Bose-Einstein Condensation and Quantum Chaos Laboratory
Oklahoma State University
 

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Bose-Einstein Condensation

On August 18, 2004 we observed our first BEC in Rubidium 87 atoms. A nice explanation of what BEC is and why it is interesting can be found at the physics 2000 website. Our setup is different from most BEC experiments in that we use an “all-optical” technique to reach the condensation conditions. What this means is that we load our atoms into a trap which is formed by the overlap of two focused laser beams. The atoms are trapped because they are attracted to regions where the laser intensity is the highest. This happens to be where the two laser beams intersect.

To make a BEC one needs to have a dense sample of atoms at an extremely low temperature.  For the density that we have in our experiments we must reach a temperature that is only about 100 billionth of a degree above absolute zero. This low temperature is achieved through evaporative cooling, the process which will lead a cup of coffee to become cold if it is left to stand for a few minutes. In our case, this evaporation is performed by lowering the intensity of the laser beams the constitute the optical trap to let the hottest atoms escape. The atoms which remain have a lower average energy and hence a lower temperature.  With this technique we can reach the temperature necessary for a BEC to form, the only catch being that we have thrown away some of the atoms. Typically we start with about 1 million atoms and end up with about 10,000 in the Bose-Einstein condensate. The whole evaporation process is carried out in about 1.5 seconds.

At the top right is a sequence of pictures showing the formation of the BEC. These images were taken after the condensate was allowed to freely expand for 7 milliseconds. You can see that as the condensation temperature is crossed the peak goes up dramatically. This is because the condensate is expanding at a much slower rate than the non-condensate (also known as a thermal cloud).

The lower picture shows what happens when the condensate is allowed to expand for different amounts of time. In this situation a thermal cloud of atoms would expand uniformly in all directions, so that after 12 milliseconds we should observe a spherical atomic cloud. Instead the condensate turns into a cigar shaped object. This effect is produced by the interactions between the atoms which are much stronger in the vertical direction. This is because we confine the atoms more tightly along this axis.

 


A sequence of images showing the formation of the condensate as the temperature of the atoms is reduced.

 

Here is a movie showing the formation of the condensate using two trapping laser beams.


Sequence of frames showing the evolution of the condensate after it is released from the optical trap in which it is made. Unlike a thermal cloud, the BEC expands asymmetrically due to the interactions between the atoms.  You can also see that gravity is causing the BEC to accelerate (the downwards direction is also down in the picture).

 

 

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