Chemistry & Biochemistry Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2752
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Item How Non-Hermitian Superfluids are Special? Theory and Experiments(2024) Tao, Junheng; Spielman, Ian Bairstow; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ultracold atoms emerge as a promising advanced platform for researching the principles of quantum mechanics. Its development of scientific understanding and technology enriches the toolbox for quantum simulations and quantum computations. In this dissertation work, we describe the methods we applied to build our new high-resolution 87Rb Bose-Einstein condensate (BEC) machine integrated with versatile quantum control and measurement tools. Then we describe the applications of these tools to the research of novel superfluidity and non-Hermitian physics. Superfluids and normal fluids were often studied in the context of Landau’s two-fluid model, where the normal fluid stemmed from thermally excited atoms in a superfluid background. But can there be normal fluids in the ground state of a pure BEC, at near zero temperature? Our work addressed the understanding of this scenario, and then measured the anisotropic superfluid density in a density-modulated BEC, where the result matched the prediction of the Leggett formula proposed for supersolids. We further considered and measured this BEC in rotation and found a non-classical moment of inertia that sometimes turns negative. We distinguished the roles of superfluid and normal fluid flows, and linked some features to the dipolar and spin-orbit coupled supersolids. As a second direction, we describe our capability to create non-Hermiticity with Raman lasers, digital-micromirror device (DMD), and microwave, and present our work in engineering the real space non-Hermitian skin effect with a spin-orbit coupled BEC. By use of a spin-dependent dissipative channel, we realized an imaginary gauge potential which led to nonreciprocal transport in the flat box trap. We studied the system dynamics by quenching the dissipation, and further prepared stationary edge states. We link our discoveries to a non-Hermitian topological class characterized by a quantized winding number. Finally, we discuss the exciting promises of using these tools to study many-body physics open quantum systems.Item Towards Low-frequency Squeezed Light and Its Applications with Four-wave Mixing in Rubidium Vapor(2020) Wu, Meng-Chang; Lett, Paul D.; Rolston, Steven L.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study a variety of mechanisms that introduce noise into squeezed light generated by a four-wave mixing (4WM) process in Rb vapor. The noise from the seeding beam itself is a general noise that appears in any squeezed light generated from a seeding process. This noise dominates in the squeezed light at acoustic and lower measurement frequencies. A second excess noise source is observed in the twin beams pumped by either a diode laser system or a Ti:sapphire laser system. This excess noise is much stronger in the diode laser systems. It is present in the twin beams at measurement frequencies when the 4WM gain is reduced toward unity. Most of this excess noise can be removed with a dual-seeded 4WM scheme. A third noise source we examine is from a two-beam coupling that degrades the squeezing of the dual-seeded 4WM process at low frequencies of the order of the atomic transition linewidth. This noise can be avoided by seeding skew rays in the 4WM gain region. This gives us an insight to solve this "cross talk" problem by imaging the source in the 4WM gain region. In addition to studying noise sources, we propose a gain-independent calibration scheme that relies on higher order correlation function for the absolute calibration of photodiodes. Having low frequency squeezing is really important if we record quantum images with a CCD camera, which has a slow shutter speed. Also, it's been very difficult for people to get low-frequency squeezing. We obtain a record level of low-frequency squeezing using a simple dual-seeding technique. With this study of noise sources we are closer to having a portable quantum light source using diode lasers.Item Optical and quantum interferences in strong field ionization and optimal control(2017) Foote, David B.; Hill, Wendell T.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)For decades, ultrafast laser pulses have been used to probe and control strong-field molecular dynamics, including in optimal control experiments. While these experiments successfully recover the optimal control pulses (OCPs), they have a limitation -- it is generally unknown how the OCP guides the target system to its final state. This thesis is concerned with "unpacking" OCPs to explain how they achieve their control goals. The OCPs that inspired this work consisted of pulse trains; a twin-peaked pulse (TPP) is the simplest example. Consequently, TPPs with variable interpeak delay and relative phase were employed in this work to study ionization, the first step in many control experiments. Two types of interference influence ionization from a TPP: optical interference (OI) between the electric fields of the two peaks, and quantum interference (QuI) between the electron wavepackets produced by the two peaks. Two sets of experiments were performed to determine what roles OI and QuI play in controlling ionization from a TPP and how they in turn influence subsequent molecular dynamics. The first set of experiments measured the total ionization yield induced by the TPPs. It was found that OI was principally responsible for changing the ion yield; QuI-induced oscillations were not observed. Small imperfections in the shape of the TPP (i.e., pedestals and subordinate peaks) were found to have a surprisingly large influence in the OI, highlighting the need for researchers in molecular control experiments to characterize the temporal profile of their pulses accurately. A time-dependent perturbation theory simulation showed that the signatures of QuI in the ionic continuum vanish when measuring {\it total} electron yield, but appear in {\it energy-resolved} electron yields. The second set of experiments measured photoelectron energy distributions from a TPP with a velocity map imager to search for QuI. The experiments were performed at high intensities (~10^14 W/cm^2) where the ponderomotive energy tends to wash out the fine energy structures of QuI. The thesis ends by proposing a modified, low-intensity experiment that will allow for the first unambiguous observation of QuI in non-resonant, multiphoton ionization.Item Dissipation in a superfluid atom circuit(2017) Lee, Jeffrey Garver; Hill, Wendell T; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Bose-Einstein condensates of weakly interacting dilute atomic gases provide a unique system with which to study phenomena associated with superfluidity. The simplicity of these systems allows us to study the fundamental physics of superfluidity without having to consider the strong interactions present in other superfluid systems such as superconductors and liquid helium. While condensate-based studies have been around for 20 years, our novel approach to confining ultracold atoms has opened a completely new range of parameter space to investigate. Armed with an ability for straightforward creation of arbitrary, time-dependent potential landscapes in which to study superfluid interactions, we were able to take a closer look at predictions of superfluid behavior that are decades old, but until now have never been tested directly. The purpose of this research was to draw direct analogies between superfluid BEC systems, which we term superfluid atom circuits, and existing superconducting circuits, thus allowing us to take advantage of much of the existing knowledge that has come from this well-studied field. Specifically, existing circuits and devices that have been created with superconductors give us insight into what might be possible someday with atom-circuit devices and inspiration to create them. In these experiments, we employed two different atom circuits; one classical (thermal ideal gas) and one quantum (ultracold superfluid). Our results show that each system is equivalent to an electronic circuit consisting of a capacitor being discharged through an inductor in series with some dissipative element. In the thermal system, dissipation can be described in terms of simple resistive flow with the resistance equivalent to ballistic, Sharvin resistance seen in electronic circuits. The superfluid measurements show that the dissipation is best described as a resistance-shunted Josephson junction, which is an analogue to similar devices in superconducting circuits. Additionally, the specific geometry of the atom circuit we used in our superfluid system allowed us to investigate directly a predicted mechanism responsible for the dissipation in superfluids caused by the generation of collective excitations, namely vortices. Direct observation of this mechanism has not previously been possible in superfluid helium and superconducting systems.Item AN APPARATUS FOR LIGHT-LESS ARTIFICIAL GAUGE FIELDS AND NEW IMAGING TECHNIQUES(2015) Perry, Abigail Reiko; Spielman, Ian B; Rolston, Steve; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The thesis presented has three components: experiments with artificial vector potentials, a new atom-chip apparatus designed and built for light-less fictitious gauge fields, and an imaging experiment. First, we introduce experiments with light-induced vector potentials using two-photon Raman coupling to simulate charged particles using charge neutral Bose-Einstein condensates (BECs). Depending on the spatial and temporal properties of the engineered vector potential, it is possible for ultracold atoms to experience different variants of an effective Lorenz force such as; magnetic fields, electric fields, and spin-orbit coupling, via coupling between an atom's internal spin and its linear momentum. In this context, we discuss the main focus of this thesis, the design and construction of an atom-chip apparatus for $^{87}$Rb BECs for experiments with light-less artificial gauge fields. Eliminating the source of heating due to spontaneous emission will open new paths to explore artificial gauge fields in alkali fermions and will be a step towards the realization of simulated topological insulators using ultracold atoms. Finally, we will describe in detail an imaging experiment performed on this new apparatus, the reconstruction of the two-dimensional column density of a BEC using multiple defocused images taken simultaneously.Item NONEQUILIBRIUM MANYBODY DYNAMICS WITH ULTRACOLD ATOMS IN OPTICAL LATTICES AND SELECTED PROBLEMS IN ATOMIC PHYSICS(2014) Brown, Roger; Porto, James V; Rolston, Steven L; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis is a collection of three separate projects ordered according to the historical development of atomic physics, covering first spectroscopy, then laser cooling, and finally the exploration of quantum dynamics of many-particle states of ultra-cold atoms in optical lattices. We begin with a description of the theory of atomic line shapes with unresolvable hyperfine structure. We apply this theory to experimentally measured spectra of the Lithium D lines and report improved determination of the absolute transition frequencies and an improved bound of the difference in 6Li-7Li nuclear charge radius. We then discuss multi-photon processes in laser cooling and report experimental implementation of multi-photon laser cooling and magneto optical trapping using short lived excited to excited transitions in 133Cs. We present a theoretical proposal to laser cool (Anti-) Hydrogen using a Doppler selective 1S-2S excitation and the Sisyphus effect on the 2S-3P transition. Finally, we detail the construction and operation of an ultracold 87Rb apparatus with a double well optical lattice. We use this lattice to prepare excited many-body states with N´eel antiferromagnetic order and to study the resulting non-equilibrium magnetization dynamics. We observe regimes where the dynamics is dominated by superexchange mediated magnetic interactions.