We study controlled methods of preparing vortex configurations in atomic Bose-Einstein condensates and their use in the studies of fundamental vortex scattering, reconnection processes, and superfluid sound emission. We explore techniques of imprinting vortex rings by means of coherently driving internal atomic transitions with electromagnetic fields which exhibit singular phase profiles. In particular, we show that a vortex ring can be prepared by two focused co-propagating Gaussian laser beams. More complex vortex systems may also be imprinted by directly superposing simpler field configurations or by programming their phase profiles on optical holograms. We analyze specific examples of two merging vortex rings in a trapped two-species ${}^{87}\mathrm{Rb}$ gas. We calculate the radiated sound energy in the reconnection process and show that the vortex relaxation and the redistribution of sound energy can be controlled by the imprinting process. As another creation technique, we study engineering pairs of two-dimensional point vortices in the condensates using a ‘light roadblock’ in ultraslow light propagation. We show how this can be used to study vortex collisions in compressible superfluids and how these collisions result in energy dissipation via phonons and, sometimes, annihilation of vortex pairs.

Polarization gating of high-order harmonic generation takes advantage of the significant reduction of harmonic generation efficiency for elliptically polarized excitation fields, in order to generate short bursts of harmonic radiation from relatively long pulses. We show that the currently used method for generation of polarization gated pulses using wave-plate combinations is inefficient, and propose an alternative method based on polarization pulse shaping techniques. This method is shown to be significantly more efficient and to enable significant shortening of the gate duration. Using this scheme, isolated attosecond pulses should be achievable with excitation pulses of a duration as long as 20 fs.

Using numerical solutions to relativistic quantum field theory with space-time resolution, we illustrate how an incoming electron wave packet with a definite spin scatters off a supercritical potential step. We show that the production rate is reduced of only those electrons that have the same spin as the incoming electron is reduced. This spin-resolved result further clarifies the importance of the Pauli-exclusion principle for the Klein paradox.

Metastable helium is buffer gas cooled, magnetically trapped, and evaporatively cooled in large numbers. ${10}^{11}$ ${}^4\mathrm{He}^{*}$ atoms are trapped at an initial temperature of $400\mathrm{mK}$ and evaporatively cooled into the ultracold regime, resulting in a cloud of $2\pm 0.5\times {10}^9$ atoms at $1.4\pm 0.2\mathrm{mK}$. Efficient evaporation indicates low collisional loss for ${}^4\mathrm{He}^{*}$ in both the ultracold and multi-partial-wave regime, in agreement with theory.

We observe a universal ionization threshold for microwave driven one-electron Rydberg states of H, Li, Na, and Rb, in an ab initio numerical treatment without adjustable parameters. This sheds new light on old experimental data and widens the scene for Anderson localization in light matter interaction.

Electronic structure methods are combined with variational fixed-nuclei electron scattering calculations and nuclear dynamics studies to characterize resonant vibrational excitation and electron attachment processes in collisions between low-energy electrons and $\mathrm{CF}$ radicals. Several low-lying negative ion states are found which give rise to strong vibrational excitation and which are expected to dominate the low-energy electron scattering cross sections. We have also studied several processes which could lead to production of negative ions (${\mathrm{F}}^{-}$ and ${\mathrm{C}}^{-}$); however, in contrast to other recent predictions, we do not find $\mathrm{CF}$ in its ground state to be a significant source of negative ion production when interacting with thermal electrons.

We investigate the interaction of ultracold antihydrogen with a conducting surface, focusing on the phenomenon of quantum reflection. We calculate the reflection probability as a function of incident atom energy. We find that, for ground-state $\overline{\mathrm{H}}$ atoms (with $T<{10}^{-5}\mathrm{K})$, the probability of reflection is $R\simeq 1-kb$, where $k$ is the momentum of the atom and $b=2174.0\mathrm{a}.\mathrm{u}.$ is a constant determined solely by the van der Waals–Casimir tail of the atom-wall interaction. We show that quantum reflection, which suppresses the direct contact of ultracold atoms with the surface, allows for the possibility of confinement and storage of sufficiently cold antihydrogen atoms. We calculate the lifetime of $\overline{\mathrm{H}}$ atoms enclosed in between solid walls. We develop a theory of $\overline{\mathrm{H}}$ in a waveguide and propose some applications, notably the measurement of retardation corrections in the long-range component of the antiatom-wall potential. We demonstrate the existence of quantized states for antihydrogen atoms falling onto conducting surface in the gravitational field of Earth. We calculate the lifetime of ultracold $\overline{\mathrm{H}}$ in its lowest gravitational state and obtain $\tau =(Mgb∕2ħ{)}^{-1}\simeq 0.1\mathrm{s}$, where $Mg$ is a gravitational force acting on the antiatom. We propose that measurement of this lifetime may provide a new test for the gravitational properties of antimatter.

We investigate fidelity for the quantum evolution of a Bose-Einstein condensate (BEC) and reveal its general property with a simple two-component BEC model. We find that, when the initial state is a coherent state, the fidelity decays with time in the ways of exponential, Gaussian, and power law, depending on the initial location, the perturbation strength, as well as the underlying mean-field classical dynamics. In this case we find a clear correspondence between the fast quantum fidelity decay and the dynamical instability of the mean-field system. With the initial state prepared as a maximally entangled state, we find that the behavior of fidelity has no classical correspondence and observe an interesting behavior of the fidelity: periodic revival, where the period is inversely proportional to the number of bosons and the perturbation strength. An experimental observation of the fidelity decay is suggested.

Multiconfigurational Hartree-Fock theory is presented and implemented in an investigation of the fragmentation of a Bose-Einstein condensate made of identical bosonic atoms in a double-well potential at zero temperature. The approach builds in the effects of the condensate mean field and of atomic correlations by describing generalized many-body states that are composed of multiple configurations which incorporate atomic interactions. Nonlinear and linear optimization is utilized in conjunction with the variational and Hylleraas-Undheim theorems to find the optimal ground and excited states of the interacting system. The resulting energy spectrum and associated eigenstates are presented as a function of double-well barrier height. Delocalized and localized single configurational states are found in the extreme limits of the simple and fragmented condensate ground states, while multiconfigurational states and macroscopic quantum superposition states are revealed throughout the full extent of barrier heights. Comparison is made to existing theories that either neglect mean field or correlation effects and it is found that contributions from both interactions are essential in order to obtain a robust microscopic understanding of the condensate’s atomic structure throughout the fragmentation process.

An unstable condensate of diatomic molecules will coherently disassociate into correlated pairs of atoms. This dissociation process exhibits very rich quantum dynamics depending on the quantum statistics of the constituent atoms. We show that in the case of bosonic atoms Bose enhancement can lead to stimulated dissociation, whereas, in the case of fermions Pauli blocking of the available states and a buildup of coherence between molecules and atom pairs can give rise to incomplete dissociation of the molecules and transient association-dissociation oscillations.

I give a simple argument that demonstrates that the state $\mid 0⟩\mid 1⟩+\mid 1⟩\mid 0⟩$, with $\mid 0⟩$ denoting a state with 0 particles (or photons) and $\mid 1⟩$ a one-particle state, is entangled in spite of recent claims to the contrary. I also discuss viewpoints on the old controversy about whether the above state can be said to display single-particle or single-photon nonlocality.

Recent experimental data for fully differential cross sections have been compared to various continuum-distorted-wave eikonal-initial-state models without much success, despite good agreement with double-differential cross sections. A four-body model is formulated here and results are presented both when the internuclear potential is omitted and when it is included. They are compared with recent experimental data for fully differential cross sections for $3.6\mathrm{MeV}∕\mathrm{u}$ ${\mathrm{Au}}^{Z_P+}+\mathrm{He}$ collisions, $Z_P=24,53$.

We study the coherent flow of a guided Bose-Einstein condensate incident over a disordered region of length $L$. We introduce a model of disordered potential that originates from magnetic fluctuations inherent to microfabricated guides. This model allows for analytical and numerical studies of realistic transport experiments. The repulsive interaction among the condensate atoms in the beam induces different transport regimes. Below some critical interaction (or for sufficiently small $L$) a stationary flow is observed. In this regime, the transmission decreases exponentially with increasing $L$. For strong interaction (or large $L$), the system displays a transition toward a time-dependent flow with an algebraic decay of the time-averaged transmission.

We describe the density profiles of confined atomic Bose gases in the high-rotation limit, in single-layer and multilayer geometries. We show that, in a local-density approximation, the density in a single layer shows a landscape of quantized steps due to the formation of incompressible liquids, which are analogous to fractional quantum Hall liquids for a two-dimensional electron gas in a strong magnetic field. In a multilayered setup we find different phases, depending on the strength of the interlayer tunneling $t$. We discuss the situation where a vortex lattice in the three-dimensional condensate (at large tunneling) undergoes quantum melting at a critical tunneling $t_{c_1}$. For tunneling well below $t_{c_1}$ one expects weakly coupled or isolated layers, each exhibiting a landscape of quantum Hall liquids. After expansion, this gives a radial density distribution with characteristic features (cusps) that provide experimental signatures of the quantum Hall liquids.

Circuit QED is a promising solid-state quantum computing architecture. It also has excellent potential as a platform for quantum control—especially quantum feedback control—experiments. However, the current scheme for measurement in circuit QED is low efficiency and has low signal-to-noise ratio for single-shot measurements. The low quality of this measurement makes the implementation of feedback difficult, and here we propose two schemes for measurement in circuit QED architectures that can significantly improve signal-to-noise ratio and potentially achieve quantum-limited measurement. Such measurements would enable the implementation of quantum feedback protocols and we illustrate this with a simple entanglement-stabilization scheme.

Dissociation of well-defined ${\mathrm{H}}_3$ Rydberg states into three ground state hydrogen atoms reveals characteristic correlation patterns in the center-of-mass motion of the three fragments. We present an extensive experimental dataset of momentum correlation maps for all lower Rydberg states of ${\mathrm{H}}_3$ and ${\mathrm{D}}_3$. In particular the states with principal quantum number $n=2$ feature simple correlation patterns with regular occurence of mutual affinities. Energetically higher-lying states typically show more complex patterns which are unique for each state. Quantum-chemical calculations on adiabatic potential energy surfaces of ${\mathrm{H}}_3$ Rydberg states are presented to illuminate the likely origin of these differences. We discuss the likely dissociation mechanisms and paths which are responsible for the observed continuum correlation.

Normalized doubly differential cross sections for the electron-impact ionization of helium at low energies are presented. The data are taken at the incident electron energies of 26.3, 28.3, 30.3, 32.5, 34.3, 36.5, and $40.7\mathrm{eV}$ and for scattering angles of 10°–130°. The measurements involve the use of the moveable target method developed at California State University Fullerton to accurately determine the continuum background in the energy-loss spectra. Normalization of experimental data is made on a relative scale to well-established experimental differential cross sections for excitation of the $n=2$ manifold of helium and then on an absolute scale to the well-established total ionization cross sections of Shah et al. [J. Phys. B 21, 2751 (1988)]. Comparisons are made with available experimental data and the results of the convergent close-coupling theory.

Perturbation theory for the Siegert pseudostates [Phys. Rev. A 58, 2077 (1998) and Phys. Rev. A 67, 032714 (2003)] is studied for the case of two energetically separated thresholds. The perturbation formulas for the one-threshold case are derived as a limiting case whereby we reconstruct More’s theory for the decaying states [Phys. Rev. A 3, 1217 (1971)] and amend an error. The perturbation formulas for the two-threshold case have additional terms due to the nonstandard orthogonality relationship of the Siegert pseudostates. We apply the theory to a two-channel model problem, and find that the rate of convergence of the perturbation expansion should be examined with the aide of the variance $D=\parallel E-{\sum }_n{\lambda }^n{E}^{\left(n\right)}\parallel$ instead of the real and imaginary parts of the perturbation energy individually.