Physics
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Item Modeling strong-field laser-atom interactions with nonlocal potentials(2017) Rensink, Thomas C.; Antonsen (Jr.), Thomas M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Atom-field interactions in the ionization regime give rise to a wide range of physical phenomena, and their study continues to be an active field of research. However, simulation of atom-field dynamics is time-consuming and computationally expensive. In this thesis, a nonlocal model potential is used in place of the Coulomb potential in the time dependent Schrodinger equation, and examined for suitabil- ity of modeling strong field-atom dynamics while offering significant reduction in computation cost. Nonlocal potentials have been used to model many physical systems, from multi-electron molecular configurations to semiconductor theory. Despite their rel- ative success, nonlocal potentials have been largely unexplored for modeling high field laser-gas interactions in the ionizing regime. This work explores the theory and numerical results of a single state gaussian nonlocal model in intense, femtosecond laser pulses, with the main findings: nonlocal potentials are useful for obtaining the photoionization rate in the tunnel and multiphoton regimes, and qualitatively char- acterize the wavefunction dynamics of irradiated atoms. The model is also examined in the context of the two-color technique for producing Terahertz (THz) frequency radiation.Item Explorations of Variable Interactions in a Cold Rubidium Rydberg Gas(2012) Robinson, Jennifer Elizabeth; Rolston, Steven L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We explore dipole-dipole interactions between cold 87Rb Rydberg atoms, their utility for quantum computing, and their potential role in the development of exotic quantum phases in optical lattice systems. Rydberg atoms can have large dipole-dipole interactions, due to the fact that they are easily polarized. We propose a new atomic state, created by admixing the Rydberg state with the ground state, in order to create an atom with a long lifetime and an intermediate dipole moment, which would be useful for experiments in optical lattices. These states could be used to probe phases of the extended Bose-Hubbard Hamiltonian, as well as create novel R-dependent interactions that are not realizable in conventional condensed matter systems. In addition to the dressed-Rydberg states, we consider the use of external DC electric fields to produce a variable interaction strength. A Stark map of the specific Rydberg levels shows the energy shift of a Rydberg atom in an electric field, as well as the dipole moment, from the slope of the curve. We study Rydberg excitation in an intermediate density regime under the effects of a variable external static electric field. We use superatom analysis and Monte Carlo simulations of a Rydberg system with dipole blockade to determine that our experimental observations are consistent with an increasing dipole-dipole interaction due to an induced dipole moment, with an enhancement due to black-body-induced transitions to nearby higher-angular-momentum states. We also investigate the Van der Waals interaction by considering the zero-field excitation rate for multiple principle quantum numbers.Item Nonequilibrium Quantum Fluctuation Forces(2010) Behunin, Ryan Orson; Hu, Bei-Lok B; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study all known and as yet unknown forces between neutral atoms and neutral atoms and surfaces. The forces arise from mutual influences mediated by an attending electromagnetic field and not from direct interaction. We allow as dynamical variables the center of mass motion of the atom (or surface Chapter 5), its internal degrees of freedom, modeled as a three dimensional harmonic oscillator (the internal degrees of freedom of the surface in chapter 4), and the quantum field treated relativistically. We adopt the methods of nonequilibrium quantum field theory (NEqQFT) to study the problem of fluctuation forces beginning from first principles. NEqQFT provides a fully dynamical description of systems far from equilibrium having the advantage of being the synthesis of quantum field theory and nonequilibrium statistical mechanics. The integration of these two paradigms is necessary for a complete study of fluctuation forces; quantum field theory for providing effects such as retardation and quantum field fluctuations, and nonequilbrium statistical mechanics for treating processes involving quantum dissipation and noises. By embarking from first principles we avoid wrong or only partially correct results from inconsistent theories that can be generated from assumptions made at lower levels of accuracy. In thermodynamic equilibrium we reproduce all the effects and forces known in the last century, such as Casimir-Polder-- between neutral atoms, Lifshitz-- between an atom and a surface and Casimir between surfaces (and the generalization of these forces to nonequilibrium stationary-states). More noteworthy is the discovery of the existence of a new type of interatomic force which we call the `entanglement force', originating from the quantum correlations of the internal degrees of freedom of entangled atoms. Fluctuation phenomena associated with quantum fields is a new frontier of future research in atom-field interaction. With NEqQFT we have derived Langevin equations which account for fluctuations of an atom's trajectory about its semi-classical value. These quantum field-induced perturbations of the atom's position could lead to measurable results such as the damping of the center-of-mass oscillations of a trapped Bose-Einstein condensate near a surface or backaction cooling of moving mirror by radiative pressure and quantum viscosity discussed respectively in Chapter 3 and 5 of this thesis. The methods introduced in this thesis for treating atom-field interactions or mirror-field interactions go beyond previous work by providing a fully dynamical description of these forces valid for arbitrary atom and surface motion, indeed the inclusion of self consistent backactions are necessary for the study of phenomena such as quantum decoherence and entanglement dynamics, including non-Markovian processes which invariably will appear when backaction is taken into consideration(especially for strong fields, low temperatures, or fast response).