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.