Hi there, I am reading something about the bose-einstein condensate, in this material http://www.physics.ox.ac.uk/qubit/fetch.asp?url=groupwebsite/papers/paper103.pdf [Broken], it states something called harmonic trap. I read other introduction materials on bose-einstein condensate, it said that to create that condensate, applying the so-called magneto-optical trap and the a far-off resonant trap and evaporation. But after we get the condensate, all those trap are removed. So what is that harmonic trap actually? When is it applied and why we need that? Thanks.