## Clayton Crocker - December 13, 2018

Dissertation Title: **Improving Ion-Photon Entanglement for Quantum Networks**

Date and Time: **Tuesday, December 13, 2:00 pm**

Location: **PSC 3150**

Dissertation Committee Chair: **Dr. Christopher Monroe (Advisor)**

Committee:

Dr. Alexey GorshkovDr. Steven Rolston

Dr. Ian Spielman

Dr. Mohammad Hafezi (Dean's Representative)

Abstract:

Increasing the number of qubits that can be controlled in a quantum system represents an essential challenge to the field of quantum computing. Quantum networks consisting of nodes for local information processing and photonic channels to distribute entanglement between different nodes represent a promising modular approach to achieve this scaling. Trapped atomic ions are an ideal candidate for quantum network nodes, with long-lived identical qubit memories that can be locally entangled through their Coulomb interaction and remotely entangled through photonic channels. In this work I will show a toolkit for using 171Yb+ and 138Ba+ ions individually or together within a quantum node. I will then demonstrate how we can generate ion-photon entanglement as a resource to connect separate nodes. I will focus some important improvements to this ion-photon entanglement which willallow us to implement it as part of a larger network. These improvements include first the use of separate memory (171Yb+ ) and photon generating (138Ba+ ) ions. Additionally, the use of separate atomic lines within 138Ba+ for excitation and collection allows us to preserve integrity of this photonic interface by ensuring the purity of the single photons that are produced. To this end I demonstrate a single-photon source for quantum networking based on a trapped 138Ba+ ion with a single photon purity of g2(0) = (8.1 ± 2.3) × 10-5 without background subtraction. Trade-offs between the photonic generation rate and the memory-photon entanglement fidelity for the case of polarization photonic qubits are also examined and optimized by tailoring the spatial mode of the collected light. These techniques should be useful in constructing larger ion-photon networks.

## Min-A Cho - December 11, 2018

Dissertation Title: **Low-Latency Searches for Gravitational Waves and their Electromagnetic Counterparts with Advanced LIGO and Virgo**

Date and Time: **Tuesday, December 11, 10:30 am**

Location: **PSC 2136**

Dissertation Committee Chair: **Dr. Peter Shawhan (Advisor)**

Committee:

Dr. Paulo Bedaque Dr. Sarah Eno

Dr. Jordan Goodman

Dr. M. Coleman Miller (Dean's Representative)

Abstract:

For the first time in history, advanced detectors are available to observe the stretching and squeezing of space---gravitational waves---from violent astrophysical events. This opens up the prospects of joint detections with instruments of traditional astronomy to create the new field of multi-messenger astrophysics. Joint detections allow us to form a coherent picture of the unfolding event as told by the various channels of information: mass and energy dynamics from gravitational waves, charged particle environments from electromagnetic radiation, and thermonuclear reactions from neutrinos.

In this work, I motivate low-latency electromagnetic and neutrino follow-up of sources known to emit gravitational radiation in the sensitivity band of ground-based interferometric detectors, Advanced LIGO and Advanced Virgo. To this end, I describe the low-latency infrastructure I developed with colleagues to select promising candidate events and enable successful follow-up of the first binary black hole merger, named GW150914, and binary neutron star coalescence, named GW170817, during the first and second observing runs.

As a review, I outline the theory behind gravitational waves and explain how the advanced detectors, low-latency searches, and data quality vetting procedures work. To highlight the newness of the field that is gravitational wave astronomy, I also share results from an offline search for a more speculative source of gravitational waves, intersecting cosmic strings, from the second observing run.

Finally, I address how LIGO/Virgo is prepared to adapt to challenges that will arise during the upcoming third observing run, an era that will be inundated with near-weekly binary black hole candidate events and near-monthly binary neutron star candidate events. We made several improvements to our low-latency infrastructure, including a new, streamlined candidate event selection process, expansions I helped develop for temporal coincidence searches with electromagnetic/neutrino triggers, and data quality products on source classification and probability of astrophysical origin to provide to our observing partners for potential compact binary coalescences. These measures will further our prospects for multi-messenger astrophysics and increase our science returns.

## Safa Motesharrei - October 29, 2018

Dissertation Title: **Carrying Capacity of two-way coupled Earth-Human Systems **

Date and Time: **Monday, October 29, 2:00 pm**

Location: **ATL 3425**

Dissertation Committee Chair: **Dist. Univ. Prof. James A. Yorke**

Committee:

Dr. Victor Yakovenko** **Dist. Univ. Prof. Eugenia Kalnay

Dr. Bill Dorland

Dr. Jelena Srebric (Dean's Representative)

Abstract:

Over the last two centuries, the impact of the Human System has grown dramatically, becoming strongly dominant within the Earth System in many different ways. Consumption, inequality, and population have increased extremely fast, especially since about 1950, threatening to overwhelm the many critical functions and ecosystems of the Earth System. Changes in the Earth System, in turn, have important feedback effects on the Human System, with costly and potentially serious consequences. However, current models do not incorporate these critical feedbacks. We argue that in order to understand the dynamics of either system, Earth System Models must be coupled with Human System Models through bidirectional couplings representing the positve, negative, and delayed feedbacks that exist in the real systems. In particular, key Human System variables, such as demographics, inequality, economic growth, and migration, are not coupled with the Earth System but are instead driven by exogenous estimates, such as the United Nations population projections. This makes current models likely to miss important feedbacks in real Earth-Human system, especially those that may result in unexpected or courtnerintuitive outcomes, and thus requiring different policy interventions from current models. The importance and imminence of sustainabilty challenges, the dominant role of the Human System in the Earth System, and the essential roles the Earth System plays for the Human System, all call for collaboration of natural scientists, social scientists, and engineers in multidisciplinary research and modeling to develop coupled Earth-Human system models for devising effective science-based policies and measures to benefit current and future generations.

## David Meyer - October 26, 2018

Dissertation Title: **Magnetic & Electric Field Sensing and Applications Based on Coherent Effects in Neutral Atoms**

Date and Time: **Friday, October 26, 12:00 pm**

Location: **Atlantic 3330 **

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

Committee:

**Dr. Fredrik Fatemi (advisor)**Dr. Frederick Wellstood

Dr. Trey Porto

Dr. Edo Waks

Abstract:

This work encompasses two projects employing coherent probing of neutral atom vapors for sensing applications:

1) exploring previously unexplained features within cold-atom Nonlinear Magneto-Optical Rotation signals with applications for multi-directional magnetometry;

2) using a warm vapor of Rydberg atoms as a radio-frequency communications receiver via Electromagnetically Induced Transparency detection.

## Jeffrey Magill - September 21, 2018

Dissertation Title: **Probing the Nature of Radiative Processes within Radio Galaxies using the Fermi Gamma-Ray Large Area Telescope**

Date and Time: **Friday, September 21, 3:00 pm**

Location: **PSC 3150**

Dissertation Committee Chair: **Prof. Peter Shawhan**

Committee:

**Dr. Julie McEnery (advisor)**Dr. Jordan Goodman

Dr. Jeremy Perkins

Dr. M. Coleman Miller

Abstract:

Radio galaxies, active galactic nuclei with misaligned relativistic jets and large diffuse extended lobe structures, are home to radiative processes which are still not well understood. In this defense, I describe my use of gamma-ray photon data from the Fermi Large Area Telescope to investigate these radiative processes in the case of two radio galaxies, Fornax A and Centaurus A. I describe my discovery of the spatially extended nature of the gamma-ray emission from Fornax A, and my observation of a gamma-ray intensity which is not consistent with the predicted process of stray energetic electrons inverse-Compton scattering with extragalactic background light photons. I describe how I positively identified a new gamma-ray spectral component from the core region of Centaurus A jointly with data from the High Energy Stereoscopic System, and how the spectral component can be explained by the addition of a second hidden zone of synchrotron self-Compton emission. I describe my discovery of fine filamentary sub-structures in the gamma-ray lobes of Centaurus A using a new imaging technique which I created, mapping out the unexpected gamma-ray emission farther from the assumed central engine than we have observed in radio. I discuss how my observations of the Centaurus A lobes suggest local re-acceleration or channels of negligible magnetic field allowing long distance high energy particle paths.

## Amit Nag - August 30, 2018

Dissertation Title: **SIGNATURE OF MAJORANA MODES AND ASPECTS OF THEIR BRAIDING**

Date and Time: **Thursday, August 30, 2:00 pm**

Location: **PHY 2205 **(CMTC Seminar Room)

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

Committee:

Dr. Mohammad Hafezi

Dr. Theodore Einstein

Dr. Maissam Barkeshli

Dr. Christopher Jarzynski

Abstract:

Majorana zero modes are emergent zero-energy quasiparticle excitations in certain superconducting systems that can be viewed as fractionalized or “half” electrons. These quasiparticles obey non-Abelian braiding statistics which is one manifestation of such half-electron character. Due to such non-Abelian braiding property, Majorana zero mode pairs hold promise as potential qubits for topological quantum computation.

It is somewhat surprising that, at least theoretically, ordinary one-dimensional semiconductor systems can be induced to host such esoteric Majorana modes as edge states if some precise experimental conditions are satisfied. Because of relative simplicity of material and experimental requirements to host Majorana modes, there has been a flurry of experimental effort to realize them in semiconductor nanowire systems. While the experimental efforts have produced preliminary evidence for presence of Majorana zero modes in these systems, a thorough confirmation is lacking. The experimental signature in question is presence of a zero-bias conductance-peak that is albeit necessary, is not a sufficient criteria to establish presence of underlying Majorana modes. Given the importance of Majorana braiding for topological quantum computation and skepticism over presence of Majorana modes in these experimental systems, it would seem natural to attempt braiding these putative Majorana modes in near future. In that case an observation of non-Abelian statistics would provide the necessary sufficient condition in favor of Majorana presence in the studied experimental systems.

This thesis has three distinct parts. First we assume perfect Majorana modes as given that can be successfully braided. In this case, we calculate the diabatic error due to finite speed of braiding when the system is coupled to a Bosonic bath. Next, we grant that the mechanism for zero-bias conductance-peak is indeed topological albeit the underlying Majorana modes are imperfect (the modes are not precisely at zero energy). We study the interplay of dissipation and such imperfect Majorana modes and their effect on probability of successful braiding. Lastly, we propose studying correlation between independent left and right conductance measurements as a means to distinguish between topological versus a non-topological mechanism underlying the observed zero-bias conductance-peak.

## Cody Ballard - August 22, 2018

Dissertation Title: **VARIABLE QUBIT-QUBIT COUPLING VIA A TUNABLE LUMPED-ELEMENT RESONATOR**

Date and Time: **Wednesday, August 22, 1:00 pm**

Location: **PSC 3150**

Dissertation Committee Chair: **Prof. Frederick Wellstood (co-advisor)**

Committee:

**Dr. Christopher Lobb (co-advisor)**

Dr. Benjamin Palmer

Dr. Kevin Osborn

Dr. Ichiro Takeuchi

Abstract:

This dissertation examines the design, fabrication, and characterization of a device with two transmon qubits coupled through a tunable superconducting resonator, having a tuning range of ≈800 MHz. To achieve tunability, the inductance in the LC resonator incorporated two single Josephson junction superconducting loops. Application of an external magnetic flux to the loops varied the total inductance of the circuit, thereby changing its resonance frequency. To isolate the system and provide a means for reading out the state of the qubit, the device was mounted in a 3D Al microwave cavity. The flux-dependent transition frequencies of the system were measured and fit to results from a coupled Hamiltonian model. I show that, as the resonator is tuned, the qubit-qubit dispersive shift ranged from an “off” value of 2χ = 0.1 MHz, allowing single qubit operations, to an “on” value of 2χ = 6 MHz, providing enough coupling to perform gates.

This dissertation also includes observations of the temperature dependence of the relaxation time T1 of three Al/AlOx/Al transmons. In some cases, T1 increased by almost a factor of two as the temperature increased from 30 to 100 mK. We found that this anomalous effect is consistent with loss from non-equilibrium quasiparticles in a transmon in which one junction electrode has a slightly smaller superconducting gap than the other. I present a model of this effect, use the model to extract the density of non-equilibrium quasiparticles in the device, and find the values of the superconducting energy gaps in the films.

## Simón Riquelme Muñoz - August 9, 2018

Dissertation Title: **Generalized Natural Inflation and the Quest for Cosmic Symmetry Breaking Patterns**

Date and Time: **Thursday, August 9, 2:00 pm**

Location: **PSC 3150**

Dissertation Committee Chair: **Prof. Zackaria Chacko**

Committee:

Dr. Raman Sundrum

Dr. Theodore Jacobson

Dr. Rabindra Mohapatra

Dr. Richard Wentworth

Abstract:

We present a two-field model that generalizes Natural Inflation, in which the inflaton is the pseudo-Goldstone boson of an approximate symmetry that is spontaneously broken, and the radial mode is dynamical. Within this model, which we designate as “Generalized Natural Inflation”, we analyze how the dynamics fundamentally depend on the mass of the radial mode and determine the size of the non-Gaussianities arising from such a scenario.

We also motivate ongoing research within the coset construction formalism that aims to clarify how the spontaneous symmetry breaking pattern of spacetime, gauge, and internal symmetries may allow us to get a deeper understanding, and an actual algebraic classification, of different possible “cosmic states”, which may allow model-independent descriptions of different phases in the evolution of our universe.

## Shavindra Premaratne - July 12, 2018

Dissertation Title: **Coherent Control of Low Anharmonicity Systems for Superconducting Quantum Computing**

Date and Time:** Thursday, July 12, 1:00 pm**

Location: **PSC 2136**

Dissertation Committee Chair: **Prof. Frederick Wellstood (co-advisor)**

Committee:

**Dr. Benjamin Palmer (co-advisor)**

Dr. Kevin Osborn

Dr. James Williams

Dr. Christopher Jarzynski

Dr. Andrew Childs

Abstract:

This dissertation describes research to coherently control quantum states of superconducting devices. In the first project, the state of an 8 GHz 3D superconducting Al cavity at 20 mK was manipulated to add a single excitation. Preparing a harmonic resonator in a state with a well-defined number of excitations (Fock states) is not possible using an external classical drive. I generated Fock states by transferring a single excitation from a 5.5 GHz transmon qubit to a cavity using Stimulated Raman Adiabatic Passage (STIRAP). I also extended the STIRAP technique to put the cavity in higher Fock states, superpositions of Fock states, and Bell states between the qubit and the cavity. Master-equation simulations of the system's density matrix were in good agreement with the data, and I obtained estimated fidelities of 89%, 68% and 43% for the first three Fock states, respectively.

The second project involved implementing an entangling gate between two Al/AlOx/Al transmon qubits that were mounted in an Al cavity and cooled to 20 mK. Pertinent system frequencies were as follows: one qubit was at 6.0 GHz, the other qubit at 6.8 GHz, the cavity at 7.7 GHz, and the qubit-qubit dispersive shift was -1 MHz. I used an all-microwave technique known as Speeding up Waveforms by Inducing Phases to Harmful Transitions (SWIPHT). The technique implemented a generalized CNOT gate, by applying a specially-shaped pulse of duration 907 ns. Using quantum process tomography, I found that the gate fidelity was 80%-82%, close to the 87% fidelity expected from decoherence in the transmons during the gate time. Details of the device fabrication, device characterization, measurement techniques, and extensive modeling of device behavior are presented, along with chi-matrix characterization of single-qubit gates and SWIPHT gates.

In this talk, I will focus on the implementation of a generalized CNOT gate using the SWIPHT technique.

## Chun-Xiao Liu - July 6, 2018

Dissertation Title:** Majorana and Andreev bound states in semiconductor-superconductor nanostructures**

Date and Time:** Friday, July 6, 11:00 am**

Location: **2202 PHY (Condensed Matter Theory & Experiment Conference Room)**

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

Committee:

Dr. Sankar Das Sarma

Dr. Victor Yakovenko

Dr. James Williams

Dr. Christopher Jarzynski

Abstract:

Majorana bound states have been a topic of active research over the last two decades. In the perspective of theoretical physics, Majorana bound states, which are their own antiparticles, are zero-energy quasi-particle excitations in exotic superconducting systems. From a technological perspective, Majorana bound states can be utilized for the implementation of fault-tolerant quantum computation due to their topological properties. For example, two well-separated Majorana bound states can form a fermionic qubit state, the quantum information of its occupancy is stored in a nonlocal way, being robust against local decoherence. Also since Majorana bound states obey non-Abelian statistics, quantum gates can be implemented by braiding. Such gate operations are robust because small deviations in braiding trajectories do not affect the braiding results.

So far the most promising platform for the realization of Majorana bound states is the one-dimensional semiconductor-superconductor nanostructures. The hallmark of the existence of Majorana bound states in such systems is a quantized zero-bias conductance peak in the tunneling spectroscopy for a normal-metal-superconductor junction. Although quantized zero-bias conductance peaks that resemble the theoretical prediction have been observed in several experimental measurements, confusing aspects of the data muddy the conclusion. One source of confusion results from the existence of another type of excitation in these systems i.e. the topologically trivial near-zero-energy Andreev bound states. These excitations mimick many behaviors of the topological Majorana bound states.

In this work, we first investigate the tunnel spectrsocopy signatures of both Majorana and Andreev bound states, and show that the Andreev-bound-state-induced conductance peak indeed resembles that of Majorana bound state in many ways. We then talk about the physical mechanism for the formation of such trivial Andreev bound states and show that their presence can be very generic. We then give multiple practical proposals to differentiate between Majorana and Andreev bound states that are doable in the context of the current normal-metal-superconductor junctions. Finally, we discuss another theoretical proposal that can directly measure the topological invariant of the superconductor for future experiments.

## David Somers - July 5, 2018

Dissertation Title: **Casimir-Lifshitz Forces and Torques**

Date and Time: **Thursday, July 5, 2:00 pm**

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

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

Committee:

Dr. Steven Rolston

Dr. Ian Appelbaum

Dr. Victor Yakovenko

Dr. Luz Martinez-Miranda

Abstract:

Quantum electromagnetic field fluctuations result in the well-documented Casimir- Lifshitz force between macroscopic objects. If the objects are anisotropic, theory predicts a corresponding Casimir-Lifshitz torque that causes the objects to rotate and align. In this work, we report the first measurements of the Casimir-Lifshitz torque, which confirm the predictions first made decades ago. The experimental design uses a nematic liquid crystal separated from a birefringent crystal by an isotropic Al_{2}O_{3} layer with a thickness ≤ 25 nm. The molecular orientation of the liquid crystal is fixed with a rubbed counterplate, and, by varying the rubbing and Al_{2}O_{3} thickness, we measured the Casimir-Lifshitz torque as a function of angle and distance.

Along the way, I developed a simpler formulation for calculating the Casimir-Lifshitz interaction in planar systems, which facilitated further theoretical study of the Casimir-Lifshitz torque. Using this method, I outline the conditions for a repulsive Casimir-Lifshitz force between birefringent materials that would allow for an angularly-dependent sign of the force. I also report an unexpected enhancement of the torque from two sources: an intermediate dielectric medium and the finite speed of light.

## Chiao-Hsuan Wang - June 11, 2018

Dissertation Title: **Photon thermalization in driven open quantum systems**

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

Location: **PSC 2136**

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

Committee:

**Dr. Jacob Taylor (advisor)**

Dr. Alexey Gorshkov

Dr. Trey Porto

Dr. Luis Orozco

Dr. Mohammad Hafezi

Abstract:

Light is a paradigmatic quantum field, with individual excitations---photons---that are the most accessible massless particles known. However, their lack of mass and extremely weak interactions mean that typically the thermal description of light is that of blackbody radiation. As the temperature of the light decreases, the overall number of photons approaches zero. Therefore, efforts for quantum optics and optical physics have mostly focused on driving systems far from equilibrium to populate sufficient numbers of photons. While lasers provide a severe example of a nonequilibrium problem, recent interests in the near-equilibrium physics of so-called photon gases, such as in Bose condensation of light or in attempts to make photonic quantum simulators, suggest one re-examine near-equilibrium cases.

In this thesis, we consider peculiar driven open quantum system scenarios where near-equilibrium dynamics can lead to equilibration of photons with a finite number, following a thermal description closer to that of an ideal gas than to black body radiation. Specifically, we show how laser cooling of a well-isolated mechanical mode or atomic motion can provide an effective bath which enables control of both the chemical potential and temperature of the resulting grand canonical ensemble of photon. We then theoretically demonstrate that Bose condensation of photons can be realized by cooling an ensemble of two-level atoms inside a cavity. Finally, we find that the engineered chemical potential for light not only admits future applications in many-body quantum simulations, facilitates preparation of near-equilibrium photonic states, but also enables an analogous voltage bias for photonic circuit elements.

## Mark Eichenlaub - May 30, 2018

Dissertation Title: **Mathematical Sensemaking Via Epistemic Games**

Date and Time: **Wednesday, May 20, 2:00 pm**

Location: **PSC 2136**

Dissertation Committee Chair: **Prof. Edward Redish**

Committee:

Dr. Eric Brewe

Dr. Michelle Girvan

Dr. Ayush Gupta

Dr. Andrew Elby

Abstract:

This thesis studies how students in an introductory physics for life sciences course at the University of Maryland learn to use the mathematical reasoning resources, especially those around understanding and manipulating algebraic expressions and equations, to solve physics problems and gain insight into how physics works. There are both qualitative and quantitative threads to this work. The qualitative work analyzes a series of problem-solving interviews, first to survey the variety of rich cognitive tools students bring to bear on the problem via case studies, then to draw a connection between the ontological metaphors students use for equations and the epistemic games they play while solving problems. The quantitative thread describes the creation and analysis of the Math Epistemic Games Survey, a math test we wrote and administered to study how students’ uptake of problem-solving strategies such as “check the extreme cases” progressed over a year-long physics course.

## Yee Lam Elim Cheung - May 2, 2018

Dissertation Title: **Measurement of Atmospheric Neutrino Oscillation Parameters Using Three Years of IceCube-DeepCore Data**

Date and Time: **Wednesday, May 2, 12:00 pm**

Location: **PSC 2204**

Dissertation Committee Chair: **Prof. Gregory Sullivan**

Committee:

Dr. Rabindra Mohapatra

Dr. Kara Hoffman

Dr. Andrew Baden

Dr. M. Coleman Miller

Abstract:

The story of neutrinos began in 1930 when Pauli proposed a hypothesized particle as a “desperate remedy” to rescue quantum theory. Although Pauli was pessimistic about the detectability of his new particle, Reins and Cowan first discovered (anti) neutrinos in 1956. Soon after, neutrinos became a puzzle for particle physicists due to a persistent deficit in observed rates by multiple experiments. This mystery was partly answered by Pontecorvo who first proposed the idea of neutrino oscillations in 1957. In 1998, the Super-Kamiokande (SK) collaboration provided the first definitive evidence of neutrino oscillations, for which both the SK and the Sudbury Neutrino Observatory (SNO) collaborations were awarded the Nobel Prize in Physics 2015.

While measuring oscillation parameters has long been a focus for numerous neutrino experiments, the IceCube Neutrino Observatory with DeepCore provides a unique window to measure atmospheric oscillation parameters. With an effective volume ~ 300 times larger than SK, DeepCore can detect atmospheric neutrinos between a few and 100 GeV. In addition, IceCube acts as a thick veto shield for DeepCore to better identify atmospheric muon backgrounds. Given that the amplitude of atmospheric neutrino oscillations is expected to be maximal at ~ 25

GeV, IceCube-DeepCore is well suited for studying atmospheric neutrino oscillations by probing this energy window for the first time.

This work presents the latest measurement of atmospheric oscillation parameters using three years of IceCube-DeepCore data. The standard three neutrino mixing and matter effect due to Earth are considered. Under the assumption of a unitary mixing matrix, a binned analysis using a modified chi^2 is performed, and sixteen systematics are taken into account. Preferring a normal neutrino mass ordering, this analysis measures the mass squared difference, Delta m^2_{23} = 2.55^{+0.12}_{-0.11} x 10^-3 eV^2, and the mixing angle, sin^2 \theta_{23} = 0.58^{+0.04}_{-0.13}. The measurement from this work is comparable to the latest measurements from other long baseline neutrino experiments.

## Antony Speranza - April 27, 2018

Dissertation Title: **Investigations on entanglement entropy in gravity**

Date and Time: **Friday, April 27, 4:00 pm**

Location: **PSC 3150**

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

Committee:

Dr. Raman Sundrum

Dr. Brian Swingle

Dr. Bei Lok Hu

Dr. Jonathan Rosenberg

Abstract:

Entanglement entropy first arose from attempts to understand the entropy of black holes, and is believed to play a crucial role in a complete description of quantum gravity. This thesis explores some proposed connections between entanglement entropy and the geometry of spacetime. One such connection is the ability to derive gravitational field equations from entanglement identities. I will discuss a specific derivation of the Einstein equation from an equilibrium condition satisfied by entanglement entropy, and explore a subtlety in the construction when the matter fields are not conformally invariant. As a further generalization, I extend the argument to include higher curvature theories of gravity, whose consideration is necessitated by the presence of subleading divergences in the entanglement entropy beyond the area law.

A deeper issue in this construction, as well as in more general considerations identifying black hole entropy with entanglement entropy, is that the entropy is ambiguous for gauge fields and gravitons. The ambiguity stems from how one handles edge modes at the entangling surface, which parameterize the gauge transformations that are broken by the presence of the boundary. The final part of this thesis is devoted to identifying the edge modes in arbitrary diffeomorphism-invariant theories. Edge modes are conjectured to provide a statistical description of the black hole entropy, and this work takes some initial steps toward checking this conjecture in higher curvature theories.

## Caroline Figgatt - March 22, 2018

Dissertation Title: **Building and Programming a Universal Ion Trap Quantum Computer**

Date and Time: **Thursday, March 22, 10:00 am**

Location: **PSC 2136**

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

Committee:

Dr. Gretchen Campbell

Dr. Trey Porto

Dr. James Williams

Dr. Andrew Childs

Abstract:

Quantum computing represents an exciting frontier in the realm of information processing; it is a promising technology that may provide future advances in a wide range of fields, from quantum chemistry to optimization problems. This thesis discusses experimental results for several quantum algorithms performed on a programmable quantum computer consisting of a linear chain of five or seven trapped 171Yb+ atomic clock ions with long coherence times and high gate fidelities. We execute modular one- and two-qubit computation gates through Raman transitions driven by a beat note between counter-propagating beams from a pulsed laser. The system's individual addressing capability provides arbitrary single-qubit rotations as well as all possible two-qubit XX-entangling gates, which are implemented using a pulse-segmentation scheme. The quantum computer can be programmed from a high-level interface to execute arbitrary quantum circuits, and comes with a toolbox of many important composite gates and quantum subroutines.

We present experimental results for a complete three-qubit Grover quantum search algorithm, a hallmark application of a quantum computer with a well-known speedup over classical searches of an unsorted database, and report better-than-classical performance. The algorithm is performed for all 8 possible single-result oracles and all 28 possible two-result oracles. Two methods of state marking are used for the oracles: a phase-flip method employed by other experimental demonstrations, and a Boolean method requiring an ancilla qubit that is directly equivalent to the state-marking scheme required to perform a classical search. All quantum solutions are shown to outperform their classical counterparts.

Performing parallel operations will be a powerful capability as deeper circuits on larger, more complex quantum computers present new challenges. Here, we perform a pair of 2-qubit gates simultaneously in a single chain of trapped ions. We employ a pre-calculated pulse shaping scheme that modulates the phase and amplitude of the Raman transitions to drive programmable high-fidelity 2-qubit XX gates in parallel by coupling to the collective modes of motion of the ion chain. Ensuring the operation yields only spin-spin interactions between the desired pairs, with neither residual spin-motion entanglement nor "crosstalk" spin-spin entanglement, is a nonlinear constraint problem, and pulse solutions are found using optimization techniques. As an application, we demonstrate the quantum full adder using a depth-4 circuit requiring the use of parallel 2-qubit operations.

## 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.