## PhD Defenses

- Details
- Published: Wednesday, 20 July 2016 16:22

## Qin Liu - March 14, 2018

Dissertation Title: **LARGE-SCALE NEURAL NETWORK MODELING: FROM NEURONAL MICROCIRCUITS TO WHOLE-BRAIN COMPLEX NETWORK DYNAMICS**

Date and Time: **Wednesday, March 14, 2:00 pm**

Location: **PSC 2136**

Dissertation Committee Chair: **Prof. Steven Anlage**

Committee:

**Dr. Barry Horwitz (advisor)**Dr. Wolfgang Losert

Dr. Edward Ott

Dr. Daniel Butts

Abstract:

Neural networks mediate human cognitive functions, such as sensory processing, memory, attention, etc. Computational modeling has been proved as a powerful tool to test hypothesis of network mechanisms underlying cognitive functions, and to understand better human neuroimaging data. The dissertation presents a large-scale neural network modeling study of human brain visual/auditory processing and how this process interacts with memory and attention.

We first modeled visual and auditory objects processing and short-term memory with local microcircuits and a large-scale recurrent network. We proposed a biologically realistic network implementation of storing multiple items in short-term memory. We then realized the effect that people involuntarily switch attention to salient distractors and are difficult to distract when attending to salient stimuli, by incorporating exogenous and endogenous attention modules. The integrated model could perform a number of cognitive tasks utilizing different cognitive functions by only changing a task-specification parameter. Based on the performance and simulated imaging results of these tasks, we proposed hypothesis for the neural mechanism beneath several important phenomena, which may be tested experimentally in the future.

Theory of complex network has been applied in the analysis of neuroimaging data, as it provides a topological abstraction of human brain. We constructed functional connectivity networks for various simulated experimental conditions. A number of important network properties were studied, including the scale-free property, the global efficiency, modular structure, and explored their relations with task complexity. We showed that these network properties and their dynamics of our simulated networks matched empirical studies, and we were able to relate these network-level phenomena to the underlying model mechanisms, which verifies the validity and importance of our modeling work in testing neural network hypothesis.

## Jason Andrews - February 16, 2018

Dissertation Title: **Study of the decay B+ → K+ π0 at LHCb and mechanical development for the design of the Upstream Tracker**

Date and Time: **Friday, February 16, 1:00 pm**

Location: **PSC 3150**

Dissertation Committee Chair: **Prof. Hassan Jawahery**

Committee:

Dr. Nicholas Hadley

Dr. Zackaria Chacko

Dr. Alberto Belloni

Dr. Richard Mushotzky

Abstract:

The LHCb experiment at the Large Hadron Collider (LHC) is designed to measure the properties of particles containing charm (c) and bottom (b) quarks. This dissertation documents two major studies I have completed, one analyzing data collected by the LHCb detector, and another contributing to the design and development of an extensive upgrade to the detector.

The pattern of CP asymmetry measurements of the B → K π family of decays deviates from expectations derived from the SM, a contradiction known as the "K π puzzle." The present size of the experimental errors are such that more precise measurements in the B+ → K+ π0 decay channel are especially important. An analysis of the B+ → K+ π0 decay using data collected during Run 1 is performed. Despite low reconstruction and trigger efficiencies and enormous combinatorial backgrounds, a signal is found with a statistical significance of 3.7σ. This achievement has led to the creation of a dedicated B+ -> K+ π0 trigger, and has inspired the creation of a number of dedicated triggers for decay modes with similar topologies. A preliminary analysis of data collected during Run 2 demonstrates that the new trigger is a major success, with excellent prospects for making the world’s best measurements in the B+ → K+ π0 decay channel using the entire Run 2 data set.

Run 2 of the LHC will conclude at the end of 2018, and will be followed by Run 3, scheduled to begin in early 2021. In the interim, the LHCb detector will be upgraded to be read-out in real-time at 40 MHz, and to withstand the radiation damage associated with collecting 50 fb^(−1) of integrated luminosity by the conclusion of Run 4. A key part of this upgrade is the design and construction of a new silicon-strip tracking detector—the upstream tracker (UT). Regions at the periphery of the UT suffer from severe electrical and mechanical constraints, making a high-fidelity CAD model a critical element of the design process. The result is a mechanical integration solution that is entirely non-trivial, and which has had significant influences on the UT design. This solution and the constraints that influence it are shown in detail.

## Kiersten Ruisard - February 14, 2018

Dissertation Title: **Design of a Nonlinear Quasi-Integrable Lattice for Resonance Suppression at the University of Maryland Electron Ring**

Date and Time: **Wednesday, February 14, 10:00 am**

Location: **ERF 1207 (IREAP large conference room)**

Dissertation Committee Co-Chairs: **Profs. Thomas Antonsen **and **Timothy Koeth**

Committee:

Dr. Irving Haber

Dr. Brian Beaudoin

Dr. Andrew Baden

Dr. Patrick O'Shea

Abstract:

Conventional particle accelerators use linear focusing forces for transverse confinement. As a consequence of linearity, accelerating rings are sensitive to a myriad resonances and instabilities. At high beam intensities uncontrolled resonance-driven losses can deteriorate beam quality and cause damage or radio-activation in beam line components and surrounding areas. This is currently a major limitation of achievable current densities in state-of-the-art accelerators. Incorporating nonlinear focusing forces into machine design should provide immunity to resonances through nonlinear detuning of particle orbits from driving terms. A theory of nonlinear integrable beam optics is currently being investigated for use in accelerator rings. Such a system has potential to overcome the limits on achievable beam intensity.

This dissertation presents a plan for implementing a proof-of-principle quasi-integrable octupole lattice at the University of Maryland Electron Ring (UMER). UMER is an accelerator platform that supports the research of high-intensity beam transport. In this dissertation, two designs are presented that differ in both complexity and strength of predicted effects. A configuration with a single, relatively long octupole magnet is expected to be more stabilizing than an arrangement of many short, distributed octupoles.

Preparation for this experiment required the development and characterization of a low-intensity regime previously not operated at UMER. Additionally, required tolerances for the control of beam orbit and focusing in the proposed experiments have been determined on the basis of simulated beam dynamics. In order to achieve these tolerances, a new method for improved orbit correction is developed. Finally, a study of resonance-driven losses in the linear UMER lattice is discussed.

## Avinash Kumar - January 26, 2018

Dissertation Title: **Experiments with a superfluid BEC ring**

Date and Time: **Friday, January 26, 1:00 pm**

Location: **PSC 3150**

Dissertation Committee Chair: **Prof. Steven Rolston**

Committee:

**Dr. Gretchen Campbell (advisor)**

Dr. Christopher Lobb

Dr. Charles Clark

Dr. Christopher Jarzynski

Abstract:

The dissertation presents multiple results of our experiments with ring-shaped $^{23}$Na Bose-Einstein condensates. First, we measure the effect of temperature on the lifetime of a quantized, persistent current. We find that the persistent current lifetime decreases when the temperature is increased. We also extract the critical velocity by measuring the size of hysteresis loops. The critical velocity is found to be a strong function of temperature. Second, we implement a new technique of measuring the circulation state of a persistent current in-situ, which is minimally-destructive. This technique uses the Doppler effect. Finally, we study the dynamics of rapidly expanding rings, and explore the analogy between our experimental system and the expansion of the universe.

## Jon Balajthy - January 19, 2018

Dissertation Title: **PURITY MONITORING TECHNIQUES AND ELECTRONIC ENERGY DEPOSITION PROPERTIES IN LIQUID XENON TIME PROJECTION CHAMBERS**

Date and Time:** Friday, January 19, 3:30 pm**

Location: **PSC 3150**

Dissertation Committee Chair: **Prof. Carter Hall**

Committee:

Dr. Elizabeth Beise

Dr. Xiangdong Ji

Dr. Alberto Belloni

Dr. M. Coleman Miller

Abstract:

Currently, one of the most well motivated models of dark matter is the weakly interacting massive particle (WIMP), and the detector technology that is in the best position to observe these WIMPs is the two-phase liquid xenon time projection chamber (TPC). As liquid xenon WIMP detectors grow larger and more sensitive, the requirements placed on their signals and backgrounds become more and more stringent. We develop a technique for measuring the concentration of the radioactive 85Kr isotope in xenon. We show that we are able to detect natural krypton concentrations down to 7.7 ±2.0 parts per quadrillion (ppQ). On the signals side, we provide a measurement of the charge and light yields of beta recoils in liquid xenon. For these measurements, we use 14C and 3H calibration data collected in the LUX detector after the 2014-2016 WIMP-search run was completed. These measurements span from 43 to 491 V/cm in electric field and from 1 to 140 keVee in recoil energy. We also look for a non-statistical shape factor in the 14C spectrum. We observe a spectrum in the LUX data that is consistent with a purely statistical shape, which disagrees with a recent measurement by Kuzminov et al. by 1.8-s. However, pathologies in the LUX signals prevent us from making any strong claims on this topic.

## Jack Wimberley - December 13, 2017

Dissertation Title: **Semitauonic B_c decays and quark flavor identification methods**

Date and Time: **Wednesday, December 13, 1:00 pm**

Location: **PSC 3150**

Dissertation Committee Chair: **Prof. Hassan Jawahery**

Committee:

Dr. Nicholas Hadley

Dr. Sarah Eno

Dr. Kaustubh Agashe

Dr. Eric Slud

Abstract:

The LHCb experiment at the Large Hadron collider is a unique laboratory for studying the properties of heavy quarks. The physics program of the experiment includes studies of CP violation, measurements of CKM matrix parameters, searches for rare decays, quarkonia studies, and other flavor physics, forward physics, and new physics topics.

This dissertation presents an analysis measuring the ratio of the semileptonic branching fractions B(B_c -> J/ψ τ ν) and B(B_c -> J/ψ μ ν) of the doubly-heavy B_c meson, denoted R(J/ψ). Such semitauonic branching fraction measurements have played an increasingly prominent part of B physics research at BaBar, Belle, and LHCb. These measurements are powerful probes of the universality of the couplings of leptons (e, μ, and τ) in electroweak interactions. Currently, measurements of the quantities R(D) and R(D*) by all three experiments are in excess of precise Standard Model predictions.

A second topic of the dissertation is the creation of a new algorithm for the tagging the flavor of neutral mesons in CP violation studies, and a powerful method for calibrating these flavor tagging algorithms via binomial regression. This work ties in to the increasingly prominent use of machine learning techniques in particle physics, and the need for solid understanding of the behavior of their output.

## Stephen Ragole - December 13, 2017

Dissertation Title: **Understanding Phase Transitions, Symmetry Breaking, and Interaction Enhanced Sensing in Optomechanical and Cold Atomic Systems**

Date and Time: **Wednesday, December 13, 2:30 pm**

Location:** 2115 CSS/ATL**

Dissertation Committee Chair: **Prof. Victor Galitski**

Committee:

**Dr. Jacob Taylor (advisor)**

Dr. Gretchen Campbell

Dr. Jay Deep Sau

Dr. Mohammad Hafezi

Abstract:

We focus on the interaction of light and matter in atomic and optomechanical systems. These highly controllable and engineerable systems present access to new regimes and research opportunities that often do not exist outside the laboratory. As such, they frequently depart from more commonplace systems which are well understood. We extend our understanding of thermodynamic phase transitions, spontaneous symmetry breaking, and quantum-enhanced sensing to new regimes.

Traditionally, phase transitions are defined in thermodynamic equilibrium. However, inspired by the success of the phase transition paradigm in non-equilibrium fields, we derive an effective thermodynamics for the mechanical excitations of an optomechanical system. Noting the common frequency separation between optical and mechanical components, we study the dynamics of the mechanical modes under the influence of the steady state of the optical modes. We identify a sufficient set of constraints which allow us to define an effective equilibrium for the mechanical system. We demonstrate these constraints by studying the buckling transition in an optomechanical membrane-in-the-middle system, which spontaneously breaks a parity symmetry. Having established a thermodynamic limit, we characterize the nature of the phase transition, which can change order based on system parameters. We extend our framework, proposing an photonic systems which realizes an SO(N) symmetry breaking transition of the same nature as the membrane-in-the-middle system. While we have treated these systems in the classical limit, their open nature has pronounced effects when other noise sources are suppressed. We study the canonical optomechanical system to unravel the origin of the semiclassical force and potential on the mechanics. We find that this force, while conservative with respect to the mechanics, deeply depends on the quantum back-action due to photon loss from the cavity.

Additionally, we study the ability of cold atoms to sense rotation. We consider bosonic atoms confined to a one-dimensional ring. Employing Luttinger liquid theory to study the excitations, we find that in the strongly-repulsive regime, atomic currents can be manipulated and superposed by controlling a laser barrier. These superpositions provide a Heisenberg-limited rotation sensing method. When we include noise, the precision is reduced, but the performance still surpasses the standard quantum limit. We comment on the applicability of such a sensor for inertial sensing.

## Erin Sohr - December 11, 2017

Dissertation Title: **Student Sensemaking in Quantum Mechanics: Lessons to Teachers from Studies of Groupwork and Representation Use**

Date and Time: **Monday, December 11, 11:00 am**

Location: **PSC 3150**

Dissertation Committee Co-Chairs: **Profs. Andrew Elby** and **Ayush Gupta (advisor)**

Committee:

Dr. Janet Walkoe

Dr. Kara Hoffman

Dr. Patricia Alexander

Abstract:

This dissertation covers two distinct threads of research; both threads focus on understanding student-thinking in quantum mechanics and then draw implications for future research and instruction. The primary goal of this collection of work is, in any way possible, to improve instruction and find ways to better support students in their learning.

The first thread of research focuses on tension negotiation in collaborative group problem-solving. While group-work has become more commonplace in physics classes, this research provides instructors some means of seeing just how complicated group dynamics can be. In particular, I highlight one interactional pattern through which students resolve tension emerging in group interaction by closing conversations or conversational topics. In doing so, students leave some conceptual line of reasoning unresolved. This work provides important insights into helping instructors understand and respond to group dynamics and conversational closings. The second thread of work focuses on flexible representation use. This thread has two similar lines of research. The first focuses on how particular representations (wavefunction and external potential graphs) associated with the infinite-well and finite-well potentials can be used by students as tools to learn with. Adapting these models to new situations can lead to deeper understandings of both the model being adapted and the new situation. In some cases, the process of adaptation is not impeded by the student lacking a sophisticated understanding of the model being adapted.

The second line of research on representation use focuses on the reflexiveness of student inquiry with representations. In reflexive reasoning, the student’s sensemaking shapes, and is shaped by, the representations they draw and animate. This form of inquiry stands in contrast with traditional notions of proficiency in using representations which tend to highlight reproducing standard representational forms and then reading-out information from those forms. In this work, I highlight how this non-linear, reflexive sense-making is supported by the development of coherent, coupled systems of representations and attention to particular figural features, leading to the generation of new meaning.

## Jonathan Larson - December 8, 2017

Dissertation Title: **Innovative Scanning Probe Methods for Energy Storage Science: Elucidating the Physics of Battery Materials at the Nano-to-Microscale**

Date and Time: **Friday, December 8, 9:30 am**

Location: **CHM 0112 (Marker Seminar Room)**

Dissertation Committee Chair: **Prof. Theodore Einstein**

Committee:

Dr. Janice Reutt-Robey

Dr. Ellen Williams

Dr. James Williams

Dr. Sangbok Lee

Abstract:

Energy storage research is uniquely positioned in modern science and technology. Advancements in the field (or lack thereof), will affect the future of humans, ecosystems, environments, and economies in a positive (or negative) way. While there has been decades-long progress in the energy storage solutions of everyday portable electronic devices, major energy storage hurdles persist such as grid-scale storage, and economically palatable, safe vehicular batteries. In an attempt to tackle these massive issues, the research community is looking beyond typical storage concepts, chemistries, electrolytes, and geometries. Many of these approaches make use of nanoscale and mesoscale technologies. For example, one such promising approach replaces conventional planar electrodes with a collection of nanostructures, arraigned in dense mesoscale architectures, aiming to increase key figures of merit, like power density (via nanostructures) and energy density (via dense mesoscale architectures). Regardless of the novel approach, new techniques are needed for characterization and scientific discovery at the nano-to-microscale. In this talk, I will discuss my PhD research which has precisely targeted these needs within the energy storage community. The work has resulted in the devolvement and application of innovative scanning probe approaches for basic energy storage discovery at the nano-to-microscale. Leveraging a new class of scanning probes, invented here, “battery probes” help to enable scanning nanopipette and probe microscopy, pascalammetry with microbattery probes, inverted scanning tunneling spectroscopy, and nanoscale solid-state electrochemistry with nanobattery probes.

## Hilary Hurst - December 7, 2017

Dissertation Title: **Dynamics of Topological Defects in Hybrid Quantum Systems.**

Date and Time: **Thursday, December 7, 1:00 pm**

Location:** PSC 2136**

Dissertation Committee Chair: **Prof. Victor Galitski**

Committee:

Dr. Victor Yakovenko

Dr. Maissam Barkeshli

Dr. Ian Spielman

Dr. John Weeks

Abstract:

This dissertation focuses on dynamics and transport effects of semiclassical topological defects in systems with important quantum degrees of freedom, which we term "hybrid quantum systems". The topological defects under consideration are skyrmions and magnetic vortices in layered heterostructures of three-dimensional (3D) topological insulators (TI) and magnetic materials, and dark solitons in Bose-Einstein condensates (BEC).

We examine the proximity effect between a 3D TI and two types of insulating magnets: a chiral magnet with a single skyrmion in a ferromagnetic background, and an XY-magnet with strong easy-plane anisotropy which undergoes a vortex unbinding transition. The skyrmion magnetic texture leads to confinement of Dirac states at the skyrmion radius, resulting in a charged skyrmion that can be manipulated by an external electric field. We show that the bound states are robust in the presence of an external magnetic field. Magnetic vortices in the XY-magnet affect electronic transport at the TI surface. Scattering at classical magnetic fluctuations influences surface resistivity of the TI, and near the transition temperature we find that the resistivity has a clear maximum and scales linearly with temperature on either side of the transition. We discuss the limits of mapping the TI XY-magnet model to the classic theoretical problem of free Dirac fermions in a random magnetic field.

Secondly, we study dark solitons in a BEC coupled to thermal non-interacting impurity atoms acting as a dissipative bath. We calculate the friction coefficient due to scattering and find that it can be tuned with accessible experimental probes. We develop a general theory of stochastic dynamics of the negative-mass dark soliton and solve the corresponding Fokker-Planck equation exactly. From the time-dependent phase-space probability distribution function we find the soliton can undergo Brownian motion only in the presence of friction and a confining potential. Finally, we numerically study the ground-state properties of a spin-1 BEC gas in the "synthetic dimensions" experimental set-up. Ground state phases depend on the sign of the spin-dependent interaction parameter and the strength of the spin-orbit field. We find "charge"- and spin-density-wave phases related to helical spin order.

## Matthew Reed - December 6, 2017

Dissertation Title: **An Experimental Realization of a Griffiths Phase in 87Rb in Three Dimensions**

Date and Time: **Wednesday, December 6, 12:00 pm**

Location: **PSC 1136**

Dissertation Committee Chair: **Prof. Steven Rolston**

Committee:

Dr. Gretchen Campbell

Dr. Jay Deep Sau

Dr. Ian Spielman

Dr. Mohammad Hafezi

Abstract:

We describe a novel High Bandwidth Arbitrary Lattice Generator (HiBAL) we've created to skirt limits imposed on monochromatic standing waves of light. With its current iteration we can phase and amplitude modulate optical lattices over a broad range of wavevectors simultaneously at MHz frequencies. We characterize its behavior with a multi-Mach-Zehnder interferometer and a 0.5 NA diffraction limited imaging system, both designed and built in-house. We report lattice phase control to within a few parts in a thousand.

Disorder plays an important role in the phase diagrams of many materials. Crystal defects can cause exotic phases to coexist with the mundane in real world systems, and some phase diagrams are even dominated by the effects of disorder. We report the trapping and characterization of a Bose gas in an optical field isotropic in two dimensions and disordered in a third. We evaluate the phase diagram of our system as a function of temperature and disorder depth, and find favorable comparisons with indications of an intermediate Griffiths phase predicted by previous Monte Carlo and Renormalization Group studies separating 2D and 3D superfluid regimes.

Finally, I discuss the possibility of realizing the BKT transition in a non-orientable space. The BKT phase transition an infinite order phase transition in two dimensions from a normal gas to a superfluid mediated by vortices, which are orientable topological phase defects in two dimensions. I discuss the properties of vortices and their intractions on a Mobius strip, and describe how a relay-imaged bichromatic optical potential could be used to form a Mobius strip out of ultracold gases.

## Benjamin Reschovsky - November 20, 2017

Dissertation Title: **Studies of Ultracold Strontium Gases**

Date and Time: **Monday, November 20, 2:00 pm**

Location: **PSC 2136**

Dissertation Committee Chair: **Prof. Steven Rolston**

Committee:

**Dr. Gretchen Campbell (advisor)**

Dr. Trey Porto

Dr. Christopher Monroe

Dr. Amy Mullin

Abstract:

We describe the operation and performance of an ultracold strontium apparatus that is capable of generating quantum degenerate gases. The experiment has produced Bose-Einstein condensates (BECs) of 84Sr and 86Sr as well as degenerate Fermi gases (DFGs) of 87Sr with a reduced temperature of T/TF = 0.2 at a Fermi temperature of TF = 55 nK. Straightforward modifications could be made to allow for isotopic mixtures and BECs of the fourth stable isotope, 88Sr.

We also report on a technique to improve the continuous loading of a magnetic trap by adding a laser tuned to the 3P1 - 3S1 transition. The method increases atom number in the magnetic trap and subsequent cooling stages by up to 65% for the bosonic isotopes and up to 30% for the fermionic isotope of strontium. We optimize this trap loading strategy with respect to laser detuning, intensity, and beam size. To understand the results, we develop a one-dimensional rate equation model of the system, which is in good agreement with the data. We discuss the use of other transitions in strontium for accelerated trap loading and the application of the technique to other alkaline-earth-like atoms.

Finally, we also report on an updated investigation of photoassociation resonances relative to the 1S0 + 3P1 dissassociation limit in bosonic strontium. Multiple new resonances for 84Sr and 86Sr were measured out to binding energies of -5 GHz and several discrepancies in earlier measurements were resolved. These measurements will allow for the development of a more accurate mass-scaled model and a better theoretical understanding of the molecular potentials near the 3P1 state. We also measure the strength of the 84Sr 0u transitions in order to characterize their use as optical Feshbach resonances.

## Andika Putra - November 17, 2017

Dissertation Title: **MAGNETIZED PLANE WAVE AND STRIPE-ORDERED PHASES IN SPIN-ORBIT-COUPLED BOSE GASES**

Date and Time: **Friday, November 17, 9:00 am**

Location: **2115 CSS/ATL**

Dissertation Committee Chair: **Prof. Steven Rolston**

Committee:

**Dr. Ian B. Spielman (advisor)**

Dr. Gretchen Campbell

Dr. Frederick Wellstood

Dr. Mohammad Hafezi

Abstract:

Quantum degenerate gases have provided rich systems to simulate engineered Hamiltonians and to explore quantum many-body problems in laboratory-scale experiments. In this work, we focus our study on spin-orbit-coupled (SOC) Bose-Einstein condensates (BECs) of Rubidium-87 atoms realized using two-photon Raman coupling scheme in which various novel phases are predicted to exist due to competing energies from the atomic internal structure, coupling strength, and many-body collisions.

BECs are observed primarily using the interaction between light and matter, where it is common to probe the atoms with near-resonant light and image their shadow on a camera. This absorption imaging technique measures the integrated column density of the atoms where it is crucial to focus the imaging system. I present a systematic method to bring the ultracold atom systems into an optimal focus using power spectral density (PSD) of the atomic density-density correlation function. The spatial frequency at which the defocus-induced artifacts first appear in the PSD is maximized on focus. The focusing process thus identifies the range of spatial frequencies over which the PSD is uncontaminated by finite-thickness effects.

Next, I describe magnetic phases which exist in spin-1 spin-orbit-coupled condensates in a near-zero temperature. We observe ferromagnetic and unmagnetized phases which are stabilized by the locking between the spin and linear momentum of the system. Our measurements of both the first- and second-order transitions are in agreement with theory.

Finally, I discuss the stripe-ordered phase which occurs in SOC Bose gases favoring the miscibility configuration. The stripe phase is theoretically predicted to have excitation spectrum analogous to that of supersolidity and to exhibit spatial density modulation within specific regions of parameter space. We use the optical Bragg scattering to probe any small density modulation present in the atomic spatial distribution. I present for the very first time the observation of the stripe phase in full phase diagrams. Our measurement results of the phase boundaries are consistent with existing theory and all observations to date.

## Nightvid Cole - November 15, 2017

Dissertation Title: **CYCLOTRON RESONANCE GAIN IN THE PRESENCE OF COLLISIONS**

Date and Time: **Wednesday, November 15, 10:00 am**

Location: **IPT 1116 (IPST Conference Room)**

Dissertation Committee Chair: **Prof. Thomas Antonsen**

Committee:

Dr. Mohammad Hafezi

Dr. Edward Ott

Dr. Thomas E. Murphy

Dr. Wendell T. Hill

Abstract:

The conditions needed for the amplification of radiation by an ensemble of magnetized, relativistic electrons that are collisionally slowing down are investigated. The current study is aimed at extending the work of other researchers in developing solid-state sources of Terahertz radiation. The source type considered here is based on gyrotron-like dynamics of graphene electrons, or it can alternately be viewed as a solid state laser source which uses Landau levels as its band structure and is thus similar to a quantum cascade laser. Such sources are appealing because they offer the potential for a compact, tunable source of Terahertz radiation that could have commercial applications in scanning, communication, or energy transfer. An exploration is undertaken, using linear and nonlinear theories, of the conditions under which such sources might be viable, assuming realistic parameters. Classical physics is used, and the model involves electrons in graphene assumed to be pumped by a laser, follow classical laws of motion with the dissipation represented by a damping force term, and lose energy to the electromagnetic field as well. The graphene is assumed to be in a homogeneous magnetic field, and is sandwiched between two partially-transmissive mirrors so that the device acts as an oscillator.

This thesis incorporates the results of two approaches to the study of the problem. In the first approach, a linear model is derived semi-analytically, which is relevant to the conditions under which there is gain in the device and thus stable operation is possible, versus the regime in which there is no net gain. In the second approach, a numerical simulation is employed to explore the nonlinear regime and saturation behavior of the oscillator. The simulation and the linear model both assume the same original equations of motion for the field and particles that interact self-consistently. The model used here is very simplified, but the aim here is to elucidate the basic principles and scaling behavior of such devices, not necessarily to calculate what the exact dynamics, outputs, and parameters of a fully commercially realized device will be.

## Eric Rosenthal - November 3, 2017

Dissertation Title: **Energy Deposition in Femtosecond Filamentation: Measurements and Applications**

Date and Time: **Friday, November 3, 10:00 am**

Location: **ERF 1207 (IREAP large conference room)**

Dissertation Committee Chair: **Prof. Howard Milchberg**

Committee:

Dr. Phillip Sprangle

Dr. Gregory Nusinovich

Dr. Jared Wahlstrand

Dr. Ki-Yong Kim

Abstract:

Femtosecond filamentation is a nonlinear optical propagation regime of high peak power ultrashort laser pulses characterized by an extended and narrow core region of high intensity whose length greatly exceeds the Rayleigh range corresponding to the core diameter. Providing that a threshold power is exceeded, filamentation can occur in all transparent gaseous, liquid and solid media. In air, filamentation has found a variety of uses, including the triggering of electric discharges, spectral broadening and compression of ultrashort laser pulses, coherent supercontinuum generation, filament-induced breakdown spectroscopy, generation of THz radiation, and the generation of air waveguides. Several of these applications depend on the deposition of energy in the atmosphere by the filament. The main channels for this deposition are the plasma generated in the filament core by the intense laser field and the rotational excitation of nitrogen and oxygen molecules. The ultrafast deposition acts as a delta function-like pressure source to drive a hydrodynamic response in the air. This thesis experimentally demonstrates two applications of the filament-driven hydrodynamic response. One application is the ‘air waveguide’, which is shown to either guide a separately injected laser pulse, or act as a remote collection optic for weak optical signals. The other application is the high voltage breakdown of air, where the effect of filament-induced plasmas and hydrodynamic response on the breakdown dynamics is elucidated in detail. In all of these experiments, it is important to understand quantitatively the laser energy absorption; detailed absorption experiments were performed as a function of laser parameters. Finally, as check on simulations of filament propagation and energy deposition, we measured the axially resolved energy deposition of a filament; in the simulations, this profile is quite sensitive to the choice of the nonlinear index of refraction (n2). We found that using our measured values of n2 in the propagation simulations results in an excellent fit to the measured energy deposition profiles.

## Renxiong Wang - October 25, 2017

Dissertation Title: **SEARCH FOR NATURAL OCCURRING SUPERCONDUCTIVITY AND NOVEL PHENOMENA: MAGNETIC TRANSITIONS IN NATURAL TRANSITION METAL COMPOUNDS**

Date and Time: **Wednesday, October 25, 1:00 pm**

Location: **PHY 2126**

Dissertation Committee Chair: **Prof. Johnpierre Paglione**

Committee:

Dr. Richard Greene

Dr. Nicholas Butch

Dr. Efrain Rodriguez

Dr. Ichiro Takeuchi

Abstract:

Transition metal chalcogenides and transition metal arsenides are important families of natural mineral compounds widely distributed in the natural world. With similar structural and electronic properties of transition metal oxides, natural transition metal compounds are expected to have similar novel phenomena. With an ongoing project for searching natural superconductors in collaboration with Department of Mineral Science, Smithsonian National Museum of Natural History, we had a chance to investigate several natural minerals from the Smithsonian Museum in order to study previously unexpected naturally occurring mineral compounds for interesting ground states.

We found several interesting magnetic transitions in these natural occurring mineral samples. Some of the magnetic transitions are not reported, some of the transitions are associated with other unreported novel quantum phenomena. In this thesis, I will discuss Bornite (Cu5FeS4), Berthierite (FeSb2S4), Nagyagite (Pb5Au(Te,Sb)4S5 8), Maucherite (Ni11As8) and related experiments in detail.

Bornite (Cu5FeS4) has a semiconductor-insulator transition accompanied with an antiferromagnetic transition. As shown by our ability to tune the transition temperature and low-temperature metallicity by applying external pressure, Bornite may be a good candidate for Mott system and searching new superconductors.

Berthierite (FeSb2S4) is a quasi-1-dimensional antiferromagnet. With strong anisotropic physical properties, berthierite may provide a very good system for understanding the low dimensional magnetic material.

A Ferromagnetic order was found in natural Nagyagite (Pb5Au(Te,Sb)4S5 8) samples. The magnetic order, the weak anti-localization property with strong spin-orbital coupling and the 2-dimensional structure of this compound makes it a very interesting system for realizing topological properties in a natural compound.

The magnetic order and transitions in both natural and synthetic Maucherite (Ni11As8) samples show interesting finite-size scale effect. It gives us a different approach to understand the differences in some physical properties between natural and synthetic compounds.

Also, we will present a summary of other magnetic transitions and magnetic properties of more than 40 distinct minerals for this study and show the relation and similarities between strongly correlated transition metal oxide materials and other quantum materials. We will also make a list of other transition metal minerals that are worthy of investigation based on our research experience.

## David Wong-Campos - October 16, 2017

Dissertation Title: **DEMONSTRATION OF A QUANTUM GATE WITH ULTRAFAST LASER PULSES**

Date and Time: **Monday, October 16, 3:00 pm**

Location: **PSC 2136**

Dissertation Committee Chair: **Prof. Christopher Monroe**

Committee:

Dr. Alexey Gorshkov

Dr. Mohammad Hafezi

Dr. Luis A. Orozco

Dr. Howard Milchberg

Abstract:

One of the major problems in building a quantum computer is the development of scalable and robust methods to entangle many qubits. Quantum computers based on trapped atomic ions are one of the most mature and promising platforms for quantum information processing, exhibiting excellent coherence properties, near-perfect qubit detection efficiency and high-fidelity entangling gates. Entangling operations between multiple ions in a chain typically rely on qubit state-dependent forces that modulate their Coulomb-coupled normal modes of motion. However, scaling these operations to large qubit numbers in a single chain must account for the increasing complexity of the normal mode spectrum, and can result in a gate time slowdown or added complexity of the control forces. In this thesis, I present an alternative route to the scalability problem using optical interactions faster than any state evolution. The experiments shown here represent a proof of principle for quantum manipulation of atoms in the strong coupling regime. This work relies on spin dependent forces (SDK) with short laser pulses and use it as our fundamental building block for thermometry and non-trivial motional state preparation. Together with a robust stabilization of the ion trap and high light collection efficiency, we demonstrate two-atom entanglement with ten ultrafast pulses. Due to the nature of the interaction, the demonstrated entangling operation can be made arbitrarily fast only limited by laser engineering.

## Ayoti Patra - September 15, 2017

Dissertation Title: **Bridging quantum, classical and stochastic shortcuts to adiabaticity**

Date and Time: **Friday, September 15, 1:00 pm**

Location: **IPT 1116** (IPST conference room)

Dissertation Committee Chair: **Prof. Christopher Jarzynski**

Committee:

Dr. Alexey Gorshkov

Dr. Rajarshi Roy

Dr. Victor Yakovenko

Dr. Perinkulam Krishnaprasad

Abstract:

Adiabatic invariants -- quantities that are preserved under the slow driving of a system's external parameters -- are important in classical mechanics, quantum mechanics and thermodynamics. Adiabatic processes allow a system to be guided to evolve to a desired final state. However, the slow driving of a quantum system makes it vulnerable to environmental decoherence, and for both quantum and classical systems, it is often desirable and time-efficient to speed up a process. Shortcuts to adiabaticity are strategies for preserving adiabatic invariants under rapid driving, typically by means of an auxiliary field that suppresses excitations, otherwise generated during rapid driving. Several theoretical approaches have been developed to construct such shortcuts. In this dissertation we focus on two different approaches, namely counterdiabatic driving and fast-forward driving, which were originally developed for quantum systems. The counterdiabatic approach introduced independently by Dermirplak and Rice [J. Phys. Chem. A, 107:9937, 2003], and Berry [J. Phys. A: Math. Theor., 42:365303, 2009] formally provides an exact expression for the auxiliary Hamiltonian, which however is abstract and difficult to translate into an experimentally implementable form. By contrast, the fast-forward approach developed by Masuda and Nakamura [Proc. R. Soc. A, 466(2116):1135, 2010] provides an auxiliary potential that may be experimentally implementable but generally applies only to ground states.

The central theme of this dissertation is that classical shortcuts to adiabaticity can provide useful physical insights and lead to experimentally implementable shortcuts for analogous quantum systems. We start by studying a model system of a tilted piston to provide a proof of principle that quantum shortcuts can successfully be constructed from their classical counterparts. In the remainder of the dissertation, we develop a general approach based on flow-fields which produces simple expressions for auxiliary terms required for both counterdiabatic and fast-forward driving. We demonstrate the applicability of this approach for classical, quanutum as well as stochastic systems. We establish strong connections between counterdiabatic and fast-forward approaches, and also between shortcut protocols required for classical, quantum and stochastic systems. In particular, we show how the fast-forward approach can be extended to highly excited states of quantum systems.

## Joseph L. Garrett - August 18, 2017

Dissertation Title: **THE EFFECTS OF GEOMETRY AND PATCH POTENTIALS ON CASIMIR FORCE MEASUREMENTS**

Date and Time: **Friday, August 18, 10:00 am**

Location: **ERF 1207 (IREAP large conference room)**

Dissertation Committee Chair: **Prof. Jeremy Munday**

Committee:

Dr. Ian Applebaum

Dr. Victor Galitski

Dr. Min Ouyang

Dr. Jan Sengers

Abstract:

Electromagnetic fluctuations of the quantum vacuum cause an attractive force between surfaces, called the Casimir force. In this dissertation, the first Casimir force measurements between two gold-coated spheres are presented. The proximity force approximation (PFA) is typically used to compare experiment to theory, but it is known to deviate from the exact calculation far from the surface. Bounds are put on the size of possible deviations from the PFA by combining several sphere-sphere and sphere-plate measurements.

Electrostatic patch potentials have been postulated as a possible source of error since the first Casimir force measurements sixty years ago. Over the past decade, several theoretical models have been developed to characterize how the patch potentials contribute an additional force to the measurements. In this dissertation, Kelvin probe force microscopy (KPFM) is used to determine the effect of patch potentials on both the sphere and the plate. Patch potentials are indeed present on both surfaces, but the force calculated from the patch potentials is found to be much less than the measured force. In order to better understand how KPFM resolves patch potentials, the artifacts and sensitivities of several different KPFM implementations are tested and characterized. In addition, we introduce a new technique, called tunable spatial resolution (TSR) KPFM, to control resolution by altering the power-law separation dependence of the KPFM signal.

## Tian Li - August 10, 2017

Dissertation Title: **Optical properties of a quantum-noise limited phase-sensitive amplifier**

Date and Time: **Thursday, August 10, 2:00 pm**

Location: **PSC 2136**

Dissertation Committee Co-Chairs: **Prof. Steven Rolston **and **Prof. Paul Lett**

Committee:

Dr. William Phillips

Dr. Gretchen Campbell

Dr. Julius Goldhar

Abstract:

This thesis is a summary of investigations on the optical properties of the phase-sensitive and phase-insensitive amplifiers. Both optical amplifiers are implemented using four-wave mixing in Rb 85 atomic vapor based on a double-lambda level scheme.

We first study the effects of input phase and amplitude modulation on the output of a quantum-noise-limited phase-sensitive amplifier (PSA). We investigate the dependence of phase modulation imposed on a signal by an acousto-optical modulator on its alignment, and demonstrate a novel approach to quantifying the phase modulation by using the PSA as a diagnostic tool. We then use this method to measure the alignment-dependent phase modulation produced by an optical chopper, which arises due to diffraction effects as the chopper blade passes through the optical beam.

The phase-insensitive amplifier (PIA) has been proven to be a reliable source of two-mode squeezed light, and of quantum entanglement. In this dissertation, the PIA is utilized to generate a two-mode squeezed state, i.e., a twin-beam state, and is also used as a gain-assisted anomalous dispersion medium. We experimentally demonstrate the ability of a PSA to pre-amplify quantum correlations in twin light beams before degradation due to loss and detector inefficiency. By including a PSA before loss, one is able to preserve the correlations as well as the two-mode squeezing level. We compare the results to simulations employing a simple quantum-mechanical model and find a good agreement.

We have demonstrated that the cross-correlation between the two modes of a bipartite entangled state can be advanced by propagation through a fast-light medium. The extra noise added by a PIA has been speculated to be the mechanism that limits the advance of entanglement, preventing the mutual information from traveling superluminally. As an extension of this phase-insensitive gain assisted anomalous dispersion investigation, we explore the advance and delay of information transmitted through the PSA. We start with a two-mode squeezed state created by the PIA and measure the mutual information shared by the correlated quadratures. We then pass one of these two modes through a PSA and investigate the shift of the mutual information as a function of the PSA phase. In the case of a PSA, it is well known that no extra noise will be added to the quadrature with the correct input phase (e.g., the quadrature with the maximal amplification or the maximal deamplification). We find that there is no dispersion-like behavior at these two phases, however, the peak of mutual information could either be delayed or advanced at any other phase. We also observe an almost identical behavior when we input an amplitude modulated signal to the PSA. We are able to explain the physics of this "quasi-fast-and-slow-light" behavior utilizing a theoretical framework with distributed gain on the carrier and both positive and negative side bands but with distributed loss only on the negative side band. We obtain a good agreement between the experimental results and the theoretical simulations.

## Ranchu Mathew - August 10, 2017

Dissertation Title: **Classical and quantum dynamics of Bose-Einstein condensates**

Date and Time: **Thursday, August 10, 1:00 pm**

Location: **CSS 2115**

Dissertation Committee Chair: **Prof. Jay Deep Sau**

Committee:

**Dr. Eite Tiesinga (advisor)**

Dr. Jacob Taylor

Dr. William Dorland

Dr. Dionisios Margetis

Abstract:

After the first experimental realization of a Bose-Einstein condensate (BEC) in 1995, BECs have become a subject of intense experimental and theoretical study. In this dissertation, I present our results on the classical and quantum dynamics of BECs at zero temperature under different scenarios.

First, I consider the analog of slow light in the collision of two BECs near a Feshbach resonance. The scattering length then becomes a function of the collision energy. I derive a generalization of the Gross-Pitaevski equation for incorporating this energy dependence. In certain parameter regimes, the group velocity of a BEC traveling through another BEC decreases. I also study the feasibility of an experimental realization of this phenomena.

Second, I analyze an experiment in which a BEC in a ring-shaped trap is stirred by a rotating barrier. The phase drop across and current flow through the barrier is measured from spiral-shaped density profiles created by interfering the BEC in the ring-shaped trap and a concentric reference BEC after release from all trapping potentials. I show that a free-particle expansion is sufficient to explain the origin of the spiral pattern and relate the phase-drop to the geometry of a spiral. I also bound the expansion times for which the phase-drop can be accurately determined and study the effect of inter-atomic interactions on the expansion time scales.

Third, I study the dynamics of few-mode BECs when they become dynamically unstable after preparing an initial state at a saddle point of the Hamiltonian. I study the dynamics within the truncated Wigner approximation (TWA) and find that due to phase-space mixing, the expectation value of an observable relaxes to a steady-state value. Using the action-angle formalism, we derive analytical expressions for the steady-state value and the time evolution towards this value. We apply these general results to two systems: a condensate in a double-well potential and a spin-1 (spinor) condensate.

Finally, I study quantum corrections beyond the TWA in the semiclassical limit. I derive general expressions for the dynamics of an observable by using the van Vleck-Gutzwiller propagator and find that the interference of classical paths leads to non-perturbative corrections. As a case study, I consider a single-mode nonlinear oscillator; this system displays collapse and revival of observables. We find that the interference of classical paths, which is absent in the TWA, leads to revivals.

## Joyce Coppock - August 1, 2017

Dissertation Title: **Optical and magnetic measurements of a levitated, gyroscopically stabilized graphene nanoplatelet**

Date and Time: **Tuesday, August 1, 2:00 pm**

Location: **PSC 3150**

Dissertation Committee Chair: **Prof. Frederick Wellstood**

Committee:

Dr. Richard Greene

Dr. Christopher Lobb**Dr. Bruce Kane (advisor)**

Dr. Ichiro Takeuchi

Abstract:

Levitation of nanoscale particles is an increasingly popular technique in studies ranging from the investigation of material properties to fundamental tests of quantum mechanics. Two-dimensional materials have been extensively studied while attached to substrates, but have rarely been levitated. This work will discuss the design of a system for levitating a charged, micron-scale, multilayer graphene nanoplatelet in a quadrupole electric field trap in high vacuum, enabling sensitive mechanical and magnetic measurements.

The levitated nanoplatelet is gyroscopically stabilized by locking its frequency of rotation to an applied radio frequency (rf) electric field. The stabilized nanoplatelet is extremely sensitive to external torques, and the residual slow dynamics of the direction of the axis of rotation are determined by an applied magnetic field. Optical data on the interaction of the platelet with the magnetic field are presented. Two mechanisms of interaction are observed: a diamagnetic polarizability and a magnetic moment proportional to the frequency of rotation.

## Chang-Hun Lee - July 26, 2017

Dissertation Title: **Left-right symmetric model and its TeV-scale phenomenology**

Date and Time: **Wednesday, July 26, 1:00 pm**

Location: **PSC 3150**

Dissertation Committee Chair: **Prof. Rabindra Mohapatra**

Committee:

Dr. Raman Sundrum

Dr. Sarah Eno

Dr. Kaustubh Agashe

Dr. Niranjan Ramachandran

Abstract:

The Standard Model of particle physics is a chiral theory with a broken parity symmetry, and the left-right symmetric model is an extension of the SM with the parity symmetry restored at high energies. Its extended particle content allows us not only to find the solution to the parity problem of the SM but also to solve the problem of understanding the neutrino masses via the seesaw mechanism. If the scale of parity restoration is in the few TeV range, we can expect new physics signals that are not present in the Standard Model in planned future experiments. We investigate the TeV-scale phenomenology of the various classes of left-right symmetric models, focusing on the charged lepton flavour violation, neutrinoless double beta decay, electric dipole moments of charged leptons, and leptogenesis.

## Sungwoo Hong - July 13, 2017

Dissertation Title: **A NATURAL EXTENSION OF MINIMAL WARPED HIGHER-DIMENSIONAL COMPACTIFICATIONS: THEORY AND PHENOMENOLOGY**

Date and Time: **Thursday, July 13, 11:00 am**

Location: **PSC 3150**

Dissertation Committee Chair: **Prof. Kaustubh Agashe**

Committee:

Dr. Raman Sundrum

Dr. Rabindra Mohapatra

Dr. Sarah Eno

Dr. Zackaria Chacko

Dr. Richard Wentworth

Abstract:

Warped higher-dimensional compactifications with “bulk” standard model, or their AdS/CFT dual as the purely 4D scenario of Higgs compositeness and partial compositeness, offer an elegant approach to resolving the electroweak hierarchy problem as well as the origins of flavor structure. However, low-energy electroweak/flavor/CP constraints and the absence of non-standard physics at LHC Run 1 suggest that a “little hierarchy problem” remains, and that the new physics underlying naturalness may lie out of LHC reach. Assuming this to be the case, we show that there is a simple and natural extension of the minimal warped model in the Randall-Sundrum framework, in which matter, gauge and gravitational fields propagate modestly different degrees into the IR of the warped dimension, resulting in rich and striking consequences for the LHC (and beyond).

The LHC-accessible part of the new physics is AdS/CFT dual to the mechanism of “vectorlike confinement”, with TeV-scale Kaluza-Klein excitations of the gauge and gravitational fields dual to spin-0,1,2 composites. Unlike the minimal warped model, these low-lying excitations have predominantly flavor-blind and flavor/CP-safe interactions with the standard model. In addition, the usual leading decay modes of the lightest KK gauge bosons into top and Higgs bosons are suppressed. This effect permits erstwhile subdominant channels to become significant. These include flavor-universal decays to SM fermions and Higgs bosons, and a novel channel — decay to a radion and a SM gauge boson, followed by radion decay to a pair of SM gauge bosons.

Remarkably, this scenario also predicts small deviations from flavor-blindness originating from virtual effects of Higgs/top compositeness at ∼ O(10) TeV, with subdominant resonance decays into Higgs/top-rich final states, giving the LHC an early “preview” of the nature of the resolution of the hierarchy problem. Discoveries of this type at LHC Run 2 would thereby anticipate (and set a target for) even more explicit explorations of Higgs compositeness at a 100 TeV collider, or for next-generation flavor tests.

## Nihal Jhajj - July 7, 2017

Dissertation Title: **HYDRODYNAMIC AND ELECTRODYNAMIC IMPLICATIONS OF OPTICAL FEMTOSECOND FILAMENTATION**

Date and Time: **Thursday, July 7, 10:00 am**

Location: **ERF 1207 (IREAP large conference room)**

Dissertation Committee Chair: **Prof. Howard Milchberg**

Committee:

Dr. Phillip Sprangle

Dr. James Drake

Dr. Daniel Lathrop

Dr. Ki-Yong Kim

Abstract:

Filamentation is a nonlinear propagation mode of high peak power optical pulses in dielectric media, where nonlinearities arising from the interaction between light and matter can overcome diffraction, resulting in the self-channeling of the pulse. The large optical field present in filaments enables efficient nonlinear conversion for electromagnetic sources spanning from terahertz to x-rays. Filamentation in air is of particular interest for remote applications, where filaments are unique in their ability to deliver high field intensities (up to 10^14 W/cm^2) at kilometer distances, allowing for ranged applications such as LIDAR and laser induced breakdown spectroscopy. This dissertation presents research exploring two topics: i) how to use filaments to inscribe optical guiding structures into the air with millisecond lifetimes, enabling the generation of line-of-sight atmospheric waveguides, and ii), the discovery of the spatiotemporal optical vortex (STOV), a new and previously unobserved type of optical vortex present in all filamenting beams.

## George Hine - May 25, 2017

Dissertation Title: **COMPACT LASER-DRIVEN ELECTRON AND PROTON ACCELERATION WITH LOW ENERGY LASERS**

Date and Time: **Thursday, May 25, 3:30 pm**

Location: **ERF 1207 (IREAP large conference room)**

Dissertation Committee Chair: **Prof. Howard Milchberg**

Committee:

Dr. Gretchen Campbell

Dr. Phillip Sprangle

Dr. Timothy Koeth

Dr. Ki-Yong Kim

Abstract:

Laser-driven particle accelerators offer many advantages over conventional particle accelerators. The most significant of these is the magnitude of the accelerating gradient and, consequently, the compactness of the accelerating structure. In this dissertation, experimental and computational advances in laser-based particle acceleration in three intensity regimes are presented. All mechanisms investigated herein are accessible by "tabletop" ultrashort terawatt-class laser systems found in many university labs, with the intention of making them available to more compact and high repetition rate laser systems. The first mechanism considered is the acceleration of electrons in a preformed plasma "slow-wave" guiding structure. Experimental advances in the generation of these plasma guiding structures are presented. The second mechanism is the laser-wakefield acceleration of electrons in the self-modulated regime. A high-density gas target is implemented experimentally leading to electron acceleration at low laser pulse energy. Consequences of operating in this regime are investigated numerically. The third mechanism is the acceleration of protons by a laser-generated magnetic structure. A numerical investigation is performed identifying operating regimes for experimental realizations of this mechanism.

## Jacob Tosado - May 11, 2017

Dissertation Title: **Investigation of Graphene and other Low Dimensional Materials**

Date and Time: **Thursday, May 11, 4:00 pm**

Location: **PSC 3402**

Dissertation Committee Chair: **Prof. Ellen Williams**

Committee:

Dr. Min Ouyang

Dr. Theodore Einstein

Dr. Michael Fuhrer

Dr. Janice Reutt-Robey

Abstract:

This thesis describes experiments to characterize defects in two-dimensional materials and understand their effect on electrical conductivity. Defects limit the electrical conductivity through a material by scattering electrons. Understanding the physics of defects is therefore essential to building materials and structures with novel electronic properties. This dissertation has focused on low dimensional materials because they are simple thereby allowing for more advanced theory and they will act as a foundation for understanding higher dimensional systems.

High resolution x-ray photoelectron spectroscopy (XPS) and near edge x-ray absorption fine structure spectroscopy (NEXAFS) were used to determine the character of vacancy defects in graphene. Vacancies were induced in graphene on a thermally oxidized silicon substrate using argon ion bombardment. XPS of the carbon 1s core level of pristine graphene shows a C 1s spectrum consistent with a single C 1s peak broadened both instrumentally and by a Doniach-Sunjic type effect. As defects are created, the resulting spectrum is deconvolved into two peaks. The first retains the same spectral width as that for the pristine graphene but with a reduced intensity. The second peak, which is broader and at a slightly higher binding energy (~200 meV), increases in intensity with increasing defect concentration. This second peak is identified as the experimental XPS signature of defective graphene. The observation is somewhat at odds with theoretical calculations of XPS spectra for graphene with various vacancy arrangements, which generally produce C 1s peaks shifted to lower binding energy. Instead, the emergence of this second peak, together with the emergence of a single sharp resonance seen near the vacuum level in the NEXAFS spectra, is interpreted as a distribution of molecular-like states forming on the surface.

Preliminary efforts were made to characterize defects in semiconducting single layer MoS2 using scanning tunneling microscopy (STM) and spectroscopy (STS). Techniques for obtaining a clean MoS2 surface suitable for ultra-high vacuum STM were developed, and preliminary characterization of the single layer WS2 surface by STM and STS was carried out. The local density of states of MoS2, as measured by STS, shows the semiconducting bandgap as well as signatures of donor and acceptor states within the gap.

## Gina Quan - April 27, 2017

Dissertation Title: **Becoming a Physicist: How Identities and Practices Shape Physics Trajectories**

Date and Time: **Thursday, April 27, 2:30 pm**

Location: **PHY 1305F (the Toll Room)**

Dissertation Committee Co-Chairs: **Prof. Andrew Elby and Prof. Chandra Turpen**

Committee:

Dr. Edward Redish

Dr. Ayush Gupta

Dr. James Williams

Dr. Derek Richardson

Abstract:

This dissertation studies the relationships and processes which shape students' participation within the discipline of physics. Studying this early disciplinary participation gives insight to how students are supported in or pushed out of physics, which is an important step in cultivating a diverse set of physics students. This research occurs within two learning environments that we co-developed: a physics camp for high school girls and a seminar for undergraduate physics majors to get started in physics research. Using situated learning theory, we conceptualized physics learning to be intertwined with participation in physics practices and identity development. This theoretical perspective draws our attention to relationships between students and the physics community. Specifically, we study how students come to engage in the practices of the community and who they are within the physics community. We find that students' interactions with faculty and peers impact the extent to which students engage in authentic physics practices. These interactions also impact the extent to which students develop identities as physicists. We present implications of these findings for the design of physics learning spaces. Understanding this process of how students become members of the physics community will provide valuable insights into fostering a diverse set of successful trajectories in physics.

## Kenneth Wright - April 24, 2017

Dissertation Title: **MANIPULATION OF THE QUANTUM MOTION OF TRAPPED ATOMIC IONS VIA STIMULATED RAMAN TRANSITIONS**

Date and Time: **Monday, April 24, 12:15 pm**

Location: **CSS 2115 (Atlantic Building)**

Dissertation Committee Chair: **Prof. Christopher Monroe**

Committee:

Dr. Gretchen Campbell

Dr. Luis Orozco

Dr. James Williams

Dr. Andrew Childs

Abstract:

Trapped ions have been a staple resource of quantum simulation for the past several decades. By taking advantage of the spin motion coupling provided by the Coulomb interaction, trapped ions have been used to study quantum phase transitions of highly frustrated spins, many body localization, as well as discrete time crystals. However, all of these simulations involve decoupling the ion motion from spin at the end of the experimental procedure. Here we present progress towards driving bosonic interference between occupied phonon modes of vibration.

This thesis details a tool box for manipulating the motional states of a chain of trapped ions. Taking advantage of spin motion interaction of tightly trapped chains of Yb ions with two photon Raman transition, we will show how to prepare a specific number state of a given normal mode of motion. This is achieved without traditional individual addressing but instead by using composite pulse sequences and ion transport. This will involve a stage of quantum state distillation, and we also show preservation of phonon and spin coherence after this distillation step. This Fock state preparation sets the stage to observe bosonic interference of different phonon modes.

We will use stimulated Raman transitions to create a parametric drive; this drive will couple different normal modes of motion. To observe the bosonic nature of the phonons, we perform a Hong-Ou-Mandel (HOM) interference experiment on two singly occupied normal modes. We use the same spin motion coupling to read out the spin states of individual ions as a witness for this interaction. We also describe a process to use stimulated rapid adiabatic passage (STIRAP) to read out normal mode occupation. The toolbox presented here will be useful for future experiments towards boson sampling using trapped ions.

## Nathaniel Steinsultz - April 10, 2017

Dissertation Title: **NITROGEN-VACANCY COUPLING IN NANODIAMOND HYBRID NANOSTRUCTURES**

Date and Time: **Monday, April 10, 2:00 pm**

Location: **PHY 1305F **(John S. Toll Physics Building)

Committee Chair: **Prof. Min Ouyang**

Committee:

Dr. Steven Anlage

Dr. Christopher Lobb

Dr. Luis Orozco

Dr. John Cumings

Abstract:

Nitrogen-vacancy centers (NVs) are an atomic defect in diamond which possess remarkable fluorescence and spin properties which can be used for multiple metrological applications, particularly when the NV is hosted in nanodiamond, which can be easily integrated with a variety of nanoscale systems. A new class of nanodiamond hybrid nanostructures was developed using bottom-up synthesis methods. In this work, coupling between NV centers and plasmonic, excitonic and magnetic nanoparticles in these nanodiamond hybrid nanostructures is investigated using fluorescence lifetime measurements, spin relaxometry measurements and modeled using finite element method (FEM) and Monte Carlo simulations. This work not only characterizes the properties of these nanodiamond-hybrid nanostructures but also facilitates design guidelines for future hybrid structures with enhanced metrological and imaging capabilities.

## Setiawan - April 7, 2017

Dissertation Title: **Topological Superconductivity and Majorana Zero Modes**

Date and Time: **Friday, April 7, 1:00 pm**

Location: **PHY 2205 **(John S. Toll Physics Building)

Committee Chair: **Prof. Sankar Das Sarma**

Committee:

Dr. Jay Deep Sau

Dr. Maissam Barkeshli

Dr. Theodore Einstein

Dr. Christopher Jarzynski

Abstract:

Recent years have seen a surge interest in realizing Majorana zero modes in condensed matter system. Majorana zero modes are zero-energy quasiparticle excitations which are their own anti-particles. The topologically degenerate Hilbert space and non-Abelian statistics associated with Majorana zero modes renders them useful for realizing topological quantum computation. These Majorana zero modes can be found at the boundary of a topological superconductor. While preliminary evidence for Majorana zero modes in form of zero-bias conductance peaks have already been observed, confirmatory signatures of Majorana zero modes are still lacking.

In this thesis, we theoretically investigate several signatures of Majorana zero modes, thereby suggesting improvement and directions that can be pursued for an unambiguous identification of the Majorana zero modes. We begin by studying analytically the differential conductance of the normal-metal--topological superconductor junction across the topological transition within the Blonder-Tinkham-Klapwijk formalism. We show that despite being quantized in the topological regime, the zero-bias conductance only develops as a peak in the conductance spectra for sufficiently small junction transparencies, or for small and large spin-orbit coupling strength. We proceed to investigate the signatures of the Majorana zero modes in superconductor--normal-metal--superconductor junctions and show that the conductance quantization in this junction is.not robust against increasing junction transparency. Finally, we propose a dynamical scheme to study the short-lived topological phases in ultracold systems by first preparing the systems in its long-lived non-topological phases and then driving it into the topological phases and back. We find that the excitations' momentum distributions exhibit Stuckelberg oscillations and Kibble-Zurek scaling characteristic of the topological quantum phase transition, thus provides a bulk probe for the topological phase.

## Pablo Andrés Solano Palma - March 31, 2017

Dissertation Title: **Quantum Optics in Optical Nanofibers**

Date and Time: **Friday, March 31, 9:00 am**

Location: **PSC 2136**

Committee Chair: **Prof. Luis Orozco**

Committee:

Dr. Mohammad Hafezi

Dr. Steven Rolston

Dr. William Phillips

Dr. Jeremy Munday

Dr. Edo Waks

Abstract:

The study of atom-light interaction is the core of quantum optics and a central part of atomic physics. Systems composed of atoms interacting among each other through the electromagnetic field can be used from fundamental research to practical applications. Experimental realizations of these systems benefit from three distinct attributes: large atom-light coupling, trapping an control of atomic ensembles, and engineering and manipulation of the electromagnetic field. Optical waveguides provide a platform that achieves these three goals. In particular, optical nanofibers are an excellent candidate. They produce a high confinement of the electromagnetic field that improves atom-light coupling, guiding the field that mediates the interactions between atoms, while allowing trapping of the atoms close to it.

This thesis uses an optical nanofiber for quantum optics experiments, demonstrating its possibilities for enabling special atom-light interactions. We trap atoms near the optical nanofiber surface, and characterize the trap in a non-destructive manner. We show how the presence of the nanofiber modifies the fundamental atomic property of spontaneous emission, by altering the electromagnetic environment of the atom. Finally, we use the nanofiber to prepare collective states of atoms around it. These states can radiate faster or slower than a single atom (super and subradiance). The observation of subradiance of a few atoms, a rather elusive effect, evidences nanofibers as a strong candidate for future quantum optics experiments. Moreover, we show how the guided field mediates interaction between atoms hundreds of wavelengths apart, creating macroscopically delocalized collective states.

## 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:

## 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:

## 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.