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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Superfluid critical velocity of a Bose-Einstein condensate in a flat potential

S. K. Schnelle, Kristian James Weegink, C. Feng, Leif Humbert, Erik van Ooijen, Matthew J. Davis, N. R. Heckenberg, Halina H. Rubinsztein-Dunlop

Published Jun 1, 2009• 1 citation
Publisher

Abstract

The loading of ultra-cold atoms into two-dimensional, time-averaged optical potentials was demonstrated. The time-averaging scheme allows us to make arbitrary and two dimensional potentials, where the potential shape and depth can be dynamically controlled. Traps are produced by rapidly scanning a red-detuned laser focus in any desired pattern using a two-dimensional acousto-optical modulator (AOM). Smooth optical potentials are created when the scanning speed is much higher than any trapping frequency in system, resulting in a time-averaged dipole potential for the atoms. The depth of the potential can be controlled at any time and place by modulating the amplitude of the applied radio frequency to the AOM.

DOI
10.1109/cleoe-eqec.2009.5192435

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The loading of ultra-cold atoms into two-dimensional, time-averaged optical potentials was demonstrated. The time-averaging scheme allows us to make arbitrary and two dimensional potentials, where the potential shape and depth can be dynamically controlled. Traps are produced by rapidly scanning a red-detuned laser focus in any desired pattern using a two-dimensional acousto-optical modulator (AOM). Smooth optical potentials are created when the scanning speed is much higher than any trapping frequency in system, resulting in a time-averaged dipole potential for the atoms. The depth of the potential can be controlled at any time and place by modulating the amplitude of the applied radio frequency to the AOM.
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We propose and investigate a technique for generating smooth two-dimensional potentials for ultra-cold atoms based on the rapid scanning of a far-detuned laser beam using a two-dimensional acousto-optical modulator (AOM). We demonstrate the implementation of a feed-forward mechanism for fast and accurate control of the spatial intensity of the laser beam, resulting in improved homogeneity for the atom trap. This technique could be used to generate a smooth toroidal trap that would be useful for static and dynamic experiments on superfluidity and persistent currents with ultra-cold atoms.
This thesis presents work on Bose-Einstein condensates in non-harmonic optical potentials. First, a new trap design developed at the University of Queensland is presented that allows the creation of nearly arbitrary two-dimensional potential landscapes by spatially scanning a far red-detuned laser beam using a two-dimensional acousto-optic modulator. In conjunction with a feed-forward technique this trap is capable of producing optical traps which have the necessary stability to be used in ultra-cold atom research. Different geometries are presented. In particular toroidal trap geometries are discussed which are interesting because they offer the possibility for a multiply connected Bose-Einstein condensate. The trap also offers the possibility of dynamic potentials which have been employed to measure the critical velocity of superfluidity in Bose-Einstein condensates. Secondly, measurements on condensation dynamics are presented which use an optical dimple potential superimposed upon a harmonic magnetic trap. In the experiments the dimple potential is ramped on slowly or turned on suddenly for a range of dimple depths and widths and the condensate fraction and temperature are measured as a function of hold times. The measurements taken are compared to equilibrium thermodynamics and quantum kinetic theory and for the first time a quantitative model is presented that allows correct prediction of condensate fractions and temperatures in good agreement with the measurements. Also presented are first results in condensate rethermalization experiments where a dimple potential is suddenly switched off and the condensate left to rethermalize in a much wider magnetic potential. The measurements show a sudden breakdown in condensate fraction followed by a subsequent recovery. So far no theory exists to properly interpret the results as quantum kinetic theory cannot be applied to the problem. Lastly, progress on experiments to measure the critical velocity of superfluidity in Bose-Einstein condensates is reported. Calculations have been carried out that show the creation of solitons and vortices as mechanisms to dissipate energy leading to a breakdown of superfluid behaviour at velocities lower than that predicted by Landau’s theory of superfluidity, the first theory to describe superfluidity. Progress on an experiment is presented that uses the scanning beam trap developed as part of this project to create a line potential with a moving barrier to measure the critical velocity and show solitons and vortices as the mechanisms for energy dissipation as seen in the calculations.
Bose-Einstein condensates are superfluids which when engineered under suitable laboratory conditions support dissipationless flows and facilitate the development of quantum enhanced applications. Realising those suitable conditions requires experimental technologies which enable the complete dynamical control of the superfluid density and velocity profiles. This thesis conceives and demonstrates several dynamical controls using optical potentials formed using commercially available spatial-light-modulator technologies, and constitutes important progress toward many future experiments and the realisation of practical applications.Experimental systems and control sequences are designed using complimentary acousto-optic deflection and digital micromirror spatial light modulator technologies, rather than magnetic field based approaches, given their inherently favourable spatial and temporal resolutions. An experimental apparatus which includes a time-averaged acousto-optic controlled potential is constructed to generate toroidal potentials suitable for developing inertial rotation sensors. Density and phase manipulation strategies are developed using that apparatus which prepare condensates with arbitrarily controllable wavefunctions. Complementary digital micromirror based adaptations are subsequently developed during this project.Corrugations across the condensate density distributions are reduced using an iteratively corrected spatial light modulator configuration. Rigorous and generalised scan requirements are established for engineering time-averaged potentials; numerical methods are conceived which facilitate the simulation of these systems. The combination of the feedforward density control and time-averaging phase control enable the deterministic preparation of persistent currents around multiply connected condensates. These capabilities enable experiments into many-body groundstates, superfluid turbulence, atomtronics and metrology.Spatial light modulator trapping sequences geared toward the study of Onsager vortices, vortex dipole optics, inertial rotation sensing and wake turbulence are specifically developed. Several upgraded experimental systems are finally designed using knowledge gained during this thesis, which enable these various applications in future. This thesis therefore constitutes significant progress toward several important experimental investigations using dynamically controllable optical trapping technologies.
The final stage in the creation of a Bose-Einstein condensate (BEC) is trapping and compressing in a strong magnetic trap. When the spin of an atom changes, it is lost from this magnetic trap. To be able to do experiments with varying atom spin, one can use an optical dipole trap, which traps atoms independent of their spin. This thesis presents a far red-detuned optical dipole trap, which is characterized by trapping BEC's. Furthermore, an approach to rapidly scan this trap is presented, creating a 'quasi-2D' potential. With this potential we attempt to create a horizontal sheet of condensed atoms, where the depth of the sheet of atoms is smaller than the range of the spin dynamics. This would enable a unique visualization of the spin dynamics using spin-dependent imaging.
We manipulate a Bose--Einstein condensate using the optical trap created by the diffraction of a laser beam on a fast ferroelectric liquid crystal spatial light modulator. The modulator acts as a phase grating which can generate arbitrary diffraction patterns and be rapidly reconfigured at rates up to 1kHz1\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}1kHz to create smooth, time-varying optical potentials. The flexibility of the device is demonstrated with our experimental results for splitting a Bose--Einstein condensate and independently transporting the separate parts of the atomic cloud.