Experimental study of novel quantum dynamics in a gaseous Bose-Einstein condensate

通过在稀原子气体中使用玻色-爱因斯坦凝聚体（EEC），我们实验探索了超流体中的一种新的量子现象：多电荷涡旋的相干衰减动力学。通过反转阱磁场，在^<87>Rb（F=2,m=2）原子的BEC中拓扑形成了电荷为4的量子化涡旋。虽然Rb BEC中的涡旋形成受到重力的强烈干扰，但我们通过补偿EEC周围的重力成功地形成了涡旋。利用远蓝失谐激光束产生的光偶极子力势实现了重力补偿。这种新开发的附加光学技术也使我们能够在BEC的中心不断地产生涡流。为了观察多电荷涡旋的衰变过程，我们将BEC置于陷阱场中，观察了多电荷涡旋的时间演化过程。结果，我们成功地观察到涡旋的分裂显然是BEC横截面上原子数密度的线性耗竭。采用层析吸收成像技术监测其横截面。除了磁阱外，我们还应用了远共振光阱来防止电场反转后BEC的变形。观察到的线性耗竭被解释为一条由四个单荷电涡组成的线，这是在C_2旋转对称扰动下四荷电涡的理论预期。从理论分析上解释了实验中选择性地促进这种衰变模式的原因：涡旋位置与EEC中心的轻微偏离都会增强这种衰变模式的不稳定性，因为涡旋会受到平均场相互作用的影响，这是C_2旋转对称性的扰动。

By using a Bose-Einstein condensate (EEC) in a dilute atomic gas, we experimentally explored a novel quantum phenomenon in a superfluid : coherent decay dynamics of a multiply charged vortex. The quantized vortex with a charge of 4 was topologically formed in a BEC of ^<87>Rb(F=2,m=2) atoms by reversing the trap magnetic field. Although this vortex formation in the Rb BEC was strongly disturbed by gravity, we achieved successful vortex formation by compensating the gravity around the EEC. The gravity compensation was realized with optical dipole force potential created by a far blue-detuned laser beam. This additional optical technique, which was newly developed, also enabled us to create the vortex constantly at the center of the BEC.To observe decay processes of a multiply charged vortex, we held the BEC in the trap field and observed the time-evolution of the vortex. As a result, we succeeded in observing the splitting of the vortex apparently as a linear depletion of the atom number density in the cross section of the BEC. The cross section was monitored by tomographic absorption imaging technique. We also applied a far off-resonance optical trap, in addition to the magnetic trap, for preventing the deformation of the BEC after the field reversal.The linear depletion observed is explained as a line of four singly-charged vortices, which is expected theoretically for a quadruply-charged vortex under the perturbation in the C_2 rotational symmetry. From theoretical analysis, we also sexplained the reason why this decay mode was selectively promoted in the experiment : Even a slight deviation of the vortex position from the EEC center enhanced the instability along this decay mode, because the vortex felt mean field interaction as the perturbation in the C_2 rotational symmetry.