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arXiv:0912.3393 · Physical Review A

Trapping ultracold atoms in a time-averaged adiabatic potential

M. Gildemeister, E. Nugent, Benjamin E. Sherlock, Marcin Kubasik, Benjamin Thomas Sheard, Christopher J. Foot

Published Mar 22, 2010• 32 citations
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Abstract

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 $^{87}\mathrm{Rb}$ to quantum degeneracy and observed condensate lifetimes of longer than $3$ s.

DOI
10.1103/physreva.81.031402

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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×\else\texttimes\fi1045\ifmmode\times\else\texttimes\fi{}{10}^{4}5\ifmmode×\else\texttimes\fi104 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.
We report the experimental realization of a versatile ring trap for ultracold atoms. The ring geometry is created by the time-averaged adiabatic potential resulting from the application of an oscillating magnetic bias field to a rf-dressed quadrupole trap. Lifetimes for a Bose-Einstein condensate in the ring exceed 11s11\mathrm{s}11s and the ring radius was continuously varied from $50 \ensuremath{\mu}\text{m}$ to $262 \ensuremath{\mu}\text{m}$. An efficient method of loading the ring from a conventional time-averaged orbiting potential trap is presented together with a rotation scheme which introduces angular momentum into the system. The ring presents an opportunity to study the superfluid properties of a condensate in a multiply connected geometry and also has applications for matter-wave interferometry.
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.
A combination of static and oscillating magnetic fields can be used to 'dress' atoms with radiofrequency (RF), or microwave, radiation.The spatial variation of these fields can be used to create an enormous variety of traps for ultra-cold atoms and quantum gases.This article reviews the type and character of these adiabatic traps and the applications which include atom interferometry and the study of low-dimensional quantum systems.We introduce the main concepts of magnetic traps leading to adiabatic dressed traps.The concept of adiabaticity is discussed in the context of the Landau-Zener model.The first bubble trap experiment is reviewed together with the method used for loading it.Experiments based on atom chips show the production of double wells and ring traps.Dressed atom traps can be evaporatively cooled with an additional RF field, and a weak RF field can be used to probe the spectroscopy of the adiabatic potentials.Several approaches to ring traps formed from adiabatic potentials are discussed, including those based on atom chips, time-averaged adiabatic potentials and induction methods.Several proposals for adiabatic lattices with dressed atoms are also reviewed.
We present the first experimental demonstration of a multiple radio-frequency dressed potential for the configurable magnetic confinement of ultracold atoms. We load cold 87Rb^{87}\mathrm{Rb}87Rb atoms into a double-well potential with an adjustable barrier height, formed by three radio-frequencies applied to atoms in a static quadrupole magnetic field. Our multiple radio-frequency approach gives precise control over the double-well characteristics, including the depth of individual wells and the height of the barrier, and enables reliable transfer of atoms between the available trapping geometries. We characterize the multiple radio-frequency dressed system using radio-frequency spectroscopy, finding good agreement with the eigenvalues numerically calculated using Floquet theory. This method creates trapping potentials that can be reconfigured by changing the amplitudes, polarizations, and frequencies of the applied dressing fields and easily extended with additional dressing frequencies.
A theoretical investigation for implementing a scheme of forced evaporative cooling in radio-frequency (rf) adiabatic potentials is presented. Supposing the atoms to be trapped in a combination of a dc magnetic field and a rf field at frequency ${\ensuremath{\omega}}_{1}$, the cooling procedure is facilitated using a second rf source at frequency ${\ensuremath{\omega}}_{2}$. This second rf field produces a controlled coupling between the spin states dressed by ${\ensuremath{\omega}}_{1}$. The evaporation is then possible in a pulsed or continuous mode. In the pulsed case, atoms with a given energy are transferred into untrapped dressed states by abruptly switching off the ${\ensuremath{\omega}}_{2}$ coupling. In the continuous case, it is possible for energetic atoms to adiabatically follow the doubly dressed states and escape out of the trap. Our results also show that when ${\ensuremath{\omega}}_{1}$ and ${\ensuremath{\omega}}_{2}$ are separated by at least the Rabi frequency associated with ${\ensuremath{\omega}}_{1}$, additional evaporation zones appear which can make this process more efficient.