Edgar Perez- April 5, 2024
Dissertation Title: High Performance Nanophotonic Cavities and Interconnects for Optical Parametric Oscillators and Quantum Emitters
Date and Time: Friday, April 5, 10:00 am
Location: PSC 2136
Dissertation Committee Chair: Mohammad Hafezi and Kartik Srinivasan
Committee:
Yanne Chemo
Efrain Rodriguez
Edo Waks
Xiyuan Lu
Abstract:
Integrated photonic devices like photonic crystals, microring resonators, and quantum emitters produce useful states of light, like solitons or single photons, through carefully engineered light-matter interactions. However, practical devices demand advanced integration techniques to meet the needs of cutting-edge technologies. High performance nanophotonic cavities and interconnects present opportunities to solve outstanding issues in the integration of nanophotonic devices. In this dissertation I develop three core tools required for the comprehensive integration of quantum emitters: wavelength-flexible excitation sources with enough pump power to drive down stream systems, photonic interconnects to spatially link the excitation sources to emitters, and cavities that can Purcell enhance quantum emitters without sacrificing other performance metrics.
To create wavelength-flexible excitation sources, high performance χ(3) microring Optical Parametric Oscillation (OPO) is realized in silicon nitride. Microring OPOs are nonlinear frequency conversion devices that can extend the range of a high quality on-chip (or off-chip) laser source to new wavelengths. However, parasitic effects normally limit the output power and conversion efficiency of χ(3) microring OPOs. This issue is resolved by using a microring geometry with very strongly normal dispersion, that uses multiple spatial mode families to satisfy the phase and frequency matching conditions. Our OPO achieves world-class performance with a conversion efficiency of up to 29% and an on-chip output power of over 18 mW.
To create photonic interconnects, Direct Laser Writing (DLW) is used to fabricate 3-dimensional (3D) nanophotonic devices that can couple light into and out of photonic chips. In particular, polymer microlenses of 20 µm diameter are fabricated on the facet of photonic chips that increase the tolerance of the chips to misaligned input fibers by a factor of approximately 4. DLW is also used to fabricate Polymer Nanowires (PNWs) with diameters smaller than 1 µm that can directly couple photons from quantum emitters into Gaussian-like optical modes. Comparing the same quantum emitter system before and after the fabrication of a PNW, a (3±0.7)× increase in the fiber-coupled collection efficiency is measured in the system with the PNW.
To refine the design of quantum emitter cavities, a toy model is used to understand the underlying mechanisms that shape the emission profiles of Circular Bragg Gratings (CBGs). Insights from the toy model are used to guide the Bayesian optimization of high performance CBG cavities suitable for coupling to single mode fibers. I also demonstrate cavity designs with Qs on the order of 105 that can be used in future experiments in cavity quantum electrodynamics or nonlinear optics. Finally, I show that these cavities can be optimized for extraction to a cladded PNW while producing a Purcell enhancement factor of 100 with efficient extraction into the fundamental PNW mode.
The tools developed in this dissertation can be used to integrate individual quantum emitter systems or to build more complex systems, like quantum networks, that require the integration of multiple quantum emitters with multiple photonic devices.
Shiyi Sheng - April 3, 2024
Dissertation Title: Quantum Computing and Machine Learning Approaches to Many-Body Physics
Date and Time: Wednesday, April 3, 10:00 AM
Location: PSC 3150
Dissertation Committee Chair: Professor Paulo F. Bedaque
Committee:
Professor Thomas Cohen
Professor Zackaria Chacko
Professor Manuel Franco Sevilla
Professor Mohammad Hafezi
Abstract:
Lattice field theory provides a framework for which to explore properties of quantum field theories non-perturbatively. However for certain lattice calculations, for example when considering real-time dynamics or fermionic systems at finite density, sign problems occur which render those calculations intractable. One approach to solving the sign problem is to avoid it altogether by instead considering a simulation of the field theory on a quantum computer. For bosonic field theories, a procedure of qubitizing the bosonic fields is a necessary first step. The infinite-dimensional Hilbert space of the bosonic fields must be properly truncated as to encode those fields on a finite-dimensional Hilbert space spanned by the qubits on the quantum computer.
This thesis first discusses various strategies of making such a truncation. Ideally, the truncation yields a discrete spin system that contains a critical point in the same universality class as the untruncated field theory. That way, the physics of the original field theory is reproduced in the continuum limit of the truncated theory without needing to take a second limit of removing the truncation. Simulations of different models arising from various truncation strategies of the (1+1)-dimensional O(3) nonlinear sigma model are performed and different qubitizations for SU(2) gauge fields are considered and proposed. Due to a lack of an efficient method for solving many-body systems in more than one dimension, numerical simulations of these SU(2) qubitizations are unavailable.
The second half of the thesis explores the use of machine learning techniques in providing effective ways to solve quantum many-body problems. Neural network structures, such as feed-forward networks and restricted Boltzmann machines are universal approximators for continuous and discrete functions respectively. Therefore, they can be used as flexible wavefunction ansatze. Gradient descent algorithms can be applied to variationally search the general functional space spanned by neural-network-based ansatze for ground states of interacting, many-body systems. An ansatz is constructed explicitly for a system of indistinguishable bosons in one dimension and tested by comparing numerical results with analytic solutions of several exactly-solvable models. An extension of these neural-network ansatze to systems of identical bosons and fermions and discrete spin systems in higher dimensions would allow for concrete simulations of systems ranging from nuclei and qubitization models.
Hong Nhung Nguyen - April 3, 2024
Dissertation Title: Quantum Simulation of High-Energy Physics with Trapped Ions
Date and Time: Wednesday, April 3, 1:00 pm
Location: PSC 2136 and Zoom
Dissertation Committee Chair: Professor Alicia Kollár
Committee:
Professor Norbert Linke, Advisor and Co-Chair
Professor Zohreh Davoudi
Professor Ian Spielman
Professor Xiaodi Wu
Abstract:
Trapped ions stand out as a leading platform for quantum computing due to their long coherence times, high-fidelity quantum gates, and the ability to precisely control individual qubits, enabling scalable and precise quantum computations. This dissertation reports advances in quantum computing with trapped ions, focusing on robust and high-fidelity entanglement generation, logical qubit encoding, and applications in quantum simulations of high-energy physics.
In particular, we report the implementation of a novel pulse optimization scheme for
achieving high-fidelity entangling gates in our setup. The scheme enables a balanced trade-
off between robustness to experimental drift, laser power, and gate duration, without the
need for expensive optimization. We also demonstrate the implementation of the Shor code with different code distances on our trapped-ion quantum computer, highlighting the fault-tolerant preparation of a logical qubit with high fidelity and showcasing the potential for reliable quantum computing.
Finally, we detail an experimental quantum simulation of the Schwinger model, a quantum electrodynamics theory in 1+1 dimensions, using two, four, and six qubits, demonstrating non-perturbative effects such as pair creation over extended periods of time. We study the gate requirement for two formulations of the model using a quantum simulation algorithm, considering the trade-offs between Hamiltonian term ordering, the number of time steps, and experimental errors. We employ a symmetry-protection protocol with random unitaries and a symmetry based post-selection technique to minimize errors. This work emphasizes the importance of the integrated approach between theory, algorithms, and experiments for efficient simulation of complex physical systems like lattice gauge theories.
Elijah Willox - April 2, 2024
Dissertation Title: The Search for Coincident Gamma-Ray Emission From Fast Radio Bursts with the HAWC Observatory
Date and Time: Tuesday, April 2, 3:00 PM
Location: PSC 3150
Dissertation Committee Chair: Jordan Goodman
Committee:
Andrew Smith
Gregory Sullivan
Brian Clark
M. Coleman Miller
Abstract:
In 2007 a new class of radio transients was discovered, coming from outside of our galaxy with high fluence emitted in the radio band on millisecond timescales, emitting within an order of magnitude of the power of a gamma-ray burst or supernova. These fast radio bursts (FRBs) have since become the target of many searches across radio observatories and multiwavelength follow-up campaigns, but their origin remains unknown. In order to understand more about these fascinating events, a complete multiwavelength understanding in necessary to get a complete picture of what is creating such powerful bursts. The High Altitude Water Cherenkov (HAWC) observatory is a very-high-energy gamma-ray detector covering the range of 100 GeV to 300 TeV that is well suited to the detection of transient phenomena due its high live-time and wide field of view, and in particular for a follow-up search on FRBs to determine possible very high energy gamma-ray coincidences. The search for gamma-ray signals from FRBs consists of two searches: first is a persistent source search to identify if FRB emission ever comes from TeV gamma-ray emitting galaxies, and a transient search centered on the reported burst time and location. The results of the FRB search within the HAWC data sets the most constraining limits on the widest population ever searched in the VHE band.
Maya Amouzegar - March 29, 2024
Dissertation Title: Photon-Mediated Interactions in Lattices of Coplanar Waveguide Resonators
Date and Time: Friday, March 29, 1:30 PM
Location: PSC 3150
Dissertation Committee Chair: Professor Alicia Kollár
Committee:
Professor Mohammad Hafezi (Dean’s Representative)
Professor Steven Rolston
Professor Trey Porto
Professor Sarah Eno
Abstract:
Circuit quantum electrodynamics (circuit QED) has become one of the main platforms for quantum simulation and computation. One of its notable advantages is its ability to facilitate the study of new regimes of light-matter interactions. This is achieved due to the native strong coupling between superconducting qubits and microwave resonators, and the ability to lithographically define a large variety of resonant microwave structures, for example, photonic crystals. Such geometries allow the implementation of novel forms of photon-mediated qubit-qubit interaction, cross-Kerr qubit-mediated interactions, and studies of many-body physics. In this dissertation, I will show how coplanar waveguide (CPW) lattices can be used to create engineered photon-mediated interactions between superconducting qubits. I will discuss the design and fabrication of a quasi one-dimensional lattice of CPW resonators with unconventional bands, such as gapped and ungapped flat bands. I will then present experimental data characterizing photon-mediated interactions between tunable transmon qubits and qubit-mediated non-linear photon-photon interactions in the said lattice. Our results indicate the realization of unconventional photon-photon interactions and qubit-qubit interactions, therefore, demonstrating the utility of this platform for probing novel interactions between qubits and photons. In future design iterations, one can extend the study of these interactions to two-dimensional flat and hyperbolic lattices.
Eli Mizrachi - March 29, 2024
Dissertation Title: Studies of Ionization Backgrounds in Noble Liquid Detectors For Dark Matter Searches
Date and Time: Friday, March 29, 1:00 pm
Location: PSC 2136
Dissertation Committee Chair: Professor Carter Hall
Committee:
Research Professor Anwar Bhatti
Assistant Professor Brian Clark
Professor Sarah Penniston-Dorland
Dr. Jinkge Xu
Abstract:
Dark matter is believed to make up almost 85% of the total mass of the universe, yet its identity remains unclear. Weakly Interacting Massive Particles (WIMPs) have historically been a favored dark matter candidate, and dual-phase noble liquid time projection chambers (TPCs) have set the strongest interaction limits to date on WIMPs with a mass greater than several GeV. However, because no definitive interactions have been observed, the parameter space for conventional WIMPs is highly constrained. This has sparked greater interest in new sub-GeV dark matter models. At this mass scale, dark matter interactions with xenon or argon target media may still produce detectable signals at or near the single electron limit. However, these signals are currently obscured by delayed ionization backgrounds (“electron-trains”) which persist for seconds after an ionization event occurs. Electron-trains have been observed in many different experiments and exhibit similar characteristics, but their cause is only partially understood.
This work examines the nature of electron-trains in various contexts, as well as possible strategies to mitigate them. First, a characterization of electron-trains in the LZ experiment is presented, including new evidence of a dependence on detector conditions. The characterization also informed the development of an electron-train veto for LZ’s first WIMP search, which set world-leading limits on the spin-independent and spin-dependent WIMP-nucleon cross-sections for medium and high-mass WIMPs.
Next, to complement the analysis of LZ data, hardware upgrades were performed in XeNu, a small xenon TPC at Lawrence Livermore National Lab. These included replacing plastics with low-outgassing metal and machinable ceramic components, as well as a replacement of XeNu’s photomultiplier tube array with silicon photomultipliers. The resulting reduction in the intensity of electron-trains and better position resolution from the respective upgrades will improve future studies of low energy interactions and phenomena. Concurrent with this work, XeNu was used to perform a nuclear recoil calibration and a search for the Migdal effect, the latter of which can substantially enhance an experiment's low-mass dark matter sensitivity.
Finally, the development of CoHerent Ionization Limits in Liquid Argon and Xenon (CHILLAX), is reported. CHILLAX is a new xenon-doped, dual-phase argon test stand that has the potential to have a higher sensitivity to low-mass dark matter interactions and lower backgrounds than current liquid xenon TPCs. The system is designed to handle high (percent level) xenon concentrations in liquid argon that can enable a range of ionization signal production and collection benefits. CHILLAX demonstrated the feasibility of such concepts by achieving a world record xenon doping concentration with stable operation.
Robert Dalka - March 11, 2024
Dissertation Title: Developing Methods and Theories for Modeling Student Leadership Development and Student’s Experiences of Academic Support
Date and Time: Monday, March 11, 1:00 pm
Location: Atlantic Building 3332 and Zoom
Dissertation Committee Chair: Dr. Chandra Turpen
Committee:
Dr. Justyna P. Zwolak
Dr. Diana Sachmpazidi
Dr. Andrew Elby
Dr. Michelle Girvan
Dr. Christopher Palmer
Abstract:
This dissertation brings together two research strands that study: (a) the ways in which physics and STEM students contribute to growing capacity for institutional change within collaborative teams and (b) the support structures of graduate programs through an innovative methodology grounded in network science.
The first research strand is explored within two different team environments, one of a student-centric interinstitutional team and a second of departmental change teams. Across both of these contexts, I identify how by engaging in an interaction-based agency, students are able to jointly define their own roles and the projects they pursue. In comparing across these contexts, we identify how students navigate different leadership structures and how this can support or limit student contributions in these teams. A central contribution of this work is a model for cultivating capacity for change through shared leadership and relational agency. This model captures how capacity can be built in different domains tied to the activity systems of the work. I show how this model can help practitioners and facilitators better partner with students as well as how researchers can use this model to capture how students contribute to the work of the team.
The second research strand focuses on developing and applying an innovative methodology for network analysis of Likert-style surveys. This methodology generates a meaningful network based on survey item response similarity. I show how researchers can use modular analysis of the network to identify larger themes built from the connections of particular aspects. Additionally, I apply this methodology to provide a unique interpretation of responses to the Aspects of Student Experience Scale instrument for well-defined demographic groups to show how thematic clusters identified in the full data set re-emerge or change for different groups of respondents. These results are important for practitioners who seek to make targeted changes to their physics graduate programs in hopes of seeing particular benefits for particular groups.
This dissertation opens up lines for future work within both strands. The model for building capacity for change draws attention to the mediating processes that emerge on a team and in students’ interactions with others. This model can be developed further to include additional constructs and leadership structures, as well as explore the relevance to other academic contexts. For quantitative researchers, the network analysis for Likert-style surveys methodology is widely applicable and provides a new way to investigate the wide range of phenomena assessed by Likert-style surveys.
Evan Dowling - March 5, 2024
Dissertation Title: Feedback experiments using entangled photons for polarization control in future quantum networks
Date and Time: Tuesday, March 5, 9:00 AM
Location: ERF (IREAP) 1207
Dissertation Committee Chair: Professor Thomas E. Murphy
Committee:
Professor Rajarshi Roy, Co-Advisor
Professor Julius Goldhar
Professor Yanne Chembo
Professor Wendell T. Hill
Abstract:
Control of the measurement frames that project on polarization entangled photons is an important experimental task for near term fiber-based quantum networks. Because of the changing birefringence in optical fiber arising from temperature fluctuations or external vibrations, the polarization projection direction at the end of a fiber channel is unpredictable and varies with time. This polarization drift can cause errors in quantum information protocols, like quantum key distribution, that rely on the alignment of measurement bases between users sharing a quantum state. Polarization control within fiber is typically accomplished using feedback measurements from classical power alignment signals, multiplexed in time or wavelength with the quantum signal that coexist in the same fiber. This thesis explores ways to use only measurements on the entangled photons for polarization control and perform entanglement measures without multiplexing alignment signals. This approach is experimentally less complex and can reduce the noise within the quantum channel arising from the alignment signals. In the first part of this dissertation, we study how to use distributed measurements on polarization entangled photons for polarization drift correction in a 7.1 km deployed fiber between the University of Maryland and the Laboratory of Telecommunication Sciences for two individuals sharing a near maximally entangled Bell state, $\hat \rho = |\Psi^-\rangle\langle\Psi^-|$. In the second part of the dissertation, we examine how to use feedback measurements to maximize the violation of a Bell's inequality used as an entanglement measure. Both polarization drift correction and the maximization of a Bell's inequality violation use iterative optimization algorithms to actuate upstream polarization controllers. In the Bell's inequality investigation, three numerical methods: Bayesian optimization, Nelder-Mead simplex optimization, and stochastic gradient descent are implemented and compared against each other. For complete polarization control and Bell's inequality violation experiments, we developed a polarization and time multiplexed detection system that reduced the number of photon detectors needed and mitigates the demand on the coincidence counting electronics for real-time feedback and control.
Mingshu Zhao - March 1, 2024
Dissertation Title: Turbulence and superfluidity in atomic Bose-Einstein condensates
Date and Time: Friday, March 1, 12:00 PM
Location: PSC 3150
Dissertation Committee Chair: Daniel Lathrop
Committee:
Ian Spielman
Nathan Schine
Thomas Antonsen
Johan Larsson (Dean’s Rep)
Abstract:
In this dissertation, I investigate superfluid properties of atomic Bose-Einstein condensates (BECs) including laminar flow experiments that probe the superfluid density and turbulent flow experiments that explore connections to Kolmogorov theory. In this presentations, I focus on a novel technique to measure the BECs velocity field and apply it to turbulence. While turbulence in classical fluids has been extensively studied, there are many open questions in atomic superfluids, particularly regarding the existence of an inertial scale and the applicability of Kolmogorov scaling laws. I developed a velocimetry method, similar to particle image velocimetry using spinor impurities as tracers to measure the velocity field in a spatially resolved way. This enables the first observation of velocity structure functions (VSFs) in BECs, turbulent or otherwise. The observed VSFs reveal that superfluid turbulence in BECs conforms to Kolmogorov theory, including the so-called intermittency evident in both higher-order VSFs and the distribution of velocity increments.
Henry Elder - January 17, 2024
Dissertation Title: Nonlinear Propagation of Orbital Angular Momentum Light in Turbulence and Fiber
Date and Time: Wednesday, January 17, 3:00 PM
Location: AVW 2460
Dissertation Committee Chair: Phillip Sprangle
Committee:
Thomas Murphy
Howard Milchberg
Thomas Antonsen
Wendel Hill (Dean’s Rep)
Abstract:
Light that carries orbital angular momentum (OAM), also referred to as optical vortices or twisted light, is characterized by a helical or twisted wavefront ∝exp[imφ]. In contrast to spin angular momentum (SAM), where photons are limited to two states, OAM allows for, in principle, an infinite set of spatially orthogonal states. OAM-carrying light has found applications ranging from quantum key distribution in free space and guided-wave communication systems, particle trapping and optical tweezers, nanoscopy, and remote sensing. Understanding how OAM light propagates through complex environments, and how to efficiently generate particular OAM states, is critical for any such application.
In the first part of this dissertation, we describe how OAM light propagates through a turbulent atmosphere. We build analytic models which describe (1) the OAM mode mixing caused by turbulence, (2) the evolution of short, high-power OAM pulses undergoing the effects of self-phase modulation (SPM) and group velocity dispersion (GVD), and (3) the evolution of high-power Gaussian pulses including SPM, GVD, and turbulence. The models are compared to experimental data and nonlinear, turbulent pulse propagation simulation programs, which we have made freely available. We also explore how self-focusing can minimize certain deleterious effects of turbulence for OAM light.
The second part of this dissertation considers nonlinear effects of OAM light propagating in azimuthally symmetric waveguides. Such waveguides have so-called spin-orbit (SO) modes, which are quantized based on their total angular momentum (TAM). We develop a generalized theory of four wave mixing-based parametric amplification of SO modes and show that these processes conserve TAM, but under certain circumstances can be taken to conserve SAM and OAM independently. Our theory is validated against a nonlinear multimode beam propagation simulation program which we developed and, again, have made freely available.
Huan-Kuang Wu - January 8, 2024
Dissertation Title: Aspects of Unconventional Transport and Quasiparticle in Condensed Matter Systems
Date and Time: Monday, January 8, 1:00pm
Location: ATL 4402
Dissertation Committee Chair: Jay Sau
Committee:
Steven Anlage
Christopher Jarzynski
Johnpierre Paglione
Victor Yakovenko
Abstract:
The Boltzmann transport equation (BTE) is a successful framework for describing transport in condensed matter systems. The assumption of BTE is the localized energy excitations, or quasiparticles, and it is applicable to length scales above the mean free path of the quasiparticles. In this dissertation defense, I will cover two example systems where the assumptions of BTE fail. One is a new proposed system for Majorana zero mode whose topological phase is controlled by the phases of three s-wave superconductors. When the time reversal symmetry is restored, it can be tuned to a class DIII topological superconductor, which exhibits helical Majorana edge states. The second system I will talk about is the Josephson junction chain in the insulating regime, whose high-frequency plasmon remains coherent while the low energy excitations are Anderson-localized due to charge disorder.
Sara Nabili - January 4, 2024
Dissertation Title: Search for boosted semi-visible jets in all hadronic final states with the CMS experiment at CERN
Date and Time: Thursday, January 4, 10:30 am
Location: PSC 3150
Dissertation Committee Chair: Sarah Eno
Committee:
Thomas Cohen
Christopher Palmer
Peter Shawhan
Mohamad Al-Sheikhly
Abstract:
This dissertation represents the search for the dark sector beyond the standard model using the Compact Muon Solenoid experiment simulated Monte Carlo data of the Large Hadron Collider at CERN. The search is focused on the strongly coupled Hidden Valley models that couple with the standard model via a leptophobic (fully hadronic) Z′ mediator. The final state of resonant production consists of one large jet composed of both visible and invisible particles. This search focuses on the lower mediator mass range (mZ′ ≤ 550 GeV) with the boosted topology that recoils against the initial state radiation (ISR) jet such that its decay products are contained within a single large-diameter “semi-visible” jet. The main parameters of our model are the mediator mass, the mass of the dark mesons, and the fraction of invisible stable particles. In the event of no discovery, the exclusion limits for mediator mass of 275 to 550 GeV are expected.