Observation of vortex dipoles in an oblate Bose-Einstein condensate

📝 Abstract

We report experimental observations and numerical simulations of the formation, dynamics, and lifetimes of single and multiply charged quantized vortex dipoles in highly oblate dilute-gas Bose-Einstein condensates (BECs). We nucleate pairs of vortices of opposite charge (vortex dipoles) by forcing superfluid flow around a repulsive gaussian obstacle within the BEC. By controlling the flow velocity we determine the critical velocity for the nucleation of a single vortex dipole, with excellent agreement between experimental and numerical results. We present measurements of vortex dipole dynamics, finding that the vortex cores of opposite charge can exist for many seconds and that annihilation is inhibited in our highly oblate trap geometry. For sufficiently rapid flow velocities we find that clusters of like-charge vortices aggregate into long-lived dipolar flow structures.

💡 Analysis

We report experimental observations and numerical simulations of the formation, dynamics, and lifetimes of single and multiply charged quantized vortex dipoles in highly oblate dilute-gas Bose-Einstein condensates (BECs). We nucleate pairs of vortices of opposite charge (vortex dipoles) by forcing superfluid flow around a repulsive gaussian obstacle within the BEC. By controlling the flow velocity we determine the critical velocity for the nucleation of a single vortex dipole, with excellent agreement between experimental and numerical results. We present measurements of vortex dipole dynamics, finding that the vortex cores of opposite charge can exist for many seconds and that annihilation is inhibited in our highly oblate trap geometry. For sufficiently rapid flow velocities we find that clusters of like-charge vortices aggregate into long-lived dipolar flow structures.

📄 Content

arXiv:0912.3773v1 [cond-mat.quant-gas] 18 Dec 2009 Observation of vortex dipoles in an oblate Bose-Einstein condensate T. W. Neely,1 E. C. Samson,1 A. S. Bradley,2 M. J. Davis,3 and B. P. Anderson1,4 1College of Optical Sciences, University of Arizona, Tucson, AZ 85721, USA 2Jack Dodd Centre for Quantum Technology, Department of Physics, University of Otago, P. O. Box 56, Dunedin, New Zealand 3The University of Queensland, School of Mathematics and Physics, ARC Centre of Excellence for Quantum-Atom Optics, Qld 4072, Australia 4Department of Physics, University of Arizona, Tucson, AZ 85721, USA (Dated: December 17, 2009) We report experimental observations and numerical simulations of the formation, dynamics, and lifetimes of single and multiply charged quantized vortex dipoles in highly oblate dilute-gas Bose-Einstein condensates (BECs). We nucleate pairs of vortices of opposite charge (vortex dipoles) by forcing superfluid flow around a repulsive gaussian obstacle within the BEC. By controlling the flow velocity we determine the critical velocity for the nucleation of a single vortex dipole, with excellent agreement between experimental and numerical results. We present measurements of vortex dipole dynamics, finding that the vortex cores of opposite charge can exist for many seconds and that annihilation is inhibited in our highly oblate trap geometry. For sufficiently rapid flow velocities we find that clusters of like-charge vortices aggregate into long-lived dipolar flow structures. PACS numbers: 03.75.Kk, 03.75.Lm, 67.85.De Vortex dipoles consist of a bound pair of vortices of op- posite circulation and may exist in both classical and quan- tum fluids. Although single vortices carry angular momen- tum, vortex dipoles can be considered as basic topological structures that carry linear momentum [1] in stratified or two- dimensional fluids. Vortex dipoles are widespread in classi- cal fluid flows, appearing for example in ocean currents [2] and soap films [3], and have been described as the pri- mary vortex structures in two-dimensional chaotic flows [1]. In superfluids, the roles of quantized vortex dipoles appear less well established. Given the prevalence of vortices and antivortices in superfluid turbulence [4–6], the Berezinskii- Kosterlitz-Thouless (BKT) transition [7], and phase transition dynamics [8–11], a quantitative study of vortex dipoles will contribute to a broader and deeper understanding of superfluid phenomena. The realization of vortex dipoles in dilute Bose- Einstein condensates (BECs) is especially significant as BECs provide a clean testing ground for the microscopic physics of superfluid vortices [12–14]. In this paper we present an ex- perimental and numerical study of the formation, dynamics, and lifetimes of single and multiply charged vortex dipoles in highly oblate BECs. Numerical simulations based on the Gross-Pitaevskii equa- tion (GPE) have shown that vortex dipoles are nucleated when a superfluid moves past an impurity faster than a critical veloc- ity, above which vortex shedding induces a drag force [15, 16]. Vortex shedding is therefore believed to be a mechanism for the breakdown of superfluidity [17, 18]. Experimental studies of periodic stirring of a BEC with a laser beam have measured a critical velocity for the onset of heating and a drag force on superfluid flow [19, 20], and vortex phase singularities have been observed in the wake of a moving laser beam [21, 22]. However, a microscopic picture of vortex dipole formation and the ensuing dynamics has not been established experi- mentally. In the work reported here, single vortex dipoles are deterministically nucleated by causing a highly oblate, har- monically trapped BEC to move past a repulsive obstacle. We measure a critical velocity for vortex dipole shedding, and find good agreement with numerical simulations and earlier theory [23]. Experimentally, the nucleation process exhibits a high degree of coherence and stability, allowing us to map out the orbital dynamics of a vortex dipole. We find that vor- tex dipoles can survive for many seconds in the BEC without self-annihilation. We also provide evidence for the formation of multiply charged vortex dipoles. The creation of BECs in our lab is described elsewhere [11, 24, 25]. In the experiments reported here, we begin with a BEC of 2 × 106 atoms in a highly oblate harmonic trap. Our axially symmetric trap is created by combining a red- detuned laser light-sheet trapping potential with a magnetic trapping potential, producing a BEC with an 11:1 aspect ra- tio and a Thomas-Fermi radius of 52 µm radially, as shown in Fig. 1(a,b). The BECs are additionally penetrated by a fo- cused blue-detuned laser beam that serves as a repulsive ob- stacle; the beam has a Gaussian 1/e2 radius of 10 µm and is initially located 20 µm to the left of the minimum of the har- monic trap as shown in Fig. 1(c). To nucleate vortices we translate the harmonic potential in the horizontal (x) direction at a consta

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