Optical generation of vortices in trapped Bose-Einstein condensates (original) (raw)

Observation of vortex formation in an oscillating trapped Bose-Einstein condensate

We report on the observation of vortex formation in a Bose-Einstein condensate of 87Rb atoms. Vortices are generated by superimposing an oscillating excitation to the trapping potential introduced by an external magnetic field. For small amplitudes of the external excitation field we observe a bending of the cloud axis. Increasing the amplitude we observe formation of a growing number of vortices in the sample. Shot-to-shot variations in both vortex number and position within the condensed cloud are observed, probably due to the intrinsic vortex nucleation dynamics. We discuss the possible formation of vortices and antivortices in the sample as well as possible mechanisms for vortex nucleation.

Dynamics of vortex quadrupoles in nonrotating trapped Bose-Einstein condensates

Dynamics of vortex clusters is essential for understanding diverse superfluid phenomena. In this paper, we examine the dynamics of vortex quadrupoles in a trapped two-dimensional (2D) Bose-Einstein condensate. We find that the movement of these vortex-clusters fall into three distinct regimes which are fully described by the radial positions of the vortices in a 2D isotropic harmonic trap, or by the major radius (minor radius) of the elliptical equipotential lines decided by the vortex positions in a 2D anisotropic harmonic trap. In the " recombination " and " exchange " regimes the quadrupole structure maintains, while the vortices annihilate each other permanently in the " annihilation " regime. We find that the mechanism of the charge flipping in the " exchange " regime and the disappearance of the quadrupole structure in the " annihilation " regime are both through an intermediate state where two vortex dipoles connected through a soliton ring. We give the parameter ranges for these three regimes in coordinate space for a specific initial configuration and phase diagram of the vortex positions with respect to the Thomas-Fermi radius of the condensate. We show that the results are also applicable to systems with quantum fluctuations for the short-time evolution. Vortices could be observed in most realm of physics such as hydrodynamics, superfluids, optical fields and even cosmology. The dynamics of quantized vortices is essential for understanding diverse superfluid phenomena such as critical-current densities in superconductors 1 , quantum turbulence 2–6 and novel quantum phases 7–11 in superfluids. Vortices are also topological defects that play key roles in transport, dissipative and coherent properties 12–16 of superfluid systems. The pioneering work of Yarmchuk et al. in 1979 successfully located the ends of parallel vortex lines in superfluid Helium 17. The real-time dynamics of vortex lattice in type II super conductors was observed in 1992 18. Until 2006, direct observation of quantized vortex lines in superfluid Helium in arbitrary three-dimensional configurations has been achieved 19. The realization of Bose-Einstein condensates (BECs) provides an accessible and highly controllable platform for fundamental studies of superfluid vortex dynamics, and has been followed by various theoretical investigations and numerical analyses. It is remarkable that, comparing with other systems ruled by nonlinear Schrödinger equations , BECs are the ideal laboratory for finding these nonlinear excitations due to larger interaction strengths and easier tunable parameters. The size of vortex cores in a BEC is proportional to the healing length of the condensate,  ζ ρ = mg / 2 , where m is the mass of atoms, ρ is the density of the condensate in the absence of vortex, and g is the interatomic interaction strength. For given trap frequencies, g is proportional to the scattering length between atoms, which can be easily adjusted by using magnetic or optical Feshbach resonances 20–22. Due to the matter wave nature of condensates 23 , vortices can be detected in atomic interference. Matthews et al. 24 firstly demonstrated vortex production through an interference measurement, and later, Inouye et al. observed vortex phase singular-ities as dislocations in the interference fringes in BECs 25. As the size of the vortex cores in a trapped condensate is ordinarily several times smaller than the wavelength of light used for imaging, in experiments the condensates are usually allowed to expand to a point at which the vortex cores are large compared to the imaging resolution 24–30. Many works have been done to develop the vortex detection techniques and to understand vortex dynamics during the interference of BECs 25,31–36 , which is important for the applications of matter-wave interferometry. However, the direct, in situ observation of vortices in a trapped condensate without expansion was wachieved