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

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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.