(gravito-optical surface trap):
 

Two-Dimensional Bose-Einstein
Condensate of Cesium

ultracold.atoms
Institut für Experimentalphysik,
University of Innsbruck, and
IQOQI
Austrian Academy of Sciences,
Innsbruck, Austria

BEC of cesium in an optical surface trap

In the evening of Aug 25th, 2003, our team at the Institute for Experimental Physics at Innsbruck University has created a Bose-Einstein condensate (BEC) of cesium atoms in an optical surface trap. Our experiment is the second to reach condensation of cesium, after its first demonstration on Oct 5th, 2002, in the lab next door. Our trap strongly compresses the atomic cloud in one direction, leading to a two-dimensional condensate!

Phys. Rev. Lett. 92, 173003 (2004). cond-mat/0309536.

 

Evanescent-wave trap

Instead of using conventional traps in free space, we trap and cool the atoms in close proximity to a glass surface. A repulsive evanescent wave prevents the atoms from hitting the surface, and supports them against gravity at a distance of only a few micrometers. In order to confine the atomic motion also in the two horizontal directions, we focus an attractive laser beam onto the glass surface.

Our dipole trap is formed by a repulsive evanescent-wave close to a glass surface and an attractive focused laser beam
 

 

Evaporation

When the intensity of the attractive focused beam is slowly reduced, the hotter atoms escape from the trap, and a colder atomic sample remains after thermalization. Using this technique, we are able to reach extremely low temperatures and high phase-space densities. Under such conditions, the atomic motion can no longer be described classically, and quantum-mechanical effects arise.

We measure the kinetic energy in the vertical direction by suddenly switching the evanescent wave off, and turning it back on after a variable time. For recapture times smaller than about 0.8 ms, no atoms are lost because of the initial distance to the surface of a few micrometers. For longer times of flight, the atoms start hitting the surface and are lost. The sharp drop indicates a small velocity spread in our sample, corresponding to only 16 nK. This value is clearly smaller than the quantum mechanical level splitting for the vertical motion, given by 25nK * kB , so the 2D-regime is reached. (The actual temperature is even smaller than 16 nK, since this value is dominated by the contribution of the ground state energy).
 

 

Bose-Einstein Condensation

After evaporation, the measured values for the temperature, number of atoms (a few thousands) and trap parameters indicate a peak phase-space density clearly higher than one. In order to prove the presence of a condensate, we demonstrate the occurance of a mean-field effect, which is absent in thermal samples. In a condensate, the interaction strength affects the collective behavior of the atoms. In particular, attractive interactions lead to an implosion of the condensate and consequent inelastic loss.

 

The interaction between cesium atoms can be controlled by applying magnetic fields. Below 17 Gauss, the interaction is attractive, leading to an implosion of the condensate.
 

 

Evidence for Efimov states

A remarkable result in research could be achieved in showing the evidence for Efimov states. Together with the LevT team it was shown that trapped Cs atoms with a temperature of  below 250 nK experience a giant three-body recombination loss if they are put into the regime of negative two-body scattering lenghts ( Nature 440, 315 (2006) cond-mat/0512394).
These triatomic Efimov resonances are examined intensely by the LevT team which has resently observed Efimov states with atoms and molecules

The Team

The team at the Institut für Experimentalphysik, Innsbruck University, Austria:

(from left to right)

  • Hanns-Christoph Nägerl (staff scientist)
  • David Rychtarik (graduate student)
  • Rudi Grimm (group leader)
  • Bastian Engeser (graduate student)

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" and the FASTnet (Field Atom Surface Training Network, within the 5th Framework Programme of the European Union).

FWF

last change: 03-03-09 by BS