Multichannel Quantum Defect Theory (MQDT) is shown to be capable of producing quantitatively accurate results for low-energy atom-molecule scattering calculations. With a suitable choice of reference potential and short-range matching distance, it is possible to define a matrix that encapsulates the short-range collision dynamics. Multichannel quantum defect theory can provide an efficient alternative to full coupled-channel calculations for low-energy molecular collisions. However, the efficiency relies on interpolation of the Y matrix that encapsulates the short-range dynamics. It is shown how the phases of the MQDT reference functions may be chosen so as to remove such poles from the vicinity of a reference energy and dramatically increase the range of interpolation. For the test cases of Mg+NH and Li+NH, the resulting optimized Y matrix may be interpolated smoothly over an energy range of several Kelvin and a magnetic field range of over 1000G. Calculations at additional energies and fields can then be performed at a computational cost that is proportional to the number of channels N and not to N cubed. MQDT thus provides a promising method for carrying out low-energy molecular scattering calculations on systems where full exploration of the energy and the field dependence is currently impractical.

This thesis presents work on experiments performed using electromagnetically induced transparency (EIT) in Rubidium vapour cells. Starting with a simple three-level Λ-system, extra complexity is introduced by adding an additional control beam. This makes the switching from narrow transmitting resonances to absorbing resonances possible. Rydberg EIT is then introduced and is used to shown how the switching of such resonances using external electric fields in possible. The work then progresses towards the use of thin vapour cells to overcome the limitations of conventional systems. In the transition to thin vapour cells, the role of dipole interactions in Rydberg atoms motivates the idea of an atomic quantum dot. The Kerr effect is then characterised and used to perform phase modulation in a gas. The Rydberg state itself is then phase modulated and this is shown to give rise to frequency sidebands which offer a promising way of determining dc electric fields.

This thesis describes the observation of a novel optical non-linearity mediated by the dipole-dipole interactions in a cold gas of Rydberg atoms. Electromagnetically induced transparency (EIT) is used to map the strong dipolar interactions onto an optical transition, resulting in a cooperative effect where the optical response of a single atom is modified by the surrounding atoms due to dipole blockade. This optical non-linearity is characterised as a function of probe power and density for both attractive and repulsive interactions, demonstrating a non-linear density dependence associated with cooperativity. For the case of repulsive interactions, excellent agreement is obtained at low densities between experimental data and an interacting three-atom model. The ability to tune the interactions with an external field is also verified. This cooperative effect can be used to manipulate light at the single photon level, which is relevant for applications in quantum information processing. A theoretical model is developed to show that the non-linearity can be used to obtain a highly correlated single-photon output from a coherent laser field interacting with a single blockade region. Progress towards observing this experimentally is described, including details of the construction of a new apparatus capable of confining atoms to within a blockade radius.

No description available.

This work is concerned with elastic and inelastic scattering of ions and atoms. Semi-classical and quantal phase shift treatments are applied to the system of lithium ions in helium. A SCF-MO calculation of the interaction potential for the ground state of the system is reported and the results compared with other quantal calculations and with semi-emperical cross - sections. The cross-section for scattering through an angle greater than a given angle, the total elastic cross-section, the diffusion cross - section and the mobility are obtained and compared with experiment. The presence of orbiting is seen in the total cross - sections. It is predicted that the SCF-MO potential supports seven bound vibrational states. A classical binary encounter impulse approximation is applied to ionization of atoms. The velocity distribution of the bound atomic electrons is given by Hartree Fock wave function. Inner and outer shell ionizations cross- sections of atomic helium, lithium, oxygen, nitrogen and neon by electron and proton impact are calculated. The results are compared with other classical and quantal calculations, and where possible with experiment. Finally the excitation of atomic hydrogen by proton impact is considered within the framework of the impact parameter model. The closure approximation, which implicitly takes account of all rearrangement channels, is used to obtain excitation cross - sections into the 2s-state. The calculation is performed retaining only two states explicitly and the results are compared with those predicted by other quantal treatments. No experimental results are available for comparison.

No description available.

This thesis reports on the realisation and characterisation of a magnetically trapped ultracold atomic mixture of (^87)Rb and (^133)Cs in the F = 1, m(_F) = -1 and F = 3, m(_F) = - 3 hyperfine states respectively. A compact two-species double magneto-optical trapping (MOT) apparatus is constructed in which a pyramid MOT acts to provide an independent flux of both atomic species for capture in the ultra-high vacuum science region of the apparatus. For the two-species science MOT in which this atom flux is captured, interspecies light assisted inelastic collisions are found to be a highly significant loss mechanism. A novel optical pressure spatial displacement technique is developed to minimise such losses, allowing near independent simultaneous loading of up to ~ 8 x 10(^8) (^87)Rb and ~ 3 x 10(^8) (^133)Cs atoms into an Ioffe-Pritchard 'baseball' magnetic trap at magnetic biasfields of 166.70(6) and 165.50(6) G respectively. At the loaded 87Rb and 133Cs atom number densities of 1.78(6) x 10(^9) and 2.53(6) X 10(^9) cm-3 respectively the magnetic trap lifetime of each atomic species is shown to be 100(10) s and independent of the presence of the second atomic species. Radio-frequency evaporative cooling trajectories for (^87)Rb and (^133)Cs of 129 s duration are separately optimised under single species magnetic trap operation to achieve phase-space densities of 6(1) x 10(^-7) and 3(1) X 10(^-4) respectively at temperatures of 7.6(1) μK and 520(10) nK.(^133)Cs Feshbach resonances at 118.06(8) and 133.4(1) G are characterised through the measurement of magnetic field dependent losses at the increased phase-space density. Implementation of simultaneous evaporative cooling following the single species trajectories is found to be ineffective below ~10 μK due to the increased thermal load imposed upon the (^133)Cs atoms as the(^87)Rb single species elastic collision cross section approaches the low energy limit. Following simultaneous evaporation to ~ 15 μK thermalisation of the mixtures axial and radial temperature components suggests a (^87)Rb-(^133)Cs interspecies elastic collision rate 3(1) and 7(1) times greater than the calculated single species (^133)Cs and (^87)Rb elastic collision rates respectively. An interspecies Feshbach resonance search is undertaken by measuring the number of atoms of each species remaining in the magnetic trap as a function of applied magnetic field following simultaneous evaporation. The absence of magnetic field dependent losses in conjunction with analysis of the measurement sensitivity demonstrates that no interspecies Feshbach resonances wider than 1 G with two-body inelastic collision rate constants greater than 5 X 10(^-10) cm(^3) s(^-1) are present over the magnetic field range 166 < B < 370 G in the trapped states. The sensitivity of this measurement is found to be highly dependent upon the magnetic field induced differential gravitational sag of the mixtures components.

Cold gases of Rydberg atoms are an ideal system in which to study the novel effects of strong interatomic interactions. This thesis describes the design and construction of the world's first experiment to study Rydberg states in a cold gas of an alkaline earth metal, in this particular case strontium. We have studied a wide range of Rydberg states, and have developed a sensitive ``step-scan" spectroscopic technique that detects the spontaneous ionization of the Rydberg gas. The step-scan method is used to acquire Stark maps, and these measurements verify a single-electron model for calculating dipole matrix-elements. From the matrix-elements, interaction strengths between strontium Rydberg atoms have been calculated for the first time. The presence of two valence electrons in an alkaline earth metal, such as strontium, offers a new angle on the study of Rydberg atoms. We create doubly excited ``autoionizing" states, the first such study in a cold gas. Autoionization is used as a high yield probe of Rydberg states, and enables a study of excitation dynamics with nanosecond time-resolution. We show that autoionization can quantitatively identify and elucidate state mixing in the Rydberg gas, and probe population transfer at the very onset of ultra-cold plasma formation.

This thesis describes the development of a new technique for measuring the spatial distribution of Rydberg atoms in a cold strontium gas. Strontium atoms are cooled and trapped in a magneto-optical trap and coherently excited to Rydberg states in a two-photon, three-level ladder scheme. Several methods of stabilizing the frequency to the cooling transition are discussed and characterized. A frequency stabilization scheme based on electromagnetically-induced transparency for the second laser required for Rydberg excitation is also explained. The Rydberg population dynamics are studied experimentally and modeled using an optical Bloch equation simulation. The divalent nature of strontium allows doubly excited “autoionizing” states to be accessed using resonant optical excitation. These states ionize in subnanosecond timescales, with the ions recorded on a micro-channel plate being proportional to the amount of Rydberg atoms. Translation of an autoionizing laser focused to a waist of 10 μm gives a spatially resolved Rydberg signal. A two-dimensional map of the Rydberg spatial distribution has been made using this autoionizing microscopy technique.

Monte Carlo event generators, such as Herwig++, provide a full simulation of events at collider experiments. They give a fully exclusive description of hadronic final states and are therefore crucial tools for the planning of future experiments and analysing of data from existing experiments. The key component that allows this description of high-multiplicity final states is the parton shower. There has been much recent progress improving the parton shower description of hard radiation using exact matrix elements. This thesis describes research into implementing and improving such methods within the Herwig++ event generator. In Chapter 1, the parton-shower formalism is reviewed and the structure of event generators described. Chapter 2 details the specifics of the \textsf{Herwig++} parton shower. In Chapters 3 and 4, the POWHEG next-to-leading-order matching procedure is described, and work implementing the scheme within Herwig++ is presented. The method is implemented for the processes e+e- to hadrons and Drell-Yan vector boson production and the results are compared to experimental data from LEP and the Tevatron. This work includes the first full implementation of the truncated shower. A description of the development and implementation of a modified matrix-element merging scheme is presented in Chapter 5. This scheme is based on CKKW merging but uses an extension of the POWHEG idea to improve the method using truncated showers. The method is implemented first for final-state radiation in e+e- to hadrons and then, in Chapter 6, extended to include initial-state radiation in Drell-Yan vector boson production.