Light storage and retrieval with nuclear spin and a study of anisotropic inelastic collisions
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Authors
Lu, Meiju
Issue Date
2011
Type
Dissertation
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Keywords
Buffer-gas cooling , electromagnetically-induced transparency , inelastic collisions , nuclear spin , optical pumping , slow/stopped light
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Abstract
Helium buffer-gas cooling is combined with laser ablation to produce large numbers of atoms and molecules at a cryogenic temperature. Absorption spectroscopy is employed to observe the target species and optical pumping is used to manipulate them. In this thesis, the first observation of electromagnetically induced transparency (EIT) in a sample of ground-state ($^1 S_0$) atomic ytterbium at 6 K is reported. The transparency is produced due to coherence between the optical field and the nuclear spin state of the $^{173}$Yb nucleus. Because the nuclear spin states interact very weakly with their environment they are resistant to decoherence due to inelastic collisions and inhomogeneous fields. Consequently, atomic ensembles of pure nuclear spin states may be a superior medium for a variety of nonlinear optics and quantum information experiments. For example, the information carried by a light pulse is stored in the nuclear spin of ground-state ($^1 S_0$) $^{173}$Yb by using EIT. Storage times of hundreds of milliseconds are observed at 4 K in our system, which has a competitive performance compared to other systems using electronic spin. The second topic of this thesis is the cold atom-helium collisions which are not only important for the fundamental studies, but also help to explore the field of ultracold atoms. A combined experimental and theoretical study of angular momentum depolarization in cold collisions of $^2 P$-state atoms is presented. It is shown that collision-induced Zeeman relaxation of ground-state ($^2 P_{1/2}^{o}$) gallium and indium atoms in cold $^4$He gas is dramatically small compared to atoms in $^2 P_{3/2}^{o}$ states. These results suggest the feasibility of sympathetic cooling and magnetic trapping of $^2 P_{1/2}^{o}$-state atoms. The inelastic collisions between cold titanium atoms and helium gas cause transitions between the fine-structure levels of the $3d^2 4s^2$ $^3 F_J$ electronic ground state of atomic titanium over a temperature range from 5 to 20 K. The Ti-He inelastic collision cross-section is significantly smaller than cross-sections measured for collisions of non-transition-metal atoms with noble gas atoms. The theoretical calculations of the inelastic cross-sections reproduce the magnitude and temperature dependence of the measurements, and attribute the suppression of inelastic collisions to titanium's ''submerged'' $d$-shell valence electrons. Finally, large numbers of $X~^3Delta_1$ TiO molecules at a translational temperature of 5 K are generated. Their cold collisions with helium, including their elastic and inelastic scattering cross-sections, are investigated and measured. As expected for $^3Delta$ molecules, which have large spin-rotation couplings, TiO's inelastic $m$-changing collision cross-section is large: on the same order as its momentum transfer cross-section.
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In Copyright(All Rights Reserved)