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Bose-Einstein Condensation of Rubidium 87

Grimm graph

On May 27, 2003, our team at the Institute for Experimental Physics at Innsbruck University achieved Bose-Einstein condensation (BEC) of rubidium 87 in the ground state F=2, mF = 2.

The two pictures are false color and greyscale images of clouds of rubidium atoms at condensation. The narrow peak of the condensate grows out of the broad thermal atom cloud at the critical temperature Tc. These images are taken after 15 ms of time of flight. The pictures are taken along the long axis (z- axis) of our cigar shaped condensate.

Three condensate wavepackets appear instead of a single one because some atoms flip their spin when we turn off the magnetic trap fields. This is due to eddy currents in the metal mount of the magnetic quadrupole coils which produce uncontrolled magnetic fields when the magnetic coils are switched off. The resulting various spin components (here mF = 2, 1, 0) are separated by a magnetic field gradient (Stern-Gerlach effect).

The sickel shaped deformation of the wavepackets is due to the mean field interaction between the atoms.

Trap/condensate parameters:

  • Trapping frequencies: ωx = 2π 210 Hz; ωy = 2p 210 Hz and ωz = 2p 23 Hz
  • Start of condensation at ~ 500nK with 6x106 atoms.
  • The pure BEC contains ~ 1.2x106 atoms.
  • The magnetic offset field is 2 Gauss and our RF ramp for evaporative cooling takes 25 seconds to ramp down from 30 MHz to 1.4 MHz.

Transport apparatus

Our BEC machine features the magnetic transport concept developed in Munich [Gre00] and is designed for maximum access and flexibility to host demanding experiments. Our apparative concept is particularly advantageous for experiments using 3D optical lattices and high resolution imaging.

The Rb atoms are first collected into a magneto-optical trap (MOT) in a compact stainless-steel cell and then loaded into a magnetic quadrupole trap. The quadrupole potential is then moved over a distance of about 49 cm into an extreme UHV (10-11 mBar) glass cell using a chain of quadrupole coils. By running suitable currents through the quadrupole coil pairs the trapping geometry of the potential is maintained during the transport process, thus minimizing heating of the trapped atom cloud. The whole apparatus is designed to be very flat (5 cm) so that the transport coils can be run with moderate currents (~100 A). Once in the glass cell, the trap is changed to Ioffe-Pritchard (or TOP) configuration and the atoms are Bose condensed following standard evaporation methods.

[Gre00] M.Greiner, I. Bloch, T.W. Hänsch, and T. Esslinger, Phys. Rev. A 63, 031401 (R) (2000).