Monroe

Committee:

Dr. Alexey Gorshkov
Dr. Mohammad Hafezi
Dr. Alan Migdall
Dr. Edo Waks

Abstract:

Trapped atomic ions are among leading platforms in quantum information processing with their long coherence times and high fidelity quantum operations. Scaling up to larger numbers of qubits is a remaining major challenge. A network of trapped ion modules offers a promising solution by keeping a manageable number of qubits within a module while photonic interfaces connect separate modules together to increase the number of controlled memory qubits. Since the generation of entanglement between qubits in different modules is probabilistic, an excessive number of connection trials might result in decoherence on the memory qubits through absorption of stray photons. This crosstalk issue could be circumvented by introducing a different atomic species as photonic qubits. Compared to a system that only utilizes single species of atoms, there are also additional advantages in a multi-species apparatus where attractive features of each atom can be employed for certain tasks.

In this thesis, I present experimental demonstrations of necessary ingredients of a multi-species module for quantum networking. In these experiments, barium ions are intended to be used as photonic communication qubits with visible photon emission lines that are more convenient for current fiber optics and detector technologies while ytterbium ions are used for storing and processing quantum information where long coherence times available in hyperfine clock states make them suitable memory qubits. The key experiments include demonstration of atom-photon entanglement using the barium qubit and utilizing the Coulomb interaction between ytterbium and barium with spin-dependent forces for transfer of information from communication to memory qubits.

David Green - November 2, 2016

Dissertation Title: ​​Measurement of the cosmic-ray proton spectrum from 54 GeV to 9.5 TeV with the Fermi Large Area Telescope

Date and Time: Wednesday, November 2, 9:00 am

Location: PSC 3150

Committee Chair: Prof. Kara Hoffman

Committee:

Dr. Elizabeth A. Hays
Dr. Jordan Goodman
Dr. Julie McEnery
Dr. M. Coleman Miller

Abstract:

Cosmic rays are a near-isotropic continuous flux of energetic particles from extraterrestrial origin. First discovered in 1912, cosmic rays span over 10 decades of energy and originate from Galactic and extragalactic sources. The Fermi Gamma-ray Space Telescope observations have recently confirmed supernova remnants (SNR) as a source class for Galactic cosmic-ray protons. Additionally, recent measurements made by AMS-02 of the cosmic-ray proton spectrum to 1.8 TeV in kinetic energy have shown an unexpected spectral break at 415 ± 117 GeV with a primary spectral index of −2.794±0.006 and a secondary spectral index of −2.702±0.047. The Fermi Large Area Telescope (LAT), one of two instruments on Fermi, has an ideal energy range for confirming a spectral break and extending a space-based cosmic-ray proton spectrum measurement to overlap with higher energy balloon-borne measurements.

In this thesis, I present the measurement of the cosmic-ray proton spectrum from 54 GeV to 9.5 TeV with the Fermi-LAT. Using the LAT's anti-coincidence detector and tracker as two independent measures of charge, I estimated a residual contamination in our proton data set of less that 5% primarily from cosmic-ray electrons and positrons. The LAT calorimeter provides an energy estimation of the electromagnetic fraction of an induced cosmic-ray proton shower. I use the charge and energy measurements to build instrument response functions, such as acceptance and response for the LAT, and measure cosmic-ray proton flux. I estimate the systematic uncertainties associated with the acceptance and the energy measurement. Using a broken power-law spectrum, I find a primary spectral index of −2.80 ± 0.03, a secondary spectral index of −2.60 ± 0.04, and an energy break of 467 ± 144 GeV. I discuss possible astrophysical and cosmic-ray physics interpretations for the observed spectral break.

Alex Jeffers - October 31, 2016

Dissertation Title: ​​3D MAGNETIC IMAGING USING SQUIDS AND SPIN-VALVE SENSORS

Date and Time: Monday, October 31, 1:00 pm

Location: PHY 0360 (CNAM Conference Room)

Committee Chair: Prof. Frederick Wellstood

Committee: 

Dr. Richard Greene
Dr. Christopher Lobb
Dr. Antonio Orozco
Dr. Ichiro Takeuchi

Abstract:

We have used 2 µm by 4 µm thin-film Cu-Mn-Ir spin-valve sensors and high Tc YBa 2 Cu 3 O 7-x dc SQUIDs to take magnetic images of test samples with current paths that meander between 1 and 5 metallization layers separated by 1 µm to 10 µm vertically. I describe the development and performance of a 3D magnetic inverse for reconstructing current paths from a magnetic image. I present results from the inverse technique that demonstrates the reconstruction of the 3D current paths from magnetic images of samples. This technique not only maps active current paths in the sample but also extracts key parameters such as the layer-to-layer separations. When imaging with 2 µm by 4 µm spin valve sensors I typically applied currents of 1mA at 95 kHz and achieved system noise of about 200 nT for a 3 ms averaging time per pixel. This enabled a vertical resolution of 1 µm and a lateral resolution of 1 µm in the top layers and 3 µm in the bottom layer. For our roughly 30 µm square SQUID sensors, I typically applied currents of 1mA at 5.3 kHz, and achieved system noise of about 200 pT for a 3 ms averaging time per pixel. The higher sensitivity compared to the spin-valve sensor allowed me to resolve more deeply buried current paths.

Shantanu Debnath - September 28, 2016

Dissertation Title: ​​A programmable five-qubit quantum computer using trapped atomic ions

Date and Time: Wednesday, September 28, 1:00 pm

Location: PSC 3150

Committee Chair: Prof. Christopher Monroe

Committee:

Dr. Steve Rolston
Dr. Eite Tiesinga
Dr. Trey Porto
Dr. Andrew Childs

Abstract:

Quantum computers can solve certain problems much more efficiently than conventional classical methods. Driven by this motivation, small scale demonstrations of quantum algorithms have been implemented across several physical platforms where each system have been adapted to run a limited number of instances of a single algorithm. Here, we present the experimental realization of a fully re-configurable quantum computer based on five trapped Yb+ ions that offers the flexibility to be programmed by the user in order to run any quantum algorithm. The computer follows an architecture where high level sequences of standard logic gates are decomposed into fundamental single- and two-qubit quantum gates that are native to the hardware consisting of a linear chain of trapped ions. Each qubit is resolved in space to implement optical addressing for the manipulation and measurement at the single qubit level. By using an array of Raman laser beams that individually address the qubits, a complete set of single-qubit and fully connected two-qubit gates can be implemented where the connectivity between qubits, being defined by the optical fields, can be reconfigured in the software thereby allowing arbitrary gate sequences to be executed. This makes the system a general purpose quantum processor where we implement several algorithms such as the Deutsch-Jozsa and Bernstein-Vazirani algorithm. We further implement a fully coherent five qubit quantum Fourier transform and apply it to solve the quantum period finding and the quantum phase estimation problem. This architecture is also shown to be scalable where the system size can be increased by simply hosting more ions inside a single processor where the number of experimental controls scale favorably.

Xunnong Xu - September 26, 2016

Dissertation Title: Quantum Optics with Optomechanical Systems in the Linear and Nonlinear Regime: With Applications in Force Sensing and Environmental Engineering

Date and Time: Monday, September 26, 2:00 PM

Location: CSS 2115

Committee Chair: Prof. Jay D. Sau

Committee:

Dr. Jacob Taylor
Dr. Mohammad Hafezi
Dr. Victor Yakovenko
Dr. Edo Waks

Abstract:

Optomechanical system, a hybrid system where mechanical and optical degrees of freedom are mutually coupled, is a new platform for studying quantum optics. Many interesting effects arise from linearized optomechanical interaction, such as the dynamical modification of the properties of the mechanical resonator and the modulation of the amplitude and phase of the light coming out of the cavity. When the single-photon optomechanical coupling is comparable to the optical and mechanical loss, it is also possible to study optomechanically induced nonlinear phenomena such as photon-blockade, Kerr nonlinearity, etc. In this talk, we study quantum optics with optomechanical systems both in the linear and nonlinear regime, with emphasis on its applications in force sensing and environmental engineering.

We first propose a mirror-in-the-middle system and show that when driving near optomechanical instability, the optomechanical interaction will generate squeezed states of the output light, which can be used to detect weak forces far below the standard quantum limit. Subsequently, we find that this particular driving scheme can also lead to enhanced optomechanical nonlinearity in a certain regime and by measuring the output field appropriately. We study the photon-blockade effect and discuss the conditions for maximum photon antibunching. We then focus on thermal noise reduction for mechanical resonators, by designing a system of two coupled resonators whose damping is primarily clamping loss. We show that optomechanical coupling to the clamping region leads to a reduction in the temperature and linewidth of the mechanical mode with increasing optical power. We also consider the Brillouin scattering induced optomechanical interaction in ring wave-guide resonators where phonon scattering via impurities is present. We find that it is possible to realize chiral transport behavior of phonons by modifying the phonon environment with optomechanics. We study a simple few-mode theory and it can explain experimental data well. Finally, we study a continuum multi-mode theory and calculate the phonon Green's function using a diagrammatic perturbative expansion, showing that a decrease in the phonon diffusion constant is possible with increasing optical pump power.

Kale Johnson - September 19, 2016

Dissertation Title: Experiments with Trapped Ions and Ultrafast Laser Pulses

Date and Time: Monday, September 19, 8:30 am

Location: PSC 2136

Committee Chair: Prof. Christopher Monroe

Committee:

Dr. William Phillips
Dr. James Williams
Dr. Alexey Gorshkov
Dr. Christopher Davis

Abstract:

Laser-cooled, trapped atomic ions have been one of the most compelling systems for the physical realization of a quantum computer. By applying qubit state dependent forces to the ions, their collective motional modes can be used as a bus to realize entangling quantum gates. Ultrafast state-dependent kicks can provide a universal set of quantum logic operations, in conjunction with ultrafast single qubit rotations, which uses only ultrafast laser pulses. This may present a clearer route to scaling a trapped ion processor. In addition to the role that spin-dependent kicks (SDKs) play in quantum computation, their utility in fundamental quantum mechanics research is also apparent. In this talk, we present a set of experiments which demonstrate some of the principle properties of SDKs including ion motion independence, high speed operations, and multi-qubit entanglement operations with speed that is not fundamentally limited by the trap oscillation frequency. These ultrafast atomic qubit manipulation tools demonstrate inherent advantages over conventional techniques, offering a fundamentally distinct regime of control and speed not previously achievable. Additionally, we use an aberration correction method to produce a diffraction limited spot from an ion in a high numerical aperture system.This allows for the highest position sensitivity of an isolated atom to date.

Wrick Sengupta - August 30, 2016

Dissertation Title: Sub-Alfvenic reduced equations in a tokamak

Date and Time: Tuesday, August 30, 1:00 pm

Location: AVW 3460, Conference Room

Committee Chair: Prof. Adil Hassam

Committee: 

Dr. James Drake
Dr. Jason TenBarge
Dr. William Dorland
Dr. Thomas Antonsen

Abstract:

Magnetized fusion experiments generally perform under conditions where ideal magnetohydrodynamic (MHD) modes are stable. It is therefore desirable to develop a reduced formalism which would order out Alfvenic frequencies. This is challenging because the sub-Alvenic phenomena are sensitive to magnetic geometries. In this work an attempt is made to develop a formalism to study plasma phenomena on time scales much longer than the Alfvenic time scales. In Part I, a reduced set of MHD equations is derived, applicable to large aspect ratio tokamaks. A major advantage is that the resulting system is 2D in space, and the system incorporates self-consistent dynamic Shafranov shifts. A limitation is that the system is valid only in radial domains where the tokamak safety factor q, is close to a rational. Various limits of our equations, including axisymmetric and subsonic limits, are considered. In the tokamak core, the system is well suited as a model to study the sawtooth discharge in the presence of Mercier modes. In Part II, we begin a reduced description of sub Alfvenic phenomena for collisionless kinetic MHD. We study the role of trapped particles dynamics in a collisionless axisymmetric toroidal systems.

Joshua Wood - August 12, 2016

Dissertation Title: An all-sky search for bursts of very high energy gamma-rays with HAWC

Date and Time: Friday, August 12, 10:00 am

Location: PSC 1136

Committee Chair: Prof. Jordan Goodman

Committee: 

Dr. Gregory Sullivan
Dr. Peter Shawhan
Dr. Julie McEnery
Dr. Christopher Reynolds

Abstract:

This dissertation reports on a new measurement for prompt gamma-ray burst (GRB) emission in the largely unexplored very-high energy (VHE) regime. GRBs are the most luminous events in the known universe. They consist of intense gamma-ray flashes coming from cosmological distances and are believed to be produced by two separate progenitor populations, collapsing massive stars and merging compact object binaries, that result in a newly formed accreting black hole. Yet many open questions remain about the physical processes behind their prompt gamma-ray emission despite the observational and theoretical advances made in the nearly 50 years since their discovery.

One major challenge has been to explain the large variety of observed GRB spectra using the most widely accepted emission model of a jetted, relativistic fireball interacting with itself and surrounding interstellar material to form internal and external shocks. While we are certain that synchrotron radiation and inverse-Compton interactions from electrons accelerated at shock fronts accounts for a large portion of the emission, it may be suppressed at the highest energies due to absorption in the emission region. Observations of the highest energy photons therefore provide direct measurements of the characteristics of the emission region.

These observations are extremely difficult to perform with satellite-based experiments due to their compact areas given the relatively low fluxes expected from VHE emission in GRB events. Thus far, only a single cutoff has been measured in the 8 year operation of the Fermi satellite. This highlights the need for ground-based observations of prompt GRB emission as the current generation of ground-based detectors have effective areas for ~100 GeV gamma-rays that are 100x the size of the Fermi satellite or larger. Yet ground-based observations of GRB events, which occur randomly throughout the sky, by Imaging Air Chereknov Telescopes have thus far been impeded by the small field-of-view and low duty cycle associated with this class of instruments.

A new ground-based wide-field extensive air shower array known as the High Altitude Water Cherenkov (HAWC) Observatory promises a new window to monitoring the ~100 GeV gamma-ray sky with the potential for detecting such a cutoff in GRBs. It represents a roughly 15 times sensitivity gain over the previous generation of wide-field gamma-ray air shower instruments and is able to detect the Crab nebula at high significance (>5 sigma) with each daily transit. Its wide field-of-view (~2 sr) and >95% uptime make it an ideal instrument for discovering gamma-ray burst (GRB) emission at ~100 GeV with an expectation for observing ~1 GRB per year based on existing measurements of GRB emission.

An all-sky, self-triggered search for VHE emission produced by GRBs with HAWC has been developed. We present the results of this search on three characteristic GRB emission timescales, 0.2 seconds, 1 second, and 10 seconds, in the first year of the fully-populated HAWC detector which is the most sensitive dataset to date. No significant detections were found, allowing us to place upper limits on the rate of GRBs containing appreciable emission in the ~100 GeV band. These constraints exclude previously unexamined parameter space.

Jacob Smith - August 11, 2016

Dissertation Title: Quantum Thermalization and Localization in a Trapped Ion Quantum Simulator

Date and Time: Thursday, August 11, 1:00 pm

Location: CSS 2115

Committee Chair: Prof. Christopher Monroe

Committee: 

Dr. Jacob Taylor
Dr. Vladimir Manucharyan
Dr. Charles Clark
Dr. Edo Waks

Abstract:

When a system thermalizes it loses all memory of its initial conditions. Even within a closed quantum system, subsystems usually thermalize using the rest of the system as a heat bath. Exceptions to quantum thermalization have been observed, but typically require inherent symmetries or noninteracting particles in the presence of static disorder. The prediction of many-body localization (MBL), in which disordered quantum systems can fail to thermalize despite strong interactions and high excitation energy, was therefore surprising and has attracted considerable theoretical attention. We experimentally generate MBL states by applying an Ising Hamiltonian with long-range interactions and programmably random disorder to ten spins initialized far from equilibrium. Using experimental and numerical methods we observe the essential signatures of MBL: initial state memory retention, Poissonian distributed energy level spacings, and evidence of long-time entanglement growth. Our platform can be scaled to more spins, where detailed modeling of MBL becomes impossible.

Rachel Lee - July 27, 2016

Dissertation Title: Guided Migration and Collective Behavior: Cell Dynamics During Cancer Progression

Date and Time: Wednesday, July 27, 12:30 pm

Location: PSC 1136

Committee Chair: Prof. Wolfgang Losert

Committee:

Dr. Michelle Girvan
Dr. Carole A. Parent
Dr. Jose Helim Aranda-Espinoza
Dr. John Fourkas

Abstract:

This dissertation focuses on gaining understanding of cell migration and collective behavior through a combination of experiment, analysis, and modeling techniques.  Cell migration is a ubiquitous process that plays an important role during embryonic development and wound healing as well as in diseases like cancer, which is a particular focus of this work.  As cancer cells become increasingly malignant, they acquire the ability to migrate away from the primary tumor and spread throughout the body to form metastatic tumors.  During this process, changes in gene expression and the surrounding tumor environment can lead to changes in cell migration characteristics. In this thesis, I analyze how cells are guided by the texture of their environment and how cells cooperate with their neighbors to move collectively. The emergent properties of collectively moving groups are a particular focus of this work as collective cell dynamics are known to change in diseases such as cancer.
 
The internal machinery for cell migration involves polymerization of the actin cytoskeleton to create protrusions that---in coordination with retraction of the rear of the cell---lead to cell motion.  This actin machinery has been previously shown to respond to the topography of the surrounding surface, leading to guided migration of amoeboid cells.  Here we show that epithelial cells on nanoscale ridge structures also show changes in the morphology of their cytoskeletons; actin is found to align with the ridge structures.  The migration of the cells is also guided preferentially along the ridge length.  These ridge structures are on length scales similar to those found in tumor microenvironments and as such provide a system for studying the response of the cells' internal migration machinery to physiologically relevant topographical cues.
 
In addition to sensing surface topography, individual cells can also be influenced by the pushing and pulling of neighboring cells.  The emergent properties of collectively migrating cells show interesting dynamics and are relevant for cancer progression, but have been less studied than the motion of individual cells.  We use Particle Image Velocimetry (PIV) to extract the motion of a collectively migrating cell sheet from time lapse images.  The resulting flow fields allow us to analyze collective behavior over multiple length and time scales.
 
To analyze the connection between individual cell properties and collective migration behavior, we compare experimental flow fields with the migration of simulated cell groups.  Our collective migration metrics allow for a quantitative comparison between experimental and simulated results.  This comparison shows that tissue-scale decreases in collective behavior can result from changes in individual cell activity without the need to postulate the existence of subpopulations of leader cells or global gradients.
 
In addition to tissue-scale trends in collective behavior, the migration of cell groups includes localized dynamic features such as cell rearrangements.  An individual cell may smoothly follow the motion of its neighbors (affine motion) or move in a more individualistic manner (non-affine motion). By decomposing individual motion into both affine and non-affine components, we measure cell rearrangements within a collective sheet.  Finally, finite-time Lyapunov exponent (FTLE) values capture the stretching of the flow field and reflect its chaotic character.
 
Applying collective migration analysis techniques to experimental data on both malignant and non-malignant human breast epithelial cells reveals differences in collective behavior that are not found from analyzing migration speeds alone.  Non-malignant cells show increased cooperative motion on long time scales whereas malignant cells remain uncooperative as time progresses.  Combining multiple analysis techniques also shows that these two cell types differ in their response to a perturbation of cell-cell adhesion through the molecule E-cadherin.  Non-malignant MCF10A cells use E-cadherin for short time coordination of collective motion, yet even with decreased E-cadherin expression, the cells remain coordinated over long time scales.  In contrast, the migration behavior of malignant and invasive MCF10CA1a cells, which already shows decreased collective dynamics on both time scales, is insensitive to the change in E-cadherin expression.

Daniel Barker - July 25, 2016

Dissertation Title: Degenerate Gases of Strontium for Studies of Quantum Magnetism

Date and Time: Monday, July 25, 12:30 pm

Location: PSC 2136

Committee Chair: Prof. Steven Rolston

Committee:

Dr. Gretchen Campbell
Dr. Eite Tiesinga
Dr. Luis Orozco
Dr. John Fourkas

Abstract:

We describe the construction and characterization of a new apparatus that can produce degenerate quantum gases of strontium. The realization of degenerate gases is an important first step toward future studies of quantum magnetism. Three of the four stable isotopes of strontium have been cooled into the degenerate regime. The experiment can make nearly pure Bose-Einstein condensates containing approximately 1x10^4 atoms, for strontium-86, and approximately 4x10^5 atoms, for strontium-84. We have also created degenerate Fermi gases of strontium-87 with a reduced temperature, T/T_F of approximately 0.2. The apparatus will be able to produce Bose-Einstein condensates of strontium-88 with straightforward modifications.
 
We also report the first experimental and theoretical results from the strontium project. We have developed a technique to accelerate the continuous loading of strontium atoms into a magnetic trap. By applying a laser addressing the 3P1 to 3S1 transition in our magneto-optical trap, the rate at which atoms populate the magnetically-trapped 3P2 state can be increased by up to 65%.
 
Quantum degenerate gases of atoms in the metastable 3P0 and 3P2 states are a promising platform for quantum simulation of systems with long-range interactions. We have performed an initial numerical study of a method to transfer the ground state degenerate gases that we can currently produce into one of the metastable states via a three-photon transition. Numerical simulations of the Optical Bloch equations governing the three-photon transition indicate that >90% of a ground state degenerate gas can be transferred into a metastable state.

Aaron Lee - July 19, 2016

Dissertation Title: Engineering a Quantum Many-body Hamiltonian with Trapped Ions

Date and Time: Tuesday, July 19, 1:00 pm

Location: PSC 2136

Committee Chair: Prof. Christopher Monroe

Committee:

Dr. Mohammad Hafezi
Dr. Ian Spielman
Dr. James Williams
Dr. Andrew Childs

Abstract:

While fault-tolerant quantum computation might still be years away, analog quantum simulators offer a way to leverage current quantum technologies to study classically intractable quantum systems. Cutting edge quantum simulators such as those utilizing ultracold atoms are beginning to study physics which surpass what is classically tractable. As the system sizes of these quantum simulators increase, there are also concurrent gains in the complexity and types of Hamiltonians which can be simulated. In this work, I describe advances toward the realization of an adaptable, tunable quantum simulator capable of surpassing classical computation. We simulate long-ranged Ising and XY spin models which can have global arbitrary transverse and longitudinal fields in addition to individual transverse fields using a linear chain of up to 24 Yb+ 171 ions confined in a linear rf Paul trap. Each qubit is encoded in the ground state hyperfine levels of an ion. Spin-spin interactions are engineered by the application of spin-dependent forces from laser fields, coupling spin to motion. Each spin can be read independently using state-dependent fluorescence. The results here add yet more tools to an ever growing quantum simulation toolbox. One of many challenges has been the coherent manipulation of individual qubits. By using a surprisingly large fourth-order Stark shifts in a clock-state qubit, we demonstrate an ability to individually manipulate spins and apply independent Hamiltonian terms, greatly increasing the range of quantum simulations which can be implemented. As quantum systems grow beyond the capability of classical numerics, a constant question is how to verify a quantum simulation. Here, I present measurements which may provide useful metrics for large system sizes and demonstrate them in a system of up to 24 ions during a classically intractable simulation. The observed values are consistent with extremely large entangled states, as much as ~95% of the system entangled. Finally, we use many of these techniques in order to generate a spin Hamiltonian which fails to thermalize during experimental time scales due to a meta-stable state which is often called prethermal. The observed prethermal state is a new form of prethermalization which arises due to long-range interactions and open boundary conditions, even in the thermodynamic limit. This prethermalization is observed in a system of up to 22 spins. We expect that system sizes can be extended up to 30 spins with only minor upgrades to the current apparatus. These results emphasize that as the technology improves, the techniques and tools developed here can potentially be used to perform simulations which will surpass the capability of even the most sophisticated classical techniques, enabling the study of a whole new regime of quantum many-body physics.

Ryan Maunu - July 14, 2016

Dissertation Title: A Search for Muon Neutrinos in Coincidence with Gamma-Ray Bursts in the Southern Hemisphere Sky Using the IceCube Neutrino Observatory

Date and Time: Thursday, July 14, 3:00 pm

Location: PSC 2136

Committee Chair: Prof. Kara Hoffman

Committee:

Dr. Gregory Sullivan
Dr. Peter Shawhan
Dr. Julie McEnery
Dr. Richard Mushotzky

Abstract:

The origin of observed ultra-high energy cosmic rays (UHECRs, energies in excess of 10^19 eV) remains unknown, as extragalactic magnetic fields deflect these charged particles from their true origin. Interactions of these UHECRs at their source would invariably produce high energy neutrinos. As these neutrinos are chargeless and nearly massless, their propagation through the universe is unimpeded and their detection can be correlated with the origin of UHECRs.

Gamma-ray bursts (GRBs) are one of the few possible origins for UHECRs, observed as short, immensely bright outbursts of gamma-rays at cosmological distances. The energy density of GRBs in the universe is capable of explaining the measured UHECR flux, making them promising UHECR sources. Interactions between UHECRs and the prompt gamma-ray emission of a GRB would produce neutrinos that would be detected in coincidence with the GRB's gamma-ray emission.

The IceCube Neutrino Observatory can be used to search for these neutrinos in coincidence with GRBs, detecting neutrinos through the Cherenkov radiation emitted by secondary charged particles produced in neutrino interactions in the South Pole glacial ice. Restricting these searches to be in coincidence with GRB gamma-ray emission, analyses can be performed with very little atmospheric background. Previous searches have focused on detecting muon tracks from muon neutrino interactions from the Northern Hemisphere, where the Earth shields IceCube's primary background of atmospheric muons, or spherical cascade events from neutrinos of all flavors from the entire sky, with no compelling neutrino signal found.

Neutrino searches from GRBs with IceCube have been extended to a search for muon tracks in the Southern Hemisphere in coincidence with 664 GRBs over five years of IceCube data in this dissertation. Though this region of the sky contains IceCube's primary background of atmospheric muons, it is also where IceCube is most sensitive to neutrinos at the very highest energies as Earth absorption in the Northern Hemisphere becomes relevant. As previous neutrino searches have strongly constrained neutrino production in GRBs, a new per-GRB analysis is introduced for the first time to discover neutrinos in coincidence with possibly rare neutrino-bright GRBs. A stacked analysis is also performed to discover a weak neutrino signal distributed over many GRBs.

Results of this search are found to be consistent with atmospheric muon backgrounds. Combining this result with previously published muon neutrino track searches in the Northern Hemisphere, cascade event searches over the entire sky, and an extension of the Northern Hemisphere track search in three additional years of IceCube data that is consistent with atmospheric backgrounds, the most stringent limits yet can be placed on prompt neutrino production in GRBs, which increasingly disfavor GRBs as primary sources of UHECRs in current GRB models.

Anton de la Fuente - July 12, 2016

Dissertation Title: Bootstrapping the 2D Ising Model

Date and Time: Tuesday, July 12, 3:00 pm

Location: PSC 3150

Committee Chair: Prof. Raman Sundrum

Committee:

Dr. Rabindra Mohapatra
Dr. Thomas Cohen
Dr. Zackaria Chacko
Dr. Jonathan Rosenberg

Abstract:

A conformal field theory is so constrained by symmetry that all its correlation functions are completely determined by its set of 2- and 3-point functions, which are in turn determined by a discrete set of numbers, known as the conformal data of the CFT. In order for the conformal data to consistently determine all higher-point functions, they must satisfy a highly nontrivial set of consistency conditions, known as the conformal bootstrap equations.  It has recently been discovered that numerical methods can efficiently identify large regions in the space of conformal data that are inconsistent with the bootstrap equations. Using these methods, we have ruled out a large region in the space of conformal data except for a tiny "island" around the 2D Ising CFT.  This gives a different perspective on the concept of universality in the study of critical phenomena.​

Matthew Adams - July 11, 2016

Dissertation Title: Magnetic and Acoustic Investigations of Turbulent Spherical Couette Flow

Date and Time: Monday, July 11, 4:00 pm

Location: ERF 1207

Committee Chair: Prof. Daniel Lathrop

Committee:

Dr. Vedran Lekic
Dr. James Duncan
Dr. James Drake
Dr. Thomas Antonsen

Abstract:

Generation of magnetic field via flows of conducting fluid, the so-called dynamo effect, is a widespread phenomenon in the universe. The experiments described here are aimed at helping to understand more about the interaction of turbulent flows with magnetic fields, and how this can lead to the spontaneous generation of large-scale magnetic fields. Specifically, the motivation for the experiments is the geodynamo of Earth's outer core, responsible for the Earth's magnetic field. The experiments are designed to be geometrically similar to Earth's core. They consist of an outer spherical shell and an inner sphere concentric with it; the working fluid lies in between them, analogous to the Earth's liquid outer core lying in between the mantle and the solid inner core. The two spheres share a rotation axis, and can be rotated differentially to drive a turbulent shear flow in the fluid. The two experiments are 60cm and 3m in diameter, respectively. For hydromagnetic investigations, liquid sodium is used as the working fluid, and an external magnetic field is applied to the experiment. Liquid sodium brings specific experimental challenges, including its opacity, making determination of the fluid velocities within the experimental volume difficult. We investigate the possibility of using frequency splittings of acoustic modes of the fluid volume to infer azimuthal velocities of the flow. This is analogous to techniques used in the field of helioseismology to determine flow patterns in the solar interior. We used gas (air or nitrogen) as the working fluid in the initial investigations due to the ease of instrumentation, and measured splittings for a variety of different inner and outer rotation rates, in the 60cm radius experiment. Working with our colleagues Vedran Lekic and Anthony Mautino, we use these splittings to put constraints on the azimuthal flow profile. Ongoing hydromagnetic investigations of the 3m experiment also give information about possible flow patterns, and preliminary investigations of acoustics in liquid sodium were carried out.​​

Christopher Verhaaren - June 8, 2016

Dissertation Title: Using the Higgs to Probe Naturalness

Date and Time: Wednesday, June 8, 3:00 pm

Location: PSC 3150

Committee Chair: Prof. Zackaria Chacko

Committee:

Dr. Raman Sundrum
Dr. Sarah Eno
Dr. Kaustubh Agashe
Dr. Alice Mignerey

Abstract:

The mass of the Higgs boson is extremely sensitive to quantum corrections from high mass states, making it 'unnaturally' light in the standard model. This 'hierarchy problem' can be solved by symmetries, which reveal themselves through new particles related, by the symmetry, to standard model fields. The Large Hadron Collider (LHC) can potentially discover these new particles. However, the particle most sensitive to the physics that affects the Higgs mass is the Higgs itself. We show that the Higgs is sometimes the best probe of the hierarchy problem at the LHC and future colliders.

If the top partners carry the color charge of the strong nuclear force, the production of Higgs pairs is affected. However, we show that on general grounds this effect is tightly correlated with single Higgs production, which the LHC is much more sensitive to. The current LHC data then implies that the most we can expect are modest enhancements in di-Higgs production. We verify this result in the context of a simplified supersymmetric model. If the top partners do not carry color charge, their direct production is greatly reduced. Nevertheless, we show that such scenarios can be revealed through Higgs dynamics. We find that many color neutral frameworks leave observable traces in Higgs couplings, which, in some cases, may be the only way to probe these theories at the LHC. Some realizations of the color neutral framework also lead to exotic decays of the Higgs with displaced vertices. These decays are so striking that the projected sensitivity for these searches, at hadron colliders, is comparable to that of searches for colored top partners. Taken together, these three case studies show the efficacy of the Higgs as a probe of naturalness.