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arXiv:1201.0718 · Physical Review A

Techniques to cool and rotate Bose-Einstein condensates in time-averaged adiabatic potentials

M. Gildemeister, Benjamin E. Sherlock, Christopher J. Foot

Published May 1, 2012• 17 citations
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Abstract

We report two novel techniques for cooling and rotating Bose-Einstein condensates in a dilute rubidium vapor that highlight the control and versatility afforded over cold atom systems by time-averaged adiabatic potentials (TAAPs). The intrinsic loss channel of the TAAP has been successfully employed to evaporatively cool a sample of trapped atoms to quantum degeneracy. The speed and efficiency of this process compares well with that of conventional forced rf evaporation. In an independent experiment, we imparted angular momentum to a cloud of atoms forming a Bose-Einstein condensate by introducing a rotating elliptical deformation to the TAAP geometry. Triangular lattices of up to 60 vortices were created. All findings reported herein result from straightforward adjustments of the magnetic fields that give rise to the TAAP.

DOI
10.1103/physreva.85.053401

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Laser Cooling and Bose-Einstein Condensation. Experimental Techniques for Bose-Einstein Condensation of Rubidium Atoms.

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The Review of Laser Engineering•Jan 1, 2000•1 citation
We report two novel techniques for cooling and rotating Bose-Einstein condensates in a dilute rubidium vapor that highlight the control and versatility afforded over cold atom systems by time-averaged adiabatic potentials (TAAPs). The intrinsic loss channel of the TAAP has been successfully employed to evaporatively cool a sample of trapped atoms to quantum degeneracy. The speed and efficiency of this process compares well with that of conventional forced rf evaporation. In an independent experiment, we imparted angular momentum to a cloud of atoms forming a Bose-Einstein condensate by introducing a rotating elliptical deformation to the TAAP geometry. Triangular lattices of up to 60 vortices were created. All findings reported herein result from straightforward adjustments of the magnetic fields that give rise to the TAAP.
We report an experimental realization of ultracold atoms confined in a time-averaged, adiabatic potential (TAAP). This trapping technique involves using a slowly oscillating ( ~ kHz) bias field to time-average the instantaneous potential given by dressing a bare magnetic potential with a high-frequency ( ~ MHz) magnetic field. The resultant potentials provide a convenient route to a variety of trapping geometries with tunable parameters. We demonstrate the TAAP trap in a standard time-averaged orbiting potential trap with additional Helmholtz coils for the introduction of the radio frequency dressing field. We have evaporatively cooled $5\ifmmode\times\else\texttimes\fi{}{10}^{4}$ atoms of 87Rb^{87}\mathrm{Rb}87Rb to quantum degeneracy and observed condensate lifetimes of longer than 333 s.
This thesis describes the experimental realisation and characterisation of three non-trivial trapping geometries for ultracold atoms. The double-well, ring and to some degree shell trap are examples of a highly versatile class of traps called time-averaged adiabatic potentials (TAAPs). In this experiment the TAAPs arise from the combination of three independent magnetic fields; a static quadrupole field dressed by a uniform radio-frequency field is time-averaged by a bias field oscillating at in the kHz regime. The result is a very smooth potential, within which ultracold atoms can be evaporatively cooled to quantum degeneracy, and subsequently manipulated into new geometries without destroying the quantum coherence.The vertically offset double-well potential provided the first example of ultracold atoms confined in a TAAP. The same potential is used to demonstrate efficient evaporative cooling across the Bose-Einstein condensate (BEC) phase transition using only the Landau-Zener loss mechanism. Switching off the time-averaging fields loads atoms from the double-well TAAP into the rf-dressed shell trap. A characterisation of this potential measured low heating rates and lifetimes of up to 58s. With efforts ongoing to increase the trap anisotropy, this potential shows promise for research into the static and rapidly rotating 2D systems. In the presence of a single time-averaging field, the shell geometry is transformed into a ring-shaped trap with an adjustable radius. The ring trap can be controllably tilted and progress towards multiply connected condensates is being made. A rotation scheme to spin up atoms in the ring trap has been demonstrated, presenting the opportunity to investigate the dynamics of superflow in degenerate quantum gases.
The construction of an apparatus capable of producing Bose-Einstein condensates marks a significant milestone in every experimental cold atom laboratory. In this thesis I describe the development of a system to create a Bose-Einstein condensate of 87Rb^{87}Rb87Rb in a Time-Orbiting Potential trap.\n\nI review the optical and magnetic techniques required to trap and cool an atomic sample under vacuum, motivating our decision to build a double MOT system comprised of a high-pressure (10−910^{-9}10−9 torr) chamber to gather atoms and a low-pressure (10−1110^{-11}10−11 torr) chamber to cool atoms to degeneracy.\n\nBy theoretically modeling the atom number and temperature inside the magnetic trap during evaporative cooling I demonstrate a simple approach to determining a cooling path that reaches the transition temperature. By making use of the condensates produced under these non-optimized conditions I determine the heating rate of the condensate in the TOP trap to be 300300300 nK/s. I further use the condensates to make a more precise measurement of the TOP trap bias field.\n\nI improve upon the conventional evaporation path used in TOP trap experiments by introducing and optimizing additional bias field compression stages in between RF evaporation ramps. I demonstrate how, by adding these additional stages, the system is capable of reaching the BEC phase transition with a final atom number of 2times1052\\times 10^{5}2times105. In contrast, RF evaporation after only a single bias field ramp has yielded condensates with only 30times10330\\times 10^330times103 atoms.
We have achieved Bose-Einstein condensation (BEC) [1,2] of Na atoms in an anisotropic TOP trap [3]. Our TOP trap has a bias field that rotates (at 20 kHz) in a plane containing the symmetry access of the quadrapole field, resulting in an anisotropic, time-averaged TOP potential with spring constants in the ratio of 1:2:4. A dark spot MOT is loaded from a Zeeman-cooled Na beam and the cloud of atoms are cooled in dark optical molasses. This produces a sample of −1010 atoms, mainly in the F = 1 hyperfine ground state at a temperature of −100 pK. We transfer about 1/3 of these atoms into a static quadrapole trap, trapping the F = 1, mF=−1 state. The quadrapole trap is stiffened to 1000 G/cm to increase the Na density and evaporated for 3 seconds with a linearly swept rf field. The TOP field is then turned on quickly to 150 G, producing a weak TOP trap. Further evaporation is performed by reducing the TOP field to 20 G, bringing in the circle of death. This 15 second evaporation period is followed by 10 seconds of rf induced evaporation, with BEC occuring at a temperature of ~1 μK, producing a condensate of ~3×106 atoms. By subsequently adiabatically expanding the trapping potential, we have reduced the temperature to −50 nK.
We demonstrate a simple scheme to achieve fast, accelerating (runaway) evaporative cooling of optically trapped atoms by tilting the optical potential with a magnetic field gradient. Runaway evaporation is possible in this trap geometry due to the weak dependence of vibration frequencies on trap depth, which preserves atomic density during the evaporation process. Using this scheme, we show that Bose-Einstein condensation with $\ensuremath{\sim}{10}^{5}$ cesium atoms can be realized in 2−−4s2--4\phantom{\rule{0.3em}{0ex}}\mathrm{s}2−−4s of forced evaporation. The evaporation speed and energetics are consistent with the three-dimensional evaporation picture, despite the fact that atoms can only leave the trap in the direction of tilt.
Experimental techniques to produce and observe a Bose-Einstein condensate of rubidium atoms are presented. The atoms are first optically cooled and trapped in an ultra-high vacuum (-10-11 torr) glass cell using a double magneto-optical trap, and then transferred to a “cloverleaf” magnetic trap. Rf-induced evaporation is performed to further cool the magnetically trapped atoms below the phase transition of Bose-Einstein condensation. The velocity distribution (inferred by absorption imaging) of the cooled atomic cloud clearly shows the expected bi-modal structure characteristic of a condensate