DRUM Collection: Physics Theses and Dissertations

Remote and Local Entanglement of Ions using Photons and Phonons

Sun, 01 Jan 2012 00:00:00 GMT

Title: Remote and Local Entanglement of Ions using Photons and Phonons Authors: Hayes, David Lee Abstract: The scaling of controlled quantum systems to large numbers of degrees of freedom is one of the long term goals of experimental quantum information science. Trapped-ion systems are one of the most promising platforms for building a quantum information processor with enough complexity to enable novel computational power, but face serious challenges in scaling up to the necessary numbers of qubits. In this thesis, I present both technical and operational advancements in the control of trapped-ion systems and their juxtaposition with photonic modes used for quantum networking. After reviewing the basic physics behind ion trapping, I then describe in detail a new method of implementing Raman transitions in atomic systems using optical frequency combs. Several dierent experimental setups along with simple theoretical models are reviewed and the system is shown to be capable of full control of the qubit-oscillator system. Two-ion entangling operations using optical frequency combs are demonstrated along with an extension of the operation designed to suppress certain experimental errors. I then give an overview of how spatially separated ions can be entangled using a photonic interconnect. Experimental results show that pulsed excitation of trapped ions provide an excellent single photon source that can be used as a heralded entangling gate between macroscopically separated systems. This heralded entangling gate is used to show a violation of a Bell inequality while keeping the detection loophole closed and can be used a source private random numbers. Finally, the coherent Coulomb force-based gates are combined with the probabilistic photon-based gates in a proof of concept experiment that shows the feasibility of a distributed ion-photon network.

Exploring Equilibrium Systems with Nonequilibrium Simulations

Sun, 01 Jan 2012 00:00:00 GMT

Title: Exploring Equilibrium Systems with Nonequilibrium Simulations Authors: Ballard, Andrew James Abstract: Equilibrium sampling is at the core of computational thermodynamics, aiding our understanding of various phenomena in the natural sciences including phase coexistence, molecular solvation, and protein folding. Despite the widespread development of novel sampling strategies over the years, efficient simulation of large complex systems remains a challenge. While the majority of current methods such as simulated tempering, replica exchange, and Monte Carlo methods rely solely on the use of equilibrium techniques, recent results in statistical physics have uncovered the possibility to sample equilibrium states through nonequilibrium simulations. In our first study we present a new replica exchange sampling strategy, "Replica Exchange with Nonequilibrium Switches," which uses nonequilibrium simulations to enhance equilibrium sampling. In our method, trial swap configurations between replicas are generated through nonequilibrium switching simulations which act to drive the replicas towards each other in phase space. By means of these switching simulations we can increase an effective overlap between replicas, enhancing the probability that these moves are accepted and ultimately leading to more effective sampling of the underlying energy landscape. Simulations on model systems reveal that our method can be beneficial in the case of low replica overlap, able to match the efficiency of traditional replica exchange while using fewer processors. We also demonstrate how our method can be applied for the calculation of solvation free energies. In a second, separate study, we investigate the dynamics leading to the dissociation of Na-Cl in water. Here we employ tools of rare event sampling to deduce the role of the surrounding water molecules in promoting the dissociation of the ion pair. We first study the thermodynamic forces leading to dissociation, finding it to be driven energetically and opposed entropically. In further analysis of the system dynamics, we deduce a) the spatial extent over which solvent fluctuations influence dissociation, b) the role of sterics and electrostatics, and c) the importance of inertia in enhancing the reaction probability.

Structured Plasma Waveguides and Deep EUV Generation Enabled by Intense Laser-Cluster Interactions

Sun, 01 Jan 2012 00:00:00 GMT

Title: Structured Plasma Waveguides and Deep EUV Generation Enabled by Intense Laser-Cluster Interactions Authors: Layer, Brian Abstract: Using the unique properties of the interaction between intense, short-pulse lasers and nanometer scale van-der-Waals bonded aggregates (or `clusters'), modulated waveguides in hydrogen, argon and nitrogen plasmas were produced and extreme ultraviolet (EUV) light was generated in deeply ionized nitrogen plasmas. A jet of clusters behaves as an array of mass-limited, solid-density targets with the average density of a gas. Two highly versatile experimental techniques are demonstrated for making preformed plasma waveguides with periodic structure within a laser-ionized cluster jet. The propagation of ultra-intense femtosecond laser pulses with intensities up to 2x1017 W/cm2 has been experimentally demonstrated in waveguides generated using both methods, limited by available laser energy. The first uses a `ring grating' to impose radial intensity modulations on the channel-generating laser pulse, which leads to axial intensity modulations at the laser focus within the cluster jet target. This creates a waveguide with axial modulations in diameter with a period between 35 μm and 2 mm, determined by the choice of ring grating. The second method creates modulated waveguides by focusing a uniform laser pulse within a jet of clusters with flow that has been modulated by periodically spaced wire obstructions. These wires make sharp, stable voids as short as 50 μm with a period as small as 200 μm within waveguides of hydrogen, nitrogen, and argon plasma. The gaps persist as the plasma expands for the full lifetime of the waveguide. This technique is useful for quasi-phase matching applications where index-modulated guides are superior to diameter modulated guides. Simulations show that these `slow wave' guiding structures could allow direct laser acceleration of electrons, achieving gradients of 80 MV/cm and 10 MV/cm for laser pulse powers of 1.9 TW and 30 GW, respectively. Results are also presented from experiments in which a nitrogen cluster jet from a cryogenically cooled gas valve was irradiated with relativistically intense (up to 2x1018 W/cm2) femtosecond laser pulses. The original purpose of these experiments was to create a transient recombination-pumped nitrogen soft x-ray laser on the 2p_{3/2_{→1s_{1/2_{(λ = 24.779 Å) and 2p_{1/2_{→1s_{1/2_{(λ = 24.785 Å) transitions in H-like nitrogen (N6+). Although no amplification was observed, trends in EUV emission from H-like, He-like and Li-like nitrogen ions in the 15 - 150 Å spectral range were measured as a function of laser intensity and cluster size. These results were compared with calculations run in a 1-D fluid laser-cluster interaction code to study the time-dependent ionization, recombination, and evolution of nitrogen cluster plasmas.}}}}}}}}

Topics in Spin-Orbit Coupling in Cold Atoms and Semiconductors

Sat, 01 Jan 2011 00:00:00 GMT

Title: Topics in Spin-Orbit Coupling in Cold Atoms and Semiconductors Authors: Anderson, Brandon Michael Abstract: This dissertation contains a collection of topics on the spin-orbit coupling of cold atoms and semiconductors. We first consider the effects of an optically induced spin-orbit interaction on a system of Bosonic atoms. The spin-orbit term couples the emergent pseudo-spin- 1/2 degree of freedom to momentum. The single particle energy spectrum has a low energy band with two minima at non-zero momentum. At low temperatures, a many-body system will condense into these minima. In the presence of vanishing interactions, the ground state wavefunction is found to be N00N state of "left" and "right"-moving state. An experimentally observable signature of the condensate is predicted that can be observed using time-of-flight imaging. We then predict a bulk manifestation of the spin-Hall effect in an inhomoge- neous spin-orbit-coupled system. The phenomena is predicted in the framework of the spin diffusion equations generalized to include arbitrary Rashba and linear and cubic Dresselhaus terms. This framework shows that a bulk spin-density wave with a wavevector oriented perpendicular to an applied electric field will induce a charge- density wave characterized by a π/2-phase shift and a non-monotonic time-varying amplitude. The optimal values of spin-orbit coupling for observation of the effect are determined. We propose a scheme for using atom interferometry to measure weak time- dependent accelerations. This proposal uses an ensemble of dilute trapped bosons with two pseudo-spin states coupled to a synthetic magnetic field, but with opposite effective charges. The synthetic field acts to couple spin to momentum continuously, which continuously imparts the acceleration on the phase of the internal states. We use time reversal pulses to reduce noise. The sensitivity of such a system is estimated to be S ∼ 10−7 m/s^2/√Hz Finally, we consider the semiclassical dynamics of a trapped spin-orbit coupled system. We find non-linear dynamics parameterized by the value of the initial displacement of the trap, and the anisotropy of the spin-orbit energy spectrum. We show that the dynamics can give a Berry's phase, and propose an experiment to measure this phase. We then propose a generalization of the 4-level scheme that allows for spin-orbit coupling described by a vector potential that is proportional to the angular momentum operator.