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Group: Wilson

Authors and contributors: Jos Martin, John Sharpe, Callum McKenzie, Nick Thomas, Andrew Wilson

[Historical report created from GSU BEC Homepage:]

"As reported by our University of Otago correspondent, BEC has now been observed in the Southern Hemisphere:

Bose-Einstein condensation has been observed at Otago using a 87Rb double MOT and TOP trap. Following evaporation we observe a rapid increase in the optical depth at the centre of the cloud and the characteristic elliptical cloud shape from time-of-flight measurements.

Experimental Details:

A grating stabilised diode laser and MOPA system run at 250 mW is used for a double MOT configuration. We load approximately 109 atoms in a high Rb vapour pressure MOT in 0.5 s. A resonant light pulse is used to transfer these atoms to the second MOT in a low pressure quartz cell, via a narrow tube allowing differential pumping and a confining hexapole guiding field. The quartz cell is pumped by a 55 L/s ion pump and a titanium sublimation pump, providing a magnetic trap lifetime in excess of 90 s. After 50 loads from the high pressue MOT, atoms in the low pressure MOT are compressed and then cooled in mollasses before being loaded into the TOP trap. TOP trap parameters are: up to 400 G/cm quadrupole, up to 50 G bias field rotating at 7 kHz. We obtain approximately 109 atoms in the TOP trap. The magnetically confined atoms are then adiabatically compressed. An evaporative cooling sequence consisting of a 40 s long exponential ramp, firstly of the zero of the magnetic field (the circle of death) and then with a radio frequency field, gives an exponential increase in the collision rate and eventually Bose condensation. Following the evaporation we reduce the curvature of the magnetic field by a factor 25, before turning the magnetic trap off and performing a time-of-flight measurement. The number of atoms in condensate is approximately 105 and the BEC transition temperature is approximately 400 nK.

As in previous experiments, our evidence for BEC is based on absorptive imaging of the atomic cloud following release from the magnetic trap and time-of-flight expansion. Performing a series of evaporation cooling runs with successively lower final rf frequency, we observe a sudden increase in the optical depth in the centre of the cloud. As well as this, 15 ms time-of-flight images of clouds evaporatively cooled below this transition point begin to show elliptical density distributions with long axes aligned with the strong axis of the magnetic trap. For the same time-of-flight conditions, clouds cooled just above the transition temperature show a circular density profile.

We expect that a number of aspects of our experimental proceedure, particularly the evaporative cooling ramp and the imaging system, can be further improved and we are currently working on this.

The Otago group combines experimental and theoretical teams.

The experimental team is led by Andrew Wilson

  • Postdocs: Jos Martin and John Sharpe
  • PhD students: Callum McKenzie and Nick Thomas
  • Leader of Atomic and Laser group: Wes Sandle
  • Associated reseachers: Don Warrington and Peter Manson

The theoretical team is led by Rob Ballagh and Crispin Gardiner (Victoria University)

For more details please email awilson@physics.otago.ac.nz, or visit our website."

Posted: 11 Aug 2005     Date: 25 Aug 1998


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