Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

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Quantum coherent phenomena in superconducting circuits and ultracold atoms
(2010) Mitra, Kaushik; Lobb, Chris J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This thesis consists of theoretical studies of superconducting qubits, and trapped bosons and fermions at ultracold temperature. In superconducting qubits I analyze the resonant properties and decoherence behavior of dc SQUID phase qubits, in which one junction acts as a phase qubit and the rest of the device provides isolation from dissipation and noise in the bias lead. Typically qubit states in phase qubits are detected by tunneling it to the voltage state. I propose an alternate non-destructive readout mechanism which relies on the difference in the magnetic flux through the SQUID loop due to state of the qubit. I also study decoherence effects in a dc SQUID phase qubit caused by the isolation circuit. When the frequency of the qubit is at least two times larger than the resonance frequency of the isolation circuit, I find that the decoherence time of the qubit is two orders of magnitude larger than the typical ohmic regime, where the frequency of the qubit is much smaller than the resonance frequency of the isolation circuit. This theory is extended to other similar superconducting quantum devices and has been applied to experiments from the group at the University of Maryland. I also demonstrate, theoretically, vacuum Rabi oscillations, analogous to circuit-QED, in superconducting qubits coupled to an environment with resonance. The result obtained gives an exact analytical expression for coherent oscillation of state between the system (the qubit) and the environment with resonance. Next I investigate ultracold atoms in harmonically confined optical lattices. They exhibit a `wedding cake structure' of alternating Mott shells with different number of bosons per site. In regions between the Mott shells, a superfluid phase emerges at low temperatures which at higher temperatures becomes a normal Bose liquid. Using finite-temperature quantum field theoretic techniques, I find analytically the properties of the superfluid, Bose liquid, and Mott insulating regions. This includes the finite temperature order parameter equation for the superfluid phase, excitation spectrum, Berezinskii-Kosterlitz-Thouless transition temperature and vortex-antivortex pair formation (in the two dimensional case), finite temperature compressibility and density - density correlation function. I also study interacting mixtures of ultracold bosonic and fermionic atoms in harmonically confined optical lattices. For a suitable choice of parameters I find emergence of superfluid and Fermi liquid (non-insulating) regions out of Bose-Mott and Fermi-band insulators, due to finite boson and fermion hopping. I also propose a possible experiment for the detection of superfluid and Fermi liquid shells through the use of Gauss-Laguerre and Gaussian beams followed by Bragg spectroscopy. Another area I explore is ultracold heteronuclear molecules such as KRb, RbCs and NaCs. I obtain the finite and zero-temperature phase diagram of bosons interacting via short range repulsive interactions and long-ranged isotropic dipolar interactions in two-dimensions. I build an analytical model for such systems that describes a first order quantum phase transition at zero temperature from a triangular crystalline phase (analogous to Wigner crystal phase of electrons) to superfluid phase. At finite temperature the crystalline phase melts, due to topological defects, to a hexatic phase where translational order is destroyed but hexagonal orientational order is preserved. Further temperature increase leads to the melting of the hexatic phase into a normal dipolar Bose liquid.
Two Experiments with Cold Atoms: I. Application of Bessel Beams for Atom Optics, and II. Spectroscopic Measurements of Rydberg Blockade Effect
(2010) Arakelyan, Ilya; Hill, III, Wendell; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In this dissertation we report the results of two experimental projects with laser-cooled rubidium atoms: I. Application of Bessel beams for atom optics, and II. Spectroscopic measurements of Rydberg blockade effect. The first part of the thesis is devoted to the development of new elements of atom optics based on blue-detuned high-order Bessel beams. Properties of a 4th order Bessel beam as an atomic guide were investigated for various parameters of the hollow beam, such as the detuning from an atomic resonance, size and the order of the Bessel beam. We extended its application to create more complicated interferometer-type structures by demonstrating a tunnel lock, a novel device that can split an atomic cloud, transport it, delay, and switch its propagation direction between two guides. We reported a first-time demonstration of an atomic beam switch based on the combination of two crossed Bessel beams. We achieved the 30% efficiency of the switch limited by the geometrical overlap between the cloud and the intersection volume of the two tunnels, and investigate the heating processes induced by the switch. We also showed other applications of crossed Bessel beams, such as a 3-D optical trap for atoms confined in the intersection volume of two hollow beams and a splitter of the atomic density. The second part of this dissertation is devoted to the spectroscopic measurements of the Rydberg blockade effect, a conditional suppression of Rydberg excitations depending on the state of a control atom. We assembled a narrow-linewidth, tunable, frequency stabilized laser system at 480 nm to excite laser-cooled rubidium atoms to Rydberg states with a high principal quantum number n ~ 50 through a two-photon transition. We applied the laser system to observe the Autler-Townes splitting of the intermediate 5p state and used the broadening of the resonance features to investigate the enhancement of Rydberg-Rydberg interactions in the presence of an external electric field.
Photon Pair Production from a Hot Atomic Ensemble in the Diamond Configuration
(2009) Willis, Richard Thomas; Rolston, Steven; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This thesis discusses four-wave mixing (4WM) in a warm ensemble of rubidium using the diamond configuration level structure. Both classical 4WM and non- classical photon-pair production are investigated. Quantum information science has spawned a great amount of experimental work on the interaction of light with collective modes of excitation in atomic ensem- bles. Plans to build quantum networks and quantum repeaters with atom ensembles take advantage of nonlinear interactions to produce and store non-classical states of light. These technologies will require photon sources that not only generate non- classical light, but also resonant, narrow band light. Here we investigate a system which could be used as such a source. We take advantage of the 4WM interaction in a warm ensemble of Rubidium atoms. Our scheme utilizes the diamond energy level configuration which, in ru- bidium, allows for correlated pairs at telecommunications wavelengths. We start by examining the properties of classical 4WM in the system. We measure the reso- nance structure and see that it can be understood in terms of velocity class selective resonant enhancement and power splitting effects. The efficiency of the process is low and limited by linear absorption of the pumps. Our observations agree with a semi-classical Maxwell-Bloch theoretical treatment. Next we observe pair generation by spontaneous 4WM from the warm ensem- ble. The temporal profile of the cross-correlation function (CCF) for the photons depends on pump-laser power and detuning. This allows us to produce biphotons with controllable spectra. A simple quantum optical theoretical treatment based on linear filtering gives qualitative agreement with the data. We show that the photon pairs are polarization entangled, clearly violating Bell's Inequality. A perturbative quantum optical treatment predicts the polariza- tion state of the pairs and agrees with our measurements. We analyze the photon statistics of the source and find the largest violation of the two beam Cauchy-Schwarz inequality from a warm atomic source yet. We cast the system as a heralded sin- gle photon source at telecommunications wavelengths and see that it is competitive with other systems in terms of spectral brightness.
Studies of atomic properties of francium and rubidium.
(2009) Perez Galvan, Adrian; Orozco, Luis A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
High precision measurements of atomic properties are excellent probes for elec- troweak interaction studies at the lowest possible energy range. The extraction of standard model coupling constants relies on a unique combination of experimen- tal measurements and theoretical atomic structure calculations. It is only through stringent comparison between experimental and theoretical values of atomic prop- erties that a successful experiment can take place. Francium, with its heavy nucleus and alkali structure that makes it amenable to laser cooling and trapping, stands as an ideal test bed for such studies. Our group has successfully created, trapped and cooled several isotopes of francium, the heaviest of the alkalies, and demonstrated that precision studies of atomic properties, such as the measurement of the 8S_1/2 excited state lifetime of 210Fr presented here, are feasible. Further work in our program of electroweak studies requires a better control of the electromagnetic environment observed by the sample of cold atoms as well as a lower background pressure (10-10 torr or better). We have designed and adapted to our previous setup a new &ldquo science &rdquo vacuum chamber that fulfills these requirements and the transport system that will transfer the francium atoms to the new chamber. We use this new experimental setup as well as a rubidium glass cell to perform precision studies of atomic and nuclear properties of rubidium. Spectroscopic studies of the most abundant isotopes of rubidium, 87Rb and 85Rb, are a vital component in our program. Performing measurements in rubidium allows us to do extensive and rigorous searches of systematics that can be later extrapolated to francium. We present a precision lifetime measurement of the 5D_3/2 state of 87Rb and a measurement of hyperfine splittings of the 6S_1/2 level of 87Rb and 85Rb. The quality of the data of the latter allows us to observe a hyperfine anomaly attributed to an isotopic difference of the magnetization distribution in the nucleus i.e. the Bohr-Weisskopf effect. The measurements we present in this work complement each other in exploring the behavior of the valence electron at different distances from the nucleus. In addition, they constitute excellent tests for the predictions of ab initio calculations using many body perturbation theory and bolster our confidence on the reliability of the experimental and theoretical tools needed for our work.
CONSTRUCTION OF APPARATUS AND FIRST EXPERIMENTS INVESTIGATING DYNAMICS OF BOSE-EINSTEIN CONDENSATES IN DISORDERED OPTICAL LATTICES
(2009) Edwards, Emily E.; Rolston, Steven L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Since the experimental achievement of Bose-Einstein condensation (BEC) in dilute gases, ultra-cold atom systems have proven to be an unparalleled test bed for condensed matter phenomena. With this in mind, our laboratory set out to build an apparatus for the study of the effects of disorder on condensed matter phase transitions using a BEC loaded into one, two, and three-dimensional lattices. My thesis is divided into two main sections. In the first section I describe in detail the design and construction of our apparatus. Our system is designed to form 87<\super>Rb condensates of approximately 105<\super> atoms. We have three possible experimental science chambers. 1. One can perform 1D lattice experiments in the chamber where the condensate is formed. There is a mirror located in vacuum, which is suitable for this purpose. 2. In a glass chamber one can do 1D, 2D, or 3D lattice experiments. Atoms are loaded into a dipole trap (optical tweezer) prior to condensation and transported approximately 20 cm to the glass cell where optical evaporation is performed to form a BEC. 3. One can transfer, using the same techniques as (2), to a chamber with a multi-channel plate detector for Rydberg atom experiments. The experimental results described in my thesis pertain to situation (1). However, I describe some details of (2) in the construction section. The first experiment presented in my thesis demonstrates the effect of disorder on the time-dependent dynamics of lattice systems. We observe that a small perturbation produces a dramatic change in the adiabaticity criteria for loading a BEC into one-dimensional optical lattice. I conclude with experiments that we expect to perform on this apparatus.
Ultracold Plasma Dynamics in a Magnetic Field
(2009) Zhang, Xianli; Rolston, Steven L.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Plasmas, often called the fourth state of matter and the most common one in the universe, have parameters varying by many orders of magnitude, from temperature of a few hundred kelvin in the Earth's ionosphere to 1016 K in the magnetosphere of a pulsar. Ultracold plasmas, produced by photoionizing a sample of laser-cooled and trapped atoms near the ionization limit, have extended traditional neutral plasma parameters by many orders of magnitude, to electron temperatures below 1 K and ion temperatures in the tens of &mu K to a few Kelvin, and densities of 105 cm-3 to 1010 cm-3. These plasmas thus provide a testing ground to study basic plasma theory in a clean and simple system with or without a magnetic field. Previous studies of ultracold plasmas have primarily concentrated on temperature measurements, collective modes and expansion dynamics in the absence of magnetic fields. This thesis presents the first study of ultracold plasma dynamics in a magnetic field. The presence of a magnetic field during the expansion can initiate various phenomena, such as plasma confinement and plasma instabilities. While the electron temperatures are very low in ultracold plasmas, we need only tens of Gauss of magnetic field to observe significant effects on the expansion dynamics. To probe the ultraocold plasma dynamics in a magnetic field, we developed a new diagnostic - projection imaging, which images the ion distribution by extracting the ions with a high voltage pulse onto a position-sensitive detector. Early in the lifetime of the plasma (< 20 &mu s), the size of the image is dominated by the time-of-flight Coulomb explosion of the dense ion cloud. For later times, we measure the 2-D Gaussian width of the ion image, obtaining the transverse expansion velocity as a function of magnetic field (up to 70 G),and observe that the transverse expansion velocity scales as B &minus1/2, explained by a nonlinear ambipolar diffusion model that involes anisotropic diffusion in two different directions. We also present the first observation of a plasma instability in an expanding ultracold plasma. We observe periodic emission of electrons from an ultracold plasma in weak, crossed magnetic and electric fields, and a strong perturbed electron density distribution in electron time-of-flight projection images. We identify this instability as a high-frequency electron drift instability due to the coupling between the electron drift wave and electron cyclotron harmonic, which has large wavenumbers corresponding to wavelengths close to the electron gyroradius.
Three-Body Recombination and Rydberg Atoms in Ultracold Plasmas
(2008-04-18) Fletcher, Robert S; Rolston, Steven; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Ultracold neutral plasmas, created by photoionizing samples of laser-cooled atoms, have well-controlled initial density and temperature parameters. With initial particle peak densities of ~10^15 m^-3, initial ion temperatures in the tens of micro-Kelvin range, and initial electron temperatures with a controllable range of 1-1000 K, these systems provide a means to study otherwise laboratory-inaccessible parameter ranges for plasma research. Furthermore, these plasmas are inhomogeneous, unconfined, and freely expanding into a vacuum. Despite the extraordinarily low electron temperatures, the electron system remains weakly coupled, although the ion system exhibits strong coupling behavior. While the initial electron temperatures are very low in ultracold plasmas, the temperature evolution has only been measured indirectly, in the earliest ~5% of the plasma lifetime, and often with large uncertainties. We present a technique that, with further theoretical support, can provide straightforward temperature measurements throughout the first fifth of the plasma lifetime. By making use of collective modes of the plasma, we fit a model of Tonks-Dattner resonances (electron sound wave propagating in the plasma) to measurements of these resonances and obtain a time-dependent electron temperature measurement for the ultracold plasma. Three-body recombination, a plasma loss process that has a rate scaling with the -9/2 power of the electron temperature, is of obvious interest in these ultracold plasma systems. Several theoretical works have predicted that the three-body recombination rate expression would need to be modified at these low electron temperatures, although the validity of these changes often hinges on the electron system being strongly coupled. We have performed the lowest temperature measurements of three-body recombination rates in a plasma and show that these measurements potentially provide a low-uncertainty means to calculate electron temperatures.
Optical Lattices and Quantum Degenerate ⁸⁷Rb in Reduced Dimensions
(2006-12-11) Huckans, John Howard; Phillips, William D.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This dissertation is about the physics of dilute gaseous Bose-Einstein condensates (BECs) confined to lower dimensions by optical lattices. The central theme of the effects of reduced dimensionality is explored within various one-dimensional (1D) and two-dimensional (2D) systems. We create a 2D BEC by adiabatically increasing the confinement of a trapping potential in one direction to the point where motion in that direction is frozen out. Doing this in two directions, we create a 1D BEC. Two experiments examine the ground state properties of a 1D and 2D system. In the 1D system (Chap. 9), a reduction in three-body recombination signals an increase in correlation resulting in a partial "fermionization" of the Bose gas. In the 2D system (Chap. 8), we measure temperature-dependent condensate phase fluctuations in the vicinity of the Berezinskii-Kosterlitz-Thouless transition. Other experiments investigate dynamic properties of reduced dimension systems. Strongly inhibited transport of a 1D gas in a lattice is observed in one experiment (Chap. 9). Another 2D experiment measures suppressed collisional decay rates due to the reduced dimensionality (Chap. 9). A final experiment (Chap. 7) examines quantum/classical correspondence in the effectively 1D dynamics of a 3D BEC. The dynamics is effectively 1D in the sense that the experiment is over before motion in the radial directions (which are not frozen out) can occur. This dissertation also describes the design and implementation of a novel 1D "accordion lattice" (Chaps. 5-6) which greatly facilitated the Berezinskii-Kosterlitz-Thouless experiment, the quantum/classical correspondence experiment, and a "superlattice" experiment conducted to assist in the calibration of the accordion lattice.
Cross-correlations and Entanglement in Cavity QED
(2006-06-30) Terraciano, Matthew Louis; Orozco, Luis A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Every quantum system subjected to measurements is an open quantum system. The cavity QED system is elegant in that it probes the interaction between two quantum systems, the atom and the field, while its loss mechanisms are well understood and can be externally monitored. The study of cross-correlations in cavity QED is important for understanding how entanglement evolves in open quantum systems. As quantum information science grows we need to learn more about entanglement and how it can be quantified and measured. Correlation functions have been used to compare an electromagnetic field (intensity) of one mode with the electromagnetic field (intensity) of the same mode at a later time or different spatial location. In quantum optics, correlation functions have been calculated and measured to probe the nonclassical field that results from the interaction of a single mode of the electromagnetic field and an ensemble of two-level atoms (the canonical cavity QED system). This field can exhibit antibunching, squeezing, and can violate inequalities required for a classical field. Entanglement in the steady state of a cavity QED system cannot be measured directly with traditional correlation functions (Hanbury-Brown and Twiss type experiments). Cross-correlations, however, interrogate directly both modes of the entangled pair, the transmitted (cavity) and the fluorescent (atom) intensities, and can act as an entanglement witness. This thesis presents the implementation of a cross-correlation measurement in a cavity QED system. The work has required the construction of an apparatus that incorporates laser cooling and trapping with quantum optics to carefully control both the external (center of mass motion) and internal (atomic state) degrees of freedom of a collection of atoms that interact with a single mode of a high finesse Fabry-Perot cavity. We examine theoretically and experimentally a new intensity cross-correlation function which probes the evolution of the cavity field conditioned on the detection of a fluorescent photon from an atom in the cavity. The results open the possibility to generalize the dynamics of entanglement as a physical resource necessary for the nascent quantum information science.
CONFINED ULTRACOLD BOSONS IN ONE DIMENSIONAL OPTICAL LATTICES
(2005-08-04) Pupillo, Guido; Williams, Carl J; Hu, Bei-Lok; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This dissertation presents my research covering the field of ultracold atoms loaded in optical lattices. The static and dynamical properties of atoms in combined periodic and parabolic potentials are studied, with a focus on the strongly interacting regimes. Because parabolic magnetic and optical potentials are routinely used to confine atoms, the results of this research are directly relevant to ongoing experimental endeavours in atomic physics. After a review of the basic theory of atoms in homogeneous periodic potentials, the equilibrium and non-equilibrium properties of non-interacting and interacting atoms in periodic plus parabolic potentials are studied. The problem of the localization of the many-body wavefunction for systems with arbitrary peak onsite density is presented in Chapters 3 and 4. The physics pertaining to the experimental realization of Mott insulator states with one or more atoms per sites in inhomogeneous lattices is elucidated by introducing an intuitive model for strongly interacting bosons in one dimension. This model is then utilized to study the decay of the dipole oscillations of atomic ensembles subject to a small displacement of the parabolic potential. Good agreement is found with results of recent experiments. Chapters 5 and 6 are dedicated to the characterization of the Mott insulator state with unit filling, which plays a central role in proposed schemes for neutral atom quantum computation. The usefulness of Bragg spectroscopy to probe the excitation spectrum of the Mott state in homogeneous lattices is analyzed in Chapter 5, where the limits of validity of linear response theory in this strongly correlated regime are delimited. In Chapter 6 the effects of finite temperature on the confined Mott insulator state are studied, and a scheme is devised for possibly estimating the system's temperature, at energies of the order of the inter-particle interaction energy. Finally, in Chapter 7, a proposal is introduced to utilize the Mott state as a robust register for neutral atom quantum computation. Unwanted residual quantum coherences inherent to the Mott insulator ground state are eliminated by a judicious choice of the trapping potentials and a selective measurement on a molecular photo-associative transition.

Theses and Dissertations from UMD

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