Session K5: Undergraduate Session

Ionization of atomic hydrogen in strong infrared laser fields

We used the matrix iteration method of Nurhuda and Faisal\footnote{M.~Nurhuda and F.H.M.~Faisal, Phys.~Rev.~A~{\bf 60} (1999) 3125.} to treat ionization of atomic hydrogen by a strong laser pulse. After testing our predictions against a variety of previous calculations, we obtained ejected-electron spectra as well as angular distributions for few-cycle infrared laser pulses with peak intensities of up to $10^{15}\,$W/cm$^2$ by using the velocity form of the electric dipole operator in connection with an efficient time-propagation scheme. We demonstrate that our results are converged with the number of partial waves used in the expansion of the total wavefunction and that they are essentially free of numerical artifacts. The results are analyzed with particular emphasis on the effect of the carrier envelope for short pulses, and we produced movies to visualize the time-dependent electron density. Choosing parameters of currently available lasers, our predictions are expected to guide and be tested by ongoing experimental investigations.   

Alignment and orientation dependence of collision induced dissociation

A single collision between an $H_2^+$ or HeH$^+$ projectile (at $\sim$ keV/amu) and an Ar atom leads predominantly to dissociative capture (DC) and collision-induced dissociation (CID). The CID process can be driven by an electronic or vibrational excitation - the latter typically occurring in close-encounter collisions. One interesting question we focus on is the dependence of these CID mechanisms on the alignment of the molecule relative to its velocity. Our experimental evidence for $H_2^+$ on Ar collisions suggests that CID is favored for perpendicular alignment. Using hetero-nuclear molecules, namely HD$^+$ and HeH$^+$, we explore also the molecular-orientation dependence, i.e. is CID enhanced when the heavy or light nucleus is the one passing closer to the target atom?   

Rubidium Optical Pumping for an Electron Spin Filter

Our group is designing a novel polarized electron source based on spin exchange between an incident beam of electrons and an optically-pumped rubidium vapor target [1,2]. An overview of the spin filter design will be provided. I will then discuss optical pumping of rubidium and techniques for measuring spin polarization. An anomalous Rb polarization reversal detected when varying the wavelength of a pump laser with a spectral width of about 6 percent of the absorption profile of the Rb D2 transition width over the absorption profile will be examined. In the rubidium electron spin filter, viable spin exchange is thought to occur in the immediate vicinity of the exit aperture of the optical pumping region. Therefore, optical techniques for mapping the spatial dependence of a pumped Rb sample will be discussed, and measurements of Rb polarization throughout the optically-pump region will be presented.\\[4pt] [1] H. Batelaan \textit{et al.}, Phys. Rev. Lett.,\textbf{ 82}, 4216 (1999).\\[0pt] [2] M.A. Rosenberry, J.P. Reyes, D. Tupa, T.J. Gay Phys. Rev. A \textbf{75}, 023401 (2007).   

Theoretical Description of Trapped Low-Dimensional Dipoles in the Strongly-Interacting Regime

Recent experimental progress on trapping and cooling dipolar molecules raises the possibility of realizing strongly correlated dipole systems, where the potential energy plays the dominant role. Dipolar systems are expected to be stable if confined in effectively two-dimensional traps. Motivated by these prospects, we calculate the equilibrium positions of a classical system of dipoles at zero temperature stochastically via the Metropolis algorithm. The numerically determined equilibrium positions can be reproduced accurately and with minimal computational effort by a local density approximation using analytical expressions derived for the homogeneous system. For a sufficiently large number of particles the approximation is excellent. Both temperature and quantum mechanical effects lead to fluctuations of the dipoles around their equilibrium positions. The transition from a regular crystalline structure to a delocalized or melted liquid-like structure is investigated by analyzing the pair distribution function and the static structure factor. We specifically focus on characterizing the self-organizing of two neighboring dipolar systems.   

Matched Bichromatic Optical Lattices for Quantum Information Processing with Ultracold Atoms

We construct a novel system to independently trap and control two atomic species in separate optical lattices for scalable quantum information processing. One species serves as qubits, storing quantum information, and one as messengers, mediating entanglement to distant qubits and performing the desired gate operations. The key component of the lattice system is a homemade blazed diffraction grating, which automatically matches the lattice constants, functioning as a beamsplitter whose output angle depends on wavelength. Laser intensities and detunings are chosen to discriminate between the two species, and the exclusively common-mode optics provide relative stability of the lattice site on the order of 10nm. Precise control of each lattice is realized by electro-optic phase modulation of the component beams, which allows messenger atoms to access many qubits in a hexagonal lattice. We demonstrate the ability to perform these steps at the timescale and fidelity required for quantum computation.   

Experimentally measurable non-monotonicity for the quantum-classical transition in nonlinear nanoelectromechanical systems (NEMS)

Current experiments are exploring the quantum-classical boundary in nonlinear oscillator systems, that is, exploring the effects of changing size and changing decoherence. One such nonlinear system, the driven damped Duffing oscillator had been previously shown to display non-monotonic behavior in phase space. In this paper, we show how this behavior can be mapped to measurable quantities in experiments. These quantities show that the quantum-classical transition is nonmonotonic in the effective size of $\hbar$. Such a system is within experimental reach possibly for atomic systems and definitely for nanoelectromechanical systems (NEMS).