<p> 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.</p>

The pairing of fermionic particles is an essential ingredient of superconductivity and of the superfluidity of 3He. While such phenomena are accurately described by BCS theory in the limit of weak pairing strength, a complete understanding remains elusive when pairing strength is increased, such as in high temperature superconductors. We create ultracold gases of trapped fermionic 6Li atoms, through which we directly observe fermionic pairing. In our system, there are no impurities whatsoever, and parameters such as the number and temperature of the trapped atoms are precisely and independently controlled. In addition, a Feshbach resonance enables the continuous tuning of interaction strength and sign between the paired atoms. This control allows us to observe the smooth crossover of a molecular Bose Einstein condensate (MBEC) to a superfluid of weakly interacting Cooper pairs. With these tools, we have performed several fundamental measurements of pairing in fermionic systems. We use optical molecular spectroscopy to precisely measure the closed-channel contribution to the many body state of paired 6Li atoms within a broad Feshbach resonance. The magnitude of this contribution is small, and supports the concept of universality for the description of broad Feshbach resonances. Moreover, the dynamics of the excitation provide clear evidence for pairing across the BEC-BCS crossover, and for the first time, into the weakly interacting BCS regime. We also prepare a polarized Fermi gas with unequal numbers of two spin states of 6Li atoms. The real-space densities of the polarized, strongly-interacting, two-component Fermi gas reveal two low temperature regimes. At the lowest temperatures, the gas separates into a phase with a uniformly paired superfluid core surrounded by a shell of normal, unpaired atoms. This phase separation is accompanied by a spatial deformation of the core. At higher temperatures, the uniformly paired core persists, though it does not deform. This temperature dependence is consistent with a tri-critical point in the phase diagram. These measurements of pairing in a polarized Fermi gas are relevant to predictions of exotic phases of quark matter and magnetized superconductors.

Continued improvement of a single-mode frequency-stabilized 1.083 $\mu$m LNA laser developed at this laboratory has resulted in greater frequency stability and higher output power. Use of this laser with the Rice Flowing Helium Afterglow apparatus has resulted in considerable insight into Penning ionization reaction processes. Also, in the present work, a modified Coherent Inc. CR899-21 Ti:Sapphire laser is being used as a source of 1.083 $\mu$m radiation. A simple technique using a confocal Fabry-Perot etalon and stabilized helium-neon laser is used to limit long-term drift in the laser output frequency to $\leq$1 MHz per day. Measurements using a beam of He(2$\sp3$S) atoms show that the laser can provide stable He(2$\sp3$S) optical pumping resulting in polarizations approaching 100%. The laser is also being employed in studies of He(2$\sp3$P) collisions at low temperatures using laser-induced fluorescence and line-shape analysis. (Abstract shortened with permission of author.)

Potassium atoms in selected high-lying Rydberg states (n $<$ 1200) are used as a tool to probe threshold behavior in electron-molecule collisions. Collisions with non-polar electron attaching molecules such as CCl$\sb4$ are dominated by electron capture in a binary interaction between the Rydberg electron and target molecule allowing the study of electron attachment at electron energies of only a few $\mu$eV. Analysis of the data shows the cross section for electron capture is consistent with the Wigner threshold law for an inelastic s-wave process. Collisions with polar molecules can lead to Rydberg atom destruction through transfer of molecular rotational energy to the Rydberg electron. Rydberg atom data obtained with polar targets are not consistent with scattering from a static dipole. However, the data are consistent with a threshold law that takes into account a rotationally-averaged induced dipole interaction that can possibly support bound or virtual states.

Many interesting physical phenomena have been observed in electron-stimulated desorption studies of the cyanide molecule on alkali halide and alkali metal surfaces. We have performed a theoretical investigation of the nature of the cyanide-surface bond and of the desorption process using an ab initio density functional theoretic method of calculating electronic structure in the local density and Born-Oppenheimer approximations. We compare our results with experiment, and offer an explanation for an anomalous non-Boltzmann, temperature-independent rotational distribution experimentally observed in cyanide desorbed from these surfaces. As a verification of the method, we have performed extensive calculations on different bare alkali halide and alkali metal clusters and compared them with experimental results.

The use of spin-labelling to study the dynamics of He (2 $\sp3$S) metastable atom ionization of atoms and simple molecules is described. Briefly, He (2 $\sp3$S) atoms created by a microwave discharge are spin-polarized in a flowing afterglow via optical pumping with circularly-polarized 1.08 $\mu$m (2 $\sp3$S) $\Rightarrow$ (2 $\sp3$P) radiation from an LNA laser. A target gas is injected into the flowtube, and electrons created in Penning ionization reactions diffuse through a differentially-pumped aperture and are energy- and spin-analyzed with a hemispherical energy analyzer in series with a retarding-potential Mott polarimeter. Data are reported for Ar, CO$\sb2$, CO, H$\sb2$O, O$\sb2$, NO, NO$\sb2$, SO$\sb2$, and Cl$\sb2$ target gases. The generally accepted model of Penning ionization, the so-called exchange model, suggests that a target electron of appropriate spin tunnels to fill the He 1s hole with simultaneous ejection of the He 2s electron, which, for polarized He (2 $\sp3$S) atoms, produces fully polarized electrons. It is found that fully polarized electrons are ejected in reactions with closed-shell, negative electron affinity targets such as Ar and CO$\sb2$, independent of the positive ion final state, in agreement with the exchange model. Substantially lower polarization is measured for open-shell targets, such as O$\sb2$ and NO, and for targets with large electron affinity, such as NO$\sb2$ and SO$\sb2$. A strongly attractive entrance channel potential is possible in these cases. The exact depolarization mechanism is unclear, but is probably related to the formation of the ionic quasi-molecule. For some targets (Cl$\sb2$, for instance), excitation transfer with subsequent autoionization of the core-excited Rydberg target state created occurs, and these reactions also produce electrons with reduced polarization.

Low energy electron interactions with C$\sb2$Cl$\sb4$ and C$\sb6$F$\sb6$ molecules have been studied by using Rydberg atoms. It is observed that the electron attachment process occurs through various channels. Collisions of Rydberg atoms with C$\sb2$Cl$\sb4$ molecules result in the formation of Cl$\sp-$ ions and C$\sb2$Cl$\sb4\sp-$ ions. The data suggest the presence of the long lived C$\sb2$Cl$\sb4\sp-$ ions and that the C$\sb2$Cl$\sb4\sp-$ ions undergo rapid autodetachment forming an electron and a neutral C$\sb2$Cl$\sb4$ molecule. Similar reactions with the C$\sb6$F$\sb6$ molecules result in the formation of both long lived C$\sb6$F$\sb6\sp-$ ions and short lived C$\sb6$F$\sb6\sp-$ ions, that suffer rapid autodetachment forming an electron and a neutral molecule. The present data are compared with the previous data obtained by Swarm and Threshold Photoelectron Spectroscopy techniques.

Absolute differential cross sections for electron capture and loss by neutral hydrogen atoms incident on various gases are presented. The measurements cover a laboratory angular range of 0.02$\sp\circ$ to 1.77$\sp\circ$ and a laboratory energy range of 2.0 to 5.0 keV. The target gases include H$\sb2$, O$\sb2$, N$\sb2$, Ar and He. Integrated experimental cross sections are compared with total cross sections reported by other investigators.

Dissociative electron attachment reactions of the type K(nd) + AB $\to$ K$\sp+$ + AB$\sp{-*}$ $\to$ K$\sp+$ + A$\sp-$ + B have been investigated for polyatomic molecules BrCN, CF$\sb2$Br$\sb2$, CF$\sb3$Br and Fe(CO)$\sb5$. Rate constants for the formation of various negative ions were measured for a range of principal quantum numbers n. At high n (n $>$ 40), the Rydberg electron can be treated as 'essentially' free electron and the present data are compared with the free electron data obtained by the TPSA technique. At lower n, the free electron model must be modified to take into account post-attachment electrostatic interactions between the product ions and effects associated with the reduced size of the Rydberg atom. The translational energy release in such reactions was measured using the TOF-PSD technique. Comparison with the excess energy of reaction for BrCN and CF$\sb2$Br$\sb2$ shows that the energy released by electron capture is well distributed among the internal modes of the intermediate negative ion prior to dissociation. In contrast, data for CF$\sb3$Br indicate that the electron capture takes place into a repulsive state resulting in the immediate dissociation of the intermediate negative ion before significant redistribution of excess energy can occur.

$C\sb{60}\sp-$ ion formation has been observed in collisions between K(np) Rydberg atoms with principal quantum numbers $\rm35\leq n\leq125,$ which corresponds to electrons with mean energies in the range $\sim$2 meV to $\sim$170 $\mu$eV, and $C\sb{60}$ molecules. This results from Rydberg electron transfer and has permitted study of the low energy electron attachment properties of Buckminsterfullerene, which is known to capture free electrons with energies up to $\sim$12 eV. Present data agree with a previous Rydberg atom study but disagree with theoretical predictions and also with previous free electron-$C\sb{60}$ studies. Results from both theory and the free electron experiments suggest that a potential barrier of $\sim$240 meV exists for electron capture by $C\sb{60},$ preventing electron capture at very low energies. The present data are compared with previous experiments and with theory, and possible explanations for the observed discrepancy, including evidence for an image-charge-bound state of $C\sb{60},$ are discussed.