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BEC at the University of Hannover

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Group: Ertmer

Authors and contributors: Kai Bongs, Sven Burger, Nils Bantleon, Gerhard Birkl and Klaus Sengstock.

[Historical report created from GSU BEC Homepage:]

"By our count, a group in Hannover, Germany is the 13th in the world to report the attainment of BEC in a gas. Our correspondent writes:

We found conclusive evidence for reaching Bose-Einstein condensation with Rb atoms in a 'cloverleaf trap'. Our trap has field gradients and curvatures which are roughly a factor of 2 larger than the ones of the original 'cloverleaf trap' by W. Ketterle. This allows us to have strong confinement in both axial and radial directions.

The experimental setup consists of a chirp-slowed and deflected beam of 87Rb atoms loading a dark spot MOT. After collecting atoms for 15 seconds the cloud is compressed, further cooled in an optical molasses, and then transferred into a magneto-static 'cloverleaf-trap'. In the trap the atoms are magnetically compressed in the two radial dimensions within 2 seconds. Evaporative cooling for about 10 seconds decreases the temperature and increases the density enough to surpass the phase-space density necessary to achieve Bose-Einstein condensation.

As in other experiments before, our evidence for BEC is based on absorptive imaging of the atoms after releasing them from the magnetic trap and allowing for times-of-flight of varying duration. Successively lowering the final frequency of the rf-knife for evaporative cooling leads to a sudden increase in the central density of the atom cloud. Time-of-flight images of atom samples created after evaporating below this transition point show elliptical density distributions with the long axis along the weak trap axis for short times (1 ms and 5 ms). For 15 ms time-of-flight the atom clouds are almost circular in shape. For still longer times-of-flight (25 ms and up) the atom clouds reveal central elliptical structures with the long axis oriented perpendicular to the weak trap axis which are characteristic for BEC in anisotropic traps. Atom samples cooled just above the transition temperature show a circular cross section for all times-of-flight above 10 ms.

Preliminary parameters for the condensates created so far:

  • number of atoms in MOT: 5 x 108
  • number of atoms in magnetic trap: 2 x 108
  • number of atoms at condensation: 106
  • number of atoms in condensate: above 105
  • transition temperature to BEC: 600 nK
  • radial gradient of 'cloverleaf trap': 320 G/cm
  • axial curvature of 'cloverleaf trap': 175 G/cm2
  • bias field of 'cloverleaf trap': 1.5 G

Since the first condensation we could observe several hundred condensates on various days.

BEC at the Institute for Quantum Optics in Hannover has been achieved during the early morning hours of August 5th by an enthusiastic team of Ph.D.-students Kai Bongs and Sven Burger, diploma-student Nils Bantleon, and post-docs Gerhard Birkl and Klaus Sengstock.

Wolfgang Ertmer"

Posted: 11 Aug 2005     Date: 05 Aug 1998

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