Expansion dynamics of a spherical Bose–Einstein condensate

We experimentally and theoretically observe the expansion behaviors of a spherical Bose-Einstein condensate. A rubidium condensate is produced in an isotropic optical dipole trap with an asphericity of 0.037. We measure the variation of the condensate size in the expansion process after switching off the trap. The free expansion of the condensate is isotropic,which is different from that of the condensate usually produced in the anisotropic trap. We derive an analytic solution of the expansion behavior based on the spherical symmetry, allowing a quantitative comparison with the experimental measurement. The interaction energy of the condensate is gradually converted into the kinetic energy during the expansion and after a long time the kinetic energy saturates at a constant value. We obtain the interaction energy of the condensate in the trap by probing the long-time expansion velocity, which agrees with the theoretical calculation. This work paves a way to explore novel quantum states of ultracold gases with the spherical symmetry.

Observation of Atomic Dynamic Behaviors in the Evaporative Cooling by In-Situ Imaging the Plugged Hole of Ultracold Atoms

We experimentally observe the dynamic evolution of atoms in the evaporative cooling, by in-situ imaging the plugged hole of ultracold atoms. Ultracold rubidium atoms confined in a magnetic trap are plugged using a blue-detuned laser beam with a waist of 20 m at a wavelength of 767 nm. We probe the variation of the atomic temperature and width versus the radio frequency in the evaporative cooling. Both the behaviors are in good agreement with the calculation of the trapping potential dressed by the rf signal above the threshold temperature,while deviating from the calculation near the phase transition. To accurately obtain the atomic width, we use the plugged hole as the reference to optimize the optical imaging system by precisely minimizing the artificial structures due to the defocus effect.