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College of Sciences

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School of Biological Sciences

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School of Chemistry and Biochemistry

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School of Earth and Atmospheric Sciences

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School of Mathematics

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School of Physics

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Now showing 1 - 9 of 9

Including a Warm Corona within the Inner Accretion Disk of Active Galactic Nuclei

(Georgia Institute of Technology, 2022-05) Xiang, Xin

Warm coronae, Comptonizing regions of warm (temperature kT ∼ 1keV), and optically thick (Thomson depth ∼ 10 - 20) gas, at the surfaces of accretion disks in active galactic nuclei (AGNs), have been proposed to explain the origin of the soft X-ray excess commonly observed in the X-ray spectra of AGNs. We calculate the X-ray emission from an irradiated constant density accretion disk atmosphere that includes heating from a warm corona, as well as illumination from an external X-ray power-law radiation, and blackbody emission from the dissipation in the accretion disk. The model accounts for the radial dependence of disk ionization, including the effects of light-bending on the illuminating X-rays. The final spectra are produced by integrating the local reflection/emission spectrum from approximately 2 to 400 gravitational radii. We demonstrate how the soft excess in AGN X-ray spectra depends on the warm corona parameters, including the heating fraction and optical depth, and the strength of the X-ray illumination. The model will be publicly released in 2022 for use in fitting AGN spectra.
Universal bound states of two- and three-body quantum systems

(Georgia Institute of Technology, 2020-07-27) Driscoll, Kevin Joseph

When there is a low-energy resonance between two particles, the typical separation of physical length scales no longer holds, and quantum systems can form states that are much larger on average than the characteristic size of the underlying interaction. Within this regime, long-wavelength theories with weak scattering break down; however, broadly applicable results can be recovered by replacing the interaction potential with appropriately chosen boundary conditions. We investigate two such systems of spinless bosons: three particles with resonant two-body interactions and two particles resonantly interacting with a flat surface. In the former case, where the Efimov effect is known to occur, we give exact expressions for the two- and three-body contacts, which describe the two- and three-body correlations in the system, at the threshold where the least bound Efimov trimer disassociates. In the latter, we find a sequence of bound states between the bosons and the surface (which produces a resonant potential) that are much larger on average than the range of interaction, and we show a new method for relating the contacts, which appear in the distribution of particles with large momenta, to the short-range correlations. The methods that we improve and develop are applicable to many systems with large scattering length and resonant potentials which open a different avenue to investigate effective three-body interactions.
Particle Image Velocimetry of Collapsing Toroidal Droplets

(Georgia Institute of Technology, 2015-08-18) Berger, Eric M.

The goal of this study is to explore the mechanism by which unstable toroidal droplets collapse inwardly. As such, particle image velocimetry methods will be employed in obtaining an experimental picture of the velocity field inside of unstable toroidal droplets as they collapse. The inward collapse exhibited by unstable toroidal droplets is unique to the geometry of the torus and is therefore physically interesting. There is currently not an available experimental picture of this collapse, so this study will attempt to fill that void. Ultimately the results of this study will be compared against the currently accessible theoretical pictures of collapsing toroidal droplets, leading to further refinements in the field.
Theory of light-matter interactions in cascade and diamond type atomic ensembles

(Georgia Institute of Technology, 2010-11-09) Jen, Hsiang-Hua

In this thesis, we investigate the quantum mechanical interaction of light with matter in the form of a gas of ultracold atoms: the atomic ensemble. We present a theoretical analysis of two problems, which involve the interaction of quantized electromagnetic fields (called signal and idler) with the atomic ensemble (i) cascade two-photon emission in an atomic ladder configuration, and (ii) photon frequency conversion in an atomic diamond configuration. The motivation of these studies comes from potential applications in long-distance quantum communication where it is desirable to generate quantum correlations between telecommunication wavelength light fields and ground level atomic coherences. In the two systems of interest, the light field produced in the upper arm of an atomic Rb level scheme is chosen to lie in the telecom window. The other field, resonant on a ground level transition, is in the near-infrared region of the spectrum. Telecom light is useful as it minimizes losses in the optical fiber transmission links of any two long-distance quantum communication device. We develop a theory of correlated signal-idler pair correlation. The analysis is complicated by the possible generation of multiple excitations in the atomic ensemble. An analytical treatment is given in the limit of a single excitation assuming adiabatic laser excitations. The analysis predicts superradiant timescales in the idler emission in agreement with experimental observation. To relax the restriction of a single excitation, we develop a different theory of cascade emission, which is solved by numerical simulation of classical stochastic differential equation using the theory of open quantum systems. The simulations are in good qualitative agreement with the analytical theory of superradiant timescales. We further analyze the feasibility of this two-photn source to realize the DLCZ protocol of the quantum repeater communication system. We provide a quantum theory of near-infrared to telecom wavelength conversion in the diamond configuration. The system provides a crucial part of a quantum-repeater memory element, which enables a "stored" near-infrared photon to be converted to a telecom wavelength for transmission without the destruction of light-atom quantum correlation. We calculate the theoretical conversion efficiency, analyzing the role of optical depth of the ensemble, pulse length, and quantum fluctuations on the process.
Cold elastic collisions of sodium and rubidium

(Georgia Institute of Technology, 2009-07-01) Breuer, John

In this thesis we numerically compute the scattering lengths and bound states for sodium-rubidium collisions at low energy. This work was motivated by experiments which aim to produce Bose-Einstein condensates (BEC) mixtures of sodium-rubidium. Elastic collision properties are important for the rethermalization of the atoms during the evaporative cooling process. Inelastic processes, which we also discuss to some extent, cause trap losses in those experiments. In order to reach the required temperature and density the elastic collision rates should be sufficiently large compared to the inelastic rates. The scattering lengths, which completely specify the elastic collision parameters at low energy, determine the miscibility and phase diagram of the sodium-rubidium condensate mixture. We calculate the scattering lengths approximately and find agreement with previous calculations indicating that miscible phases of sodium and rubidium condensates do not appear to be feasible in the absence of external fields.
Theory of Light - Atomic Ensemble Interactions: Entanglement, Storage, and Retrieval

(Georgia Institute of Technology, 2006-09-27) Jenkins, Stewart David

In this thesis, we explore the quantum dynamics of light interactions with optically thick collections of atoms. We provide a theoretical description of several recent experiments in which some key operations necessary for the implementation of quantum communication networks are demonstrated. Collective Raman scattering from an atomic ensemble is shown to produce probabilistic entanglement between the polarization of a scattered photon and an associated collective atomic excitation. The predicted correlations agree with experimental observations. We also propose a method to use cascade transitions to produce entanglement between a photon with a frequency in the telecom range (ideal for transmission over optical fibers) and a near infrared photon (ideal for storage in an atomic ensemble), and a description of the experimental demonstration is provided. We also propose and describe the implementation of a deterministic source of single photons. In addition, we generalize the theory of dark-state polaritons in ensembles of three level Lambda atoms to account for the nuclear spin degeneracy of alkali atoms. This generalized theory provides a description of the first demonstration of single photon storage and retrieval from atomic ensembles. Additionally, in the presence of a uniform magnetic field, we predict the occurrence of collapses and revivals of the photon retrieval efficiency as a function of storage time within the ensemble. These predictions are in very good agreement with subsequent experimental observations. We also exploit the ability of photon storage to entangle remote atomic qubits.
Quantum optical interactions in trapped degenerate atomic gases

(Georgia Institute of Technology, 2000-08) Berhane, Bereket H.
Kinetic theory of evaporative cooling of trapped atomic gases

(Georgia Institute of Technology, 2000-08) Geist, Wolfgang
Many-body theory of dissipative quantum optical systems

(Georgia Institute of Technology, 1995-12) Mertens, Christopher J.