Browsing by Author "Bradley, C.C."

Now showing 1 - 7 of 7
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    Analysis of in situ images of Bose-Einstein condensates of lithium
    (American Physical Society, 1997) Bradley, C.C.; Sackett, C.A.; Hulet, R.G.; Rice Quantum Institute
    The initial evidence for Bose-Einstein condensation (BEC) of Li7 consisted of image distortions that suddenly appeared as the gas was evaporatively cooled to near the transition temperature. We present results of a model of the imaging system used to produce these images. The model confirms that the distortions are a sensitive indicator of BEC, but shows that they make quantitative analysis difficult. We find that the distortions resulted from light scattered by the condensate, in conjunction with aberrations in the imaging lens.
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    Bose-Einstein Condensation of Lithium: Observation of Limited Condensate Number
    (American Physical Society, 1997) Bradley, C.C.; Sackett, C.A.; Hulet, R.G.; Rice Quantum Institute
    Bose-Einstein condensation of 7Li has been studied in a magnetically trapped gas. Because of the effectively attractive interactions between 7Li atoms, many-body quantum theory predicts that the occupation number of the condensate is limited to about 1400 atoms. We observe the condensate number to be limited to a maximum value between 650 and 1300 atoms. The measurements were made using a versatile phase-contrast imaging technique.
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    Evidence of Bose-Einstein Condensation in an Atomic Gas with Attractive Interactions
    (American Physical Society, 1995) Bradley, C.C.; Sackett, C.A.; Tollett, J.J.; Hulet, R.G.; Rice Quantum Institute
    Evidence for Bose-Einstein condensation of a gas of spin-polarized 7Li atoms is reported. Atoms confined to a permanent-magnet trap are laser cooled to 200 μK and are then evaporatively cooled to lower temperatures. Phase-space densities consistent with quantum degeneracy are measured for temperatures in the range of 100 to 400 nK. At these high phase-space densities, diffraction of a probe laser beam is observed. Modeling shows that this diffraction is a sensitive indicator of the presence of a spatially localized condensate. Although measurements of the number of condensate atoms have not been performed, the measured phase-space densities are consistent with a majority of the atoms being in the condensate, for total trap numbers as high as 2×10˄5 atoms. For 7Li, the spin-triplet s-wave scattering length is known to be negative, corresponding to an attractive interatomic interaction. Previously, Bose-Einstein condensation was predicted not to occur in such a system.
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    Observation of velocity-tuned multiphoton "Doppleron" resonances in laser-cooled atoms
    (American Physical Society, 1990) Tollett, J.J.; Chen, J.; Story, J.G.; Ritchie, N.W.M.; Bradley, C.C.; Hulet, Randall G.; Rice Quantum Institute
    An atomic beam of Li was transversely cooled using an intense standing-wave radiation field. A dramatic change in the transverse velocity distribution was observed. Structure in the resulting velocity distribution was found to be due to velocity-tuned multiphoton ‘‘Doppleron’’ resonances. The force due to seven-photon resonances is clearly resolved in the data. The data are in good agreement with theoretical predictions.
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    Optimization of evaporative cooling
    (American Physical Society, 1997) Sackett, C.A.; Bradley, C.C.; Hulet, R.G.
    Recent experiments have used forced evaporative cooling to produce Bose-Einstein condensation in dilute gases. The evaporative cooling process can be optimized to provide the maximum phase-space density with a specified number of atoms remaining. We show that this global optimization is approximately achieved by locally optimizing the cooling efficiency at each instant. We discuss how this method can be implemented, and present the results for our Li7 trap. The predicted behavior of the gas is found to agree well with experiment.
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    Permanent magnet trap for cold atoms
    (American Physical Society, 1995) Tollett, J.J.; Bradley, C.C.; Sackett, C.A.; Hulet, R.G.; Rice Quantum Institute
    We report the trapping of neutral atoms in a permanent magnet trap. Approximately 1×10˄8 ground-state lithium atoms have been confined in a nonzero magnetic-field minimum produced by six permanent magnets in an Ioffe configuration. These atoms have a kinetic temperature of 1.1 mK and a peak density of approximately 3×10˄9 cm˄−3. The trapped-atom lifetime is 240 s, limited by collisions with background gas. This trap provides an environment in which quantum-statistical effects, atomic collisions, and other ultralow-temperature phenomena can be studied.
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    Trap-loss collisions of ultracold lithium atoms
    (American Physical Society, 1995) Ritchie, N.W.M.; Abraham, E.R.I.; Xiao, Y.Y.; Bradley, C.C.; Hulet, R.G.; Julienne, P.S.; Rice Quantum Institute
    Accurate measurements are presented of the rate of trap-loss-producing collisions between ultracold magneto-optically trapped Li7 atoms for a range of trap laser intensities and frequencies. Intensities from near the atomic saturation intensity to well above it are investigated. At low intensities, fine-structure-changing collisions cause trap loss with a rate constant of ?10?10 cm3/s. At sufficiently high intensity, the trap can be deep enough to effectively freeze out the dominant fine-structure-changing collisions as a loss mechanism, enabling an accurate comparison of the radiative escape loss rate with theory. At the lowest intensities of this radiative escape regime, the measured loss rates compare favorably with those calculated using an optical Bloch equation theory and a three-dimensional model of trap depth. However, the intensity dependence of the measured rates does not show the saturation predicted by the optical Bloch equation theory. It is shown that reliable knowledge of trap depth is necessary to accurately compare experiment with theory.