Tyler Neely

The University of Arizona, Tucson, AZ, United States

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Publications (20)65.47 Total impact

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    ABSTRACT: We study conditions under which vortices in a highly oblate harmonically trapped Bose-Einstein condensate (BEC) can be stabilized due to pinning by a blue-detuned Gaussian laser beam, with particular emphasis on the potentially destabilizing effects of laser beam positioning within the BEC. Our approach involves theoretical and numerical exploration of dynamically and energetically stable pinning of vortices with winding number up to $S=6$, in correspondence with experimental observations. Stable pinning is quantified theoretically via Bogoliubov-de Gennes excitation spectrum computations and confirmed via direct numerical simulations for a range of conditions similar to those of experimental observations. The theoretical and numerical results indicate that the pinned winding number, or equivalently the winding number of the superfluid current about the laser beam, decays as a laser beam of fixed intensity moves away from the BEC center. Our theoretical analysis helps explain previous experimental observations, and helps define limits of stable vortex pinning for future experiments involving vortex manipulation by laser beams.
    arXiv:1402.6007. 02/2014;
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    ABSTRACT: Experimental stirring of a toroidally trapped Bose-Einstein condensate at high temperature generates a disordered array of quantum vortices that decays via thermal dissipation to form a macroscopic persistent current [T. W. Neely et al., Phys. Rev. Lett. 111, 235301 (2013)]. We perform 3D numerical simulations of the experimental sequence within the Stochastic Projected Gross-Pitaevskii equation using ab initio determined reservoir parameters. We find that both damping and noise are essential for describing the dynamics of the high-temperature Bose field. The theory gives a quantitative account of the formation of a persistent current, with no fitted parameters.
    Physical Review A 12/2013; 88(6):063620. · 3.04 Impact Factor
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    ABSTRACT: Fluids subjected to suitable forcing will exhibit turbulence, with characteristics strongly affected by the fluid’s physical properties and dimensionality. In this work, we explore two-dimensional (2D) quantum turbulence in an oblate Bose-Einstein condensate confined to an annular trapping potential. Experimentally, we find conditions for which small-scale stirring of the condensate generates disordered 2D vortex distributions that dissipatively evolve toward persistent currents, indicating energy transport from small to large length scales. Simulations of the experiment reveal spontaneous clustering of same-circulation vortices and an incompressible energy spectrum with k-5/3 dependence for low wave numbers k. This work links experimentally observed vortex dynamics with signatures of 2D turbulence in a compressible superfluid.
    Physical Review Letters 12/2013; 111:235301. · 7.94 Impact Factor
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    ABSTRACT: Bose-Einstein condensates of dilute gases are well-suited for investigations of vortex dynamics and turbulence in quantum fluids, yet there has been little experimental research into the approaches that may be most promising for generating states of two-dimensional turbulence in these systems. Here we give an overview of techniques for generating the large and disordered vortex distributions associated with two-dimensional quantum turbulence. We focus on describing methods explored in our Bose-Einstein condensation laboratory, and discuss the suitability of these methods for studying various aspects of two-dimensional quantum turbulence. We also summarize some of the open questions regarding our own understanding of these mechanisms of two-dimensional quantum turbulence generation in condensates. We find that while these disordered distributions of vortices can be generated by a variety of techniques, further investigation is needed to identify methods for obtaining quasi-steady-state quantum turbulence in condensates.
    03/2013;
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    ABSTRACT: We report the formation of persistent currents from the decay of turbulence in Bose- Einstein condensates (BECs). In our experiments, a BEC is pierced with a blue-detuned laser beam. By moving the trap center relative to the beam's position, vortices are stirred into the BEC, creating a quantum turbulent state. At finite temperatures, the turbulent state can decay to a persistent current about the blue-detuned laser beam that can last for up to 50 seconds; winding numbers up to 8 have been observed. Our experimental observations correspond well with numerical simulations of the non-equilibrium dynamics and calculations of vortex pinning by a laser beam. We interpret our results as evidence for an inverse energy cascade in dilute-gas BECs.
    06/2011;
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    ABSTRACT: We report the experimental generation of two-dimensional quantum turbulence in dilute-gas Bose-Einstein condensates by stirring the condensate with a laser beam, and the decay of the turbulent state to a large-scale flow in the form of a persistent current in a toroidal trap geometry. From numerical simulations of the stirring process, we characterize the dynamics of the quantized vortices in the condensate, and analyze the kinetic energy spectrum. Our observations are consistent with basic features of two-dimensional turbulence in classical incompressible fluids.
    01/2011;
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    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 trap geometry. For sufficiently rapid flow velocities, clusters of like-charge vortices aggregate into long-lived multiply charged dipolar flow structures.
    Physical Review Letters 04/2010; 104(16):160401. · 7.94 Impact Factor
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    ABSTRACT: We explore the onset of superfluid turbulence in Bose-Einstein condensates held in highly oblate traps. In our procedure, highly oblate BECs are first created in a combined optical and magnetic trap with an approximately 11:1 aspect ratio. We then modulate the the harmonic trapping frequency, introducing vortices and turbulence into the trapped gas. We explore the onset of superfluid turbulence in BECs held in both harmonic and multiply connected potential wells, comparing the characteristics of turbulence in both traps. By studying various excitation methods and re-thermalization of the gases as well as comparisons between experimental and numerical results, transitions to turbulence of ultra-cold trapped gases can be quantitatively characterized.
    05/2009;
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    ABSTRACT: By studying the dynamics of the Bose-Einstein condensation transition in highly oblate (˜11:1 aspect ratio) traps, we have measured the dependence of spontaneous vortex formation on BEC growth rate, extending our previous experimental and numerical observations of spontaneous vortex formation in weakly oblate (˜2:1 aspect ratio) traps [1]. Our condensation procedure in these highly oblate traps allows us to create BECs over a large range of growth times, from approximately 200 ms to over 2 s. By characterizing vortex formation vs. BEC growth rate, and comparing experimental and numerical results, the Kibble-Zurek mechanism for topological defect formation may be quantitatively studied in our system. [1] C.N. Weiler, T.W. Neely, D.R. Scherer, A.S. Bradley, M.J. Davis, and B.P. Anderson., Nature 455, 948 (2008).
    01/2009;
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    ABSTRACT: Phase transitions are ubiquitous in nature, and can be arranged into universality classes such that systems having unrelated microscopic physics show identical scaling behaviour near the critical point. One prominent universal element of many continuous phase transitions is the spontaneous formation of topological defects during a quench through the critical point. The microscopic dynamics of defect formation in such transitions are generally difficult to investigate, particularly for superfluids. However, Bose-Einstein condensates (BECs) offer unique experimental and theoretical opportunities for probing these details. Here we present an experimental and theoretical study of the BEC phase transition of a trapped atomic gas, in which we observe and statistically characterize the spontaneous formation of vortices during condensation. Using microscopic theories that incorporate atomic interactions and quantum and thermal fluctuations of a finite-temperature Bose gas, we simulate condensation and observe vortex formation in close quantitative agreement with our experimental results. Our studies provide further understanding of the development of coherence in superfluids, and may allow for direct investigation of universal phase transition dynamics.
    Nature 11/2008; 455(7215):948-51. · 38.60 Impact Factor
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    ABSTRACT: We experimentally observe the spontaneous creation of quantized vortices in Bose-Einstein condensates during the BEC phase transition. Numerical simulations based on the Stochastic Gross-Pitaevskii equation formalism show excellent quantitative agreement with experimental results. We will present results of ongoing experiments characterizing spontaneous vortex formation in BECs created in various trap geometries, including multiply connected and nearly two-dimensional potential wells.
    05/2008;
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    ABSTRACT: By evaporative cooling of an atomic gas through the Bose-Einstein condensation transition, we have experimentally observed and characterized the spontaneous creation of vortices in BECs during the transition. We have also observed spontaneous formation of vortices in BECs through numerical simulations of the BEC transition using theoretical methods based on the Stochastic Gross-Pitaevskii equation formalism. Our experimental and theoretical results show excellent quantitative agreement. Furthermore, our results are qualitatively consistent with the Kibble-Zurek mechanism for topological defect formation in a phase transition. A quantitative comparison of our experimental and numerical observations will be presented.
    05/2008;
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    ABSTRACT: We report on experiments and simulations of the growth of a Bose-Einstein condensate in which vortices are observed to form spontaneously in a non-rotating potential. Between 10% and 50% of experiments result in between one and three unambiguous vortex cores. We simulate a temperature quench of a Bose gas through the critical point using the stochastic Gross-Pitaevskii equation and find remarkable agreement with the experimental data. Faster cooling results in a higher probability of vortex formation, and we interpret these results in light of the Kibble-Zurek scenario for the formation of topological defects in continuous phase transitions.
    12/2007;
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    ABSTRACT: We experimentally study the growth of Bose-Einstein condensates in harmonic trapping potential and potentials shaped by light. We find that vortices naturally form in the condensates during the evaporative cooling process with the probability influenced by the trap geometry. In all cases angular momentum in not deliberately added to the system. We will discuss past and on-going experimental results.
    10/2007;
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    ABSTRACT: We have experimentally observed spontaneous generation and trapping of quantized vortices in single-component Bose- Einstein condensates. The BECs were created by a standard evaporative cooling procedure in a magnetic trap, without any additional methods of intentionally imparting angular momentum to the trapped atoms. After each BEC was formed, it was expanded such that the presence or absence of a vortex was determined. By observing numerous condensates, the statistical dependence of vortex formation on trapping and cooling parameters was examined. We will describe our experimental results and our interpretation of the vortex formation mechanism.
    06/2007;
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    ABSTRACT: We experimentally study of the growth of Bose-Einstein condensates in harmonic trapping potentials with laser-induced perturbations to the potential well. We find that some time- independent perturbations can significantly impact the growth process and final state of the BEC. In particular, in numerical simulations and our experiments, we have observed the generation of vortices and vortex-antivortex pairs as a result of creating BECs in perturbed potentials. We will describe the results of our ongoing and completed experiments (D.R. Scherer, C.N. Weiler, T.W. Neely, B.P. Anderson, cond-mat/0610187, to be published in Phys. Rev. Lett.).
    06/2007;
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    ABSTRACT: We report observations of vortex formation by merging and interfering multiple (87)Rb Bose-Einstein condensates (BECs) in a confining potential. In this experiment, a single harmonic potential well is partitioned into three sections by a barrier, enabling the simultaneous formation of three independent, uncorrelated BECs. The BECs may either automatically merge together during their growth, or for high-energy barriers, the BECs can be merged together by barrier removal after their formation. Either process may instigate vortex formation in the resulting BEC, depending on the initially indeterminate relative phases of the condensates and the merging rate.
    Physical Review Letters 04/2007; 98(11):110402. · 7.94 Impact Factor
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    ABSTRACT: We report observations of vortex formation as a result of merging together multiple $^{87}$Rb Bose-Einstein condensates (BECs) in a confining potential. In this experiment, a trapping potential is partitioned into three sections by a barrier, enabling the simultaneous formation of three independent, uncorrelated condensates. The three condensates then merge together into one BEC, either by removal of the barrier, or during the final stages of evaporative cooling if the barrier energy is low enough; both processes can naturally produce vortices within the trapped BEC. We interpret the vortex formation mechanism as originating in interference between the initially independent condensates, with indeterminate relative phases between the three initial condensates and the condensate merging rate playing critical roles in the probability of observing vortices in the final, single BEC.
    11/2006;
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    ABSTRACT: Maskless lithographic patterning of computer-generated holograms (CGHs) and diffractive optical elements enables the use of arbitrary-profile optical fields to manipulate Bose-Einstein condensates. This inexpensive and flexible method of tailoring potential wells and potential barriers for trapped condensates permits studies of condensate physics and dynamics in rarely explored regimes. We will describe the CGH creation technique used at the College of Optical Sciences and report on the progress of experiments aimed at studying phase manipulation of Rb-87 Bose-Einstein condensates using this promising tool in the atom optics toolkit.
    05/2006;
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    ABSTRACT: Computer-generated holograms (CGHs) and diffractive optical elements can be used as tools to manipulate Bose-Einstein condensates with tailored optical fields of arbitrary profiles. Using CGHs designed and created at the College of Optical Sciences, we are investigating phase manipulation and fragmentation dynamics of Bose-Einstein condensates in combined optical and magnetic trapping fields. We will briefly summarize our CGH creation technique and report on the progress of our experiments with Rb-87 Bose-Einstein condensates.
    01/2006;