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arXiv:cond-mat/0109322 · Physical Review Letters

Bose-Einstein Condensation in a Surface Microtrap

Herwig Ott, József Fortágh, G. Schlotterbeck, Alexander Grossmann, Claus Zimmermann

Published Nov 13, 2001• 375 citations
arXivHTMLPDF

Abstract

Bose-Einstein condensation has been achieved in a magnetic surface microtrap with 4 x 10(5) (87)Rb atoms. The strongly anisotropic trapping potential is generated by a microstructure which consists of microfabricated linear copper conductor of widths ranging from 3 to 30 microm. After loading a high number of atoms from a pulsed thermal source directly into a magneto-optical trap the magnetically stored atoms are transferred into the microtrap by adiabatic transformation of the trapping potential. In the microtrap the atoms are cooled to condensation using forced rf-evaporation. The complete in vacuo trap design is compatible with ultrahigh vacuum below 2 x 10(-11) mbar.

DOI
10.1103/physrevlett.87.230401

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Bose-Einstein condensation has been achieved in a magnetic surface microtrap with 4 x 10(5) (87)Rb atoms. The strongly anisotropic trapping potential is generated by a microstructure which consists of microfabricated linear copper conductor of widths ranging from 3 to 30 microm. After loading a high number of atoms from a pulsed thermal source directly into a magneto-optical trap the magnetically stored atoms are transferred into the microtrap by adiabatic transformation of the trapping potential. In the microtrap the atoms are cooled to condensation using forced rf-evaporation. The complete in vacuo trap design is compatible with ultrahigh vacuum below 2 x 10(-11) mbar.
A 87Rb^{87}\mathrm{Rb}87Rb Bose-Einstein condensate (BEC) is produced in a portable atom-chip system less than $30\ifmmode\times\else\texttimes\fi{}30\ifmmode\times\else\texttimes\fi{}15\phantom{\rule{0.3em}{0ex}}\mathrm{cm}$, where the ultrahigh vacuum is maintained by a small, 8L∕s8\phantom{\rule{0.3em}{0ex}}\mathrm{L}∕\mathrm{s}8L∕s, ion pump and nonevaporable getter. An aluminum nitride chip with lithographically patterned copper is used to seal the vacuum system, provide the electrical feedthroughs, and create the magnetic trap potentials. All cooling and trapping processes occur 0.6−−2.5mm0.6--2.5\phantom{\rule{0.3em}{0ex}}\mathrm{mm}0.6−−2.5mm from ambient laboratory air. A condensate of about 2000 87Rb^{87}\mathrm{Rb}87Rb atoms in F=2,mF=2F=2,{m}_{F}=2F=2,mF​=2 is achieved after 4.21s4.21\phantom{\rule{0.3em}{0ex}}\mathrm{s}4.21s of rf forced evaporation. A magneto-optical trap lifetime of 30s30\phantom{\rule{0.3em}{0ex}}\mathrm{s}30s indicates the vacuum near the chip surface is about ${10}^{\ensuremath{-}10}\phantom{\rule{0.3em}{0ex}}\text{torr}$. This work suggests that a chip-based BEC-compatible vacuum system can occupy a volume of less than 0.5L0.5\phantom{\rule{0.3em}{0ex}}\mathrm{L}0.5L.
We have investigated Bose-Einstein condensates and ultracold atoms in the vicinity of a surface of a magnetic microtrap. The atoms are prepared along copper conductors at distances to the surface between $300\ensuremath{\mu}\mathrm{m}$ and $20\ensuremath{\mu}\mathrm{m}.$ In this range, the lifetime decreases from 20 s to 0.7 s showing a linear dependence on the distance to the surface. The atoms manifest a weak thermal coupling to the surface, with measured heating rates remaining below 500 nK/s. In addition, we observe a periodic fragmentation of the condensate and thermal clouds when the surface is approached.
Atom chips are devices used to magnetically trap and manipulate ultracold atoms and Bose-Einstein condensates near a surface. In particular, permanent magnetic film atom chips can allow very tight confinement and intricate magnetic field designs while circumventing technical current noise. Research described in this thesis is focused on the development of a magnetic film atom chip, the production of Bose-Einstein condensates near the film surface, the characterisation of the associated magnetic potentials using rf spectroscopy of ultracold atoms and the realisation of a precision sensor based on splitting Bose-Einstein condensates in a double-well potential. The atom chip itself combines the edge of a perpendicularly magnetised GdTbFeCo film with a machined silver wire structure. A mirror magneto-optical trap collects up to 5 x 108 87Rb atoms beneath the chip surface. The current-carrying wires are then used to transfer the cloud of atoms to the magnetic film microtrap and radio frequency evaporative cooling is applied to produce Bose-Einstein condensates consisting of 1 x 105 atoms. We have identified small spatial magnetic field variations near the film surface that fragment the ultracold atom cloud. These variations originate from inhomogeneity in the film magnetisation and are characterised using a novel technique based on spatially resolved radio frequency spectroscopy of the atoms to map the magnetic field landscape over a large area. The observations agree with an analytic model for the spatial decay of random magnetic fields from the film surface. Bose-Einstein condensates in our unique potential landscape have been used as a precision sensor for potential gradients. We transfer the atoms to the central region of the chip which produces a double-well potential. A single BEC is formed far from the surface and is then dynamically split in two by moving the trap closer to the surface. After splitting, the population of atoms in each well is extremely sensitive to the asymmetry of the potential and can be used to sense tiny magnetic field gradients or changes in gravity on a small spatial scale.
We demonstrate Bose-Einstein condensation of rubidium atoms in a simple low-power magnetic trap. This trap combines the quadrupole with the Ioffe configuration ~QUIC trap! and consists of just three coils. Magneto-optically trapped Rb atoms are first loaded into the linear trapping potential of a magnetic quadrupole, which is then converted into the parabolic geometry of an Ioffe trap. During this process two spherical quadrupole traps with perpendicular axes merge and the trapped atomic cloud moves close to the magnetic field producing coils, resulting in tight confinement of the atoms while the power dissipation of the trap remains low. The magnetically trapped atoms are cooled to below the phase transition of Bose-Einstein condensation by rf-induced evaporation. @S1050-2947~98!51210-3#