Parmi les mécanismes de refroidissements d'atomes par laser, l'effet Sisyphe conduit à une situation où les atomes sont localisés aux minima d'un potentiel périodique induit par la lumière. Il est ainsi possible de créer un "cristal" optique dont les caractéristiques sont déterminées par la géométrie des faisceaux lasers. Cette thèse propose une étude expérimentale de la dynamique d'atomes alcalins (cesium, rubidium) piégés dans un réseau de lumière tridimensionnel, par une méthode de spectroscopie pompe-sonde. La localisation des atomes dans des puits de potentiels à l'échelle d'une fraction de longueur d'onde optique est démontrée par l'observation de résonances Raman stimulées entre niveaux de vibration quantiques. Ce type de résonance a également révélé le comportement paramagnétique d'un tel réseau. Un effet nouveau de propagation d'ondes de densité dans le réseau conduit à une diffusion de type Brillouin stimulée qui présente l'originalité de ne pas faire intervenir d'interaction entre les particules diffusantes. La diffusion Rayleigh stimulée donne accès à la diffusion spatiales des atomes dans le réseau. Une étude détaillée des résonances Rayleigh démontre également l'importance de la pression de radiation qu'exerce la lumière sur les atomes, mettant en évidence un effet analogue à l'effet photoréfractif observé dans des cristaux de BaTiO3 ou LiNbO3.

L'échelle naturelle de vitesse en refroidissement laser est la vitesse de recul communiquée à un atome initialement au repos lorsqu'il absorbe ou émet un photon. La méthode des résonances noires sélectives en vitesses permet de réduire la vitesse des atomes en-dessous de cette limite. Nous présentons les premiers résultats expérimentaux du refroidissement subrecul d'atomes d'hélium à trois dimensions par résonances noires. Nous obtenons des températures aussi basses que 180 nK par des movens tout optiques. Les résultats expérimentaux sont comparés avec les prédictions d'un modèle théorique utilisant les statistiques de Lévy. Les atomes refroidis par résonances noires sont préparés dans une superposition cohérente de plusieurs paquets d'ondes de de Broglie. En utilisant le transfert adiabatique induit par laser nous avons réussi à transférer la population atomique dans un seul paquet d'onde. Nous avons démontré que le transfert fonctionne aussi à deux et trois dimensions et nous avons mesuré une efficacité de l'ordre de 90% à deux dimensions et de 75% environ à trois dimensions. Le passage adiabatique nous a permis de préparer un paquet d'onde ayant une dispersion en vitesses subrecul, ainsi qu'un état de spin et une direction choisis arbitrairement. Enfin, nous avons mis en évidence les états noirs sélectifs en vitesses sur une transition J = 1 → J = 0 et nous avons développé une méthode théorique simple qui permet de prédire la structure de ces états dans l'espace des impulsions.

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Les collisions réactives entre un atome et une molécule représentent l'acte chimique élémentaire le plus simple. C'est donc à travers l'étude de ces collisions que l'on peut atteindre une compréhension fondamentale de la réaction chimique. Une technique particulièrement efficace pour ces études est la détection par fluorescence induite par laser des produits moléculaires de réaction. Cette technique de détection, sélective en état interne, permet de connaître la distribution du produit moléculaire sur les états internes, et même de mesurer pour chacun de ces états la section efficace différentielle, la vitesse étant déterminée par effet Doppler. Nous nous sommes intéressé à la collision F + I2 —-> I F + I que nous avons étudiée dans une expérience de jets croisés. La section efficace de réaction est importante et la molécule IF est facilement détectable par fluorescence induite par laser grâce à la transition (B<-X) qui intense et située dans le visible. Nous avons ainsi pu travailler avec des lasers à colorants continus monomodes. La saturation du processus d'excitation laser de cette transition a été soigneusement étudiée, de manière théorique et expérimentale. Cette étude, ainsi qu'une analyse détaillée de la spectroscopie de la transition (B<-X) nous a permis de déduire des intensités des raies de fluorescence les populations des différents niveaux rovibrationnels peuplés de l'état X, avec la plus grande cohérence interne possible. D'une part nous avons ainsi pu établir une cartographie détaillée de la distribution rotationnelle des niveaux vibrationnels v = 8 à 20 de l'état X de IF. Celle-ci s'étend jusqu'à la limite d'énergie disponible dans tous ces niveaux. Ces distributions rotationnelles présentent un aspect bimodal qui implique la coexistence de deux chemins réactionnels microscopiques différents. Cette bimodalité a été observée récemment dans une autre, étude expérimentale de cette réaction (où IF est détecté de manière non sélective, mais où sa vitesse de recul est mesurée) et prévue dans une simulation numérique de la réaction. D'autre part, nous avons montré que l'important signal observé dans des expériences antérieures sur le niveau v=0 était un artefact provoqué par des réactions de surface. Enfin, des mesures préliminaires par effet Doppler de la section efficace différentielle sont présentées. En conclusion, de nombreux résultats nouveaux onc été obtenus sur cette collision reactive. Nous avons montré, par exemple, que l'utilisation soignée; de la fluorescence induite par laser mène à des mesures précises de la distribution des molécules sur leurs états internes. La connaissance de cette réaction a ainsi pu être considérablement approfondie: celle-ci est sans doute actuellement l'une des collisions réactives les mieux décrites expérimentalement.

To utilize a single atom as a quantum bit for a quantum computer requires exquisite control over the internal and external degrees of freedom. This thesis develops techniques for controlling the external degrees of freedom of individual atoms. In the first part of this thesis, individual atoms are trapped and detected non-destructively by the addition of cooling beams in an optical lattice. This non-destructive imaging technique led to atomic storage times of two minutes in an optical lattice. The second part of thesis incorporated the individual atoms into a high finesse cavity. Inside this optical cavity, atoms are cooled and non-destructively observed for up to 10 seconds.

Element abundance trends with overall metallicity contain vital clues to the formation and evolution of the Galaxy. Abundances may be used to elucidate nucleosynthesis mechanisms and to ascertain rates of Galactic enrichment. To obtain accurate abundances, several crucial inputs such as high-quality spectroscopic observations, rigorous calculations of line transfer, and precise atomic data (e.g. transition probabilities) are necessary. The current work endeavors to improve abundance values for key elements with a four-fold approach: accumulation of hundreds of high-resolution stellar spectra in order to commence a systematic and thorough Manganese abundance derivation in cluster and halo field stars; re-determination of the neutral chromium oscillator strengths and application of this data to stellar abundance analyses; modification of a radiative line transfer code in order to yield accurate abundances from evolved stars; and semi-empirical derivation of transition probabilities to allow for the utilization of spectral features in the red visible and infrared wavelength ranges for abundance determinations. The first comprehensive investigation of manganese in globular clusters is done in this work. A subsolar Mn abundance trend for both halo globular cluster and field stars is found. The analysis shows that for the metallicity range -0.7>(Fe/H)>-2.7 stars of 19 globular clusters have a a mean relative abundance of <(Mn/Fe)>= -0.37±0.01 (σ=0.10), a value in agreement with that of the field stars: <(Mn/Fe)>= -0.36± 0.01 (σ=0.08). Remarkably, the <(Mn/Fe)> ratio remains constant in both stellar populations over a 2 orders of magnitude span in metallicity. Next, the present study employed branching fraction measurements from Fourier transform spectra in conjunction with published radiative lifetimes to determine transition probabilities for 263 lines of neutral chromium. These laboratory values are used to derive a new photospheric abundance for the Sun: log [element of](Cr I)⊙= 5.64±0.01 (σ=0:07). In addition, oscillator strengths for singly-ionized chromium recently reported by the FERRUM Project are employed to determine: log [element of](Cr II)⊙ = 5.77±0.03 (σ= 0.13). No indications of departures from LTE are found in the neutral chromium abundances. The current work then takes advantage of the fact that transition metals exhibit relatively pure LS coupling and employs standard formulae to yield semi-empirical oscillator strengths. These data were then compared to experimental gf values in order to assess accuracy. Finally, this study undertakes a new abundance investigation of the RGB and RHB stars of the M15 globular cluster. A detailed examination of the both the metallicity and n capture elements is performed. This work appears to confirm that star-to-star abundance variations do occur among the M15 giants (which was initially observed by Sneden et al. 1997, 2000). / text

Quantum information science involves the use of precise control over quantum systems to explore new technologies. However, as quantum systems are scaled up they require an ever deeper understanding of many-body physics to achieve the required degree of control. Current experiments are entering a regime which requires active control of a mesoscopic number of coupled quantum systems or quantum bits (qubits). This thesis describes several approaches to this goal and shows how mesoscopic quantum systems can be controlled and utilized for quantum information tasks. / Physics

Thermal conductivity measurements have been used to study the low lying energy levels of Cr ions in GaAs. Strong resonant phonon scattering was observed in semi-insulating (SI) and p-type samples, which is attributed to Cr 2+ or Cr 3+ ions, while the scattering in the n-type samples additional to that in undoped material was very small. From the computer fits of the thermal conductivity, zero-field ground state splittings have been deduced. A splitting at ~ 23 cm -1 is attributed to Cr 3+ ions, others at ~ 0.7 cm and 4.9 cm are due to Cr 2+. The phonon scattering in the n-type samples did not show any magnetic field dependence while big effects were observed in SI and p-type ones. This seems consistent with the results of the zero-field work. The effect of uniaxial stress on the phonon scattering has been measured in the temperature range ~2-15 K. Again no effects were seen in n-type material. The results for SI and p-type material are interpreted in terms of a static Jahn-Tellereffect of Cr 2+ ions. A preliminary investigation was made of the effect on the phonon scattering of sub-band-gap illumination. In an-type sample, the decay in the increase in the thermal resistivity produced by photoexcitation showed two parts. The first part with a characteristic time of ~ 1 hour is attributed to electron-capture at Cr 2+ ions. The second decay was very slow (persistent) with a characteristic time T » 10 5 s. This effect has tentatively been attributed to the occurrence of large lattice relaxation. The phonon scattering by the Cr ions is found to be consistent with the 'double acceptor' model for Cr in GaAs. Another model where er can act as a hole trap is discussed. Finally the effect of high magnetic fields on the thermal conductivity of acceptors in Ge was measured. From this and previous results, the g-values describing the ra8 ground state were found to be much lower than the predicted ones. A Thermally Detected EPR apparatus was designed and constructed in an attempt to check on the ground state structure of p-Ge and also p-Si but no results were obtained. This is believed to be due to the very large line widths resulting from strain splitting of the ra8 ground state.

The mini-PET system employed in this study incorporates two blocks of BOO detector crystals. Each detector module contains a 25 x 54 mm block of BOO which is segmented into a 6 x 8 array of crystal elements, 3.5 mm ( transaxially ) by 6.25 mm ( axially ) and 30 mm ( deep ). The crystals are coupled to two Hamamatsu R1548 dual photomultiplier tubes ( PMTs ). The segmented BOO detector block is currently being employed in a number of the commercially available positron tomographs. While the above mentioned block detector has significant advantages over discrete detectors, better energy resolution, sensitivity and axial resolution, it also has disadvantages such as: inter-detector scattering, and variations in sensitivity and energy and spatial resolution across the face of the detector block. In this study the variation in the efficiency, energy resolution, position of the full energy photopeaks, spatial resolution and the amount of inter-crystal scatter fraction for the individual detector crystals across the face of the detector block were investigated. The factors contributing to these variations were identified and suggestions for reducing their effects were made. For example, the inter-detector scattering and the light sharing scheme, employed in the detector block for the identification of the crystal of interaction, were found to be the cause of mispositioning the events. Two novel techniques which showed several advantages over the techniques, employed by other groups, for evaluating the amount and distribution of inter-crystal scattered events were also established. The amount of inter-crystal scatter fraction for one of the central detector crystals, utilising both of the above mentioned techniques, was obtained and found to be almost 36%. The spatial distribution of such scattered events had a FWHM of 1.4 mm which its convolution with the so called ideal LSF of the crystal deteriorated its FWHM by 0.5 mm. It was shown that the energy distribution of the majority of the inter-detector scattering events was around the 511 ke V peak and these scattering events were detected within the full energy photopeak of the true events. Hence it is not possible to reject or discard these events by means of setting a higher energy threshold. This fact, in conjunction with the contribution of these events in worsening the spatial resolution of the PET systems utilizing crystals of small dimensions, demands special consideration to be given to the problem of inter-crystal scattering when further reducing the size of the crystals. Therefore, it is recommended that prior to incorporating detector blocks, which are cut into crystals of small dimensions, in PET systems, the amount and the distribution of the scattered events and their contribution in deteriorating the spatial resolution should be investigated by employing the techniques established in this work.

In this thesis I present research in neutrino physics utilizing tools from both atomic physics and astrophysics. Recent advances in atomic physics enable a new type of beta decay experiment to measure the absolute mass scale of the neutrino using a sample of ultracold atomic tritium. These initial conditions enable the detection of the helium ion in coincidence with the beta. I construct a two-dimensional fit incorporating both the shape of the beta spectrum and the direct reconstruction of the neutrino mass peak. I present simulation results of the feasible limits on the neutrino mass achievable in this new type of tritium beta decay experiment. The same advances in atomic physics that enable the creation of an atomic source for tritium beta decay also suggest a new method of achieving large-scale isotope separation. Multiple experiments that are investigating the absolute mass scale of the neutrino through neutrinoless double beta decay could benefit from this new technique, which applies generally to many elements, including the double beta emitter Nd-150 that is particularly difficult to separate in large quantities. The method is based on an irreversible change of the mass-to-magnetic moment ratio of a particular isotope in a supersonic atomic beam, followed by a magnetic multipole whose gradients deflect and guide the atoms. I present numerical simulations of isotope separation for a range of examples and demonstrate that large-scale isotope separation should be possible using ordinary inexpensive magnets and the existing technologies of supersonic beams and lasers. Additionally I report results from a search for low-multiplicity neutrino bursts in the Sudbury Neutrino Observatory (SNO). Such bursts could indicate detection of a nearby core-collapse supernova explosion. The data were taken from November 1999 to May 2001 when the detector was filled with heavy water (Phase I), as well as data from July 2001 to August 2003 when NaCl was added to the detector (Phase II). The search was a blind analysis in which the potential backgrounds were estimated and analysis cuts were developed to eliminate such backgrounds with 90% confidence before the data were examined. The search maintained a greater than 50% detection probability for standard supernovae occurring at a distance of up to 60 kpc for Phase I and up to 70 kpc for Phase II. No low-multiplicity bursts were observed during the data-taking period. / text