Bose-Einstein Condensation of Rubidium 87
in Innsbruck

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 m_F = 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: ω_x = 2π 210 Hz; ω_y = 2p 210 Hz and ω_z = 2p 23 Hz
• Start of condensation at ~ 500nK with 6∙10⁶ atoms.
• The pure BEC contains ~ 1.2∙10⁶ 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.

Asymmetric expansion of the cigarshaped condensate due to mean field observed with absorption imaging after various times of flight. The atomic wavepacket falls in the gravitational field.

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).

Our current BEC apparatus, consisting of the MOT chamber and the UHV glass cell for condensation. The atoms are magnetically transferred into the glass cell using a chain of quadrupole coils.

BEC research worldwide

Bose-Einstein condensation in ultracold atomic gases was first realized in 1995 with ⁸⁷Rb, ²³Na, and ⁷Li. This pioneering work was honored with the Nobel prize 2001 in physics , awarded to Eric Cornell, Carl Wieman, and Wolfgang Ketterle. Since then, the research field has exploded with many groups working on BEC worldwide . Only a few more species could be condensed so far: Hydrogen (1998), ⁸⁵Rb (2000), metastable ⁴He (2001), and ⁴¹K (2001). There have been several attempts to condense cesium, the Innsbruck experiment is the first one to reach this goal.

The Team

The team at the Institut für Experimentalphysik, Innsbruck University, Austria: Matthias Theis (graduate student) Gregor Thalhammer (graduate student) Klaus Winkler (graduate student) Michael Hellwig (diploma student) George Ruff (guest professor) Johannes Hecker Denschlag (project leader) Rudi Grimm (group leader)

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Support

We are supported by the Austrian Science Fund (Fonds zur Förderung der wissenschaftlichen Forschung, FWF) in the frame of the Spezialforschungsbereich F15 "Control and Measurement of Coherent Quantum Systems".

last change: 05-12-15 by JH