BEC or bock 1

The formation of the super-cold atoms, which are a single quantum object (a wave) need three stages:

  1. Trapping of the atoms in a magnetic trap.

    Atoms have their own momenta (spins) and for this reason they can be considered as small top-spins.
    Magnetic trap is a specific field configuration - a quite shallow potential well in which atoms are trapped like top-spins in the center of a quite shallow plate.

    Trapping electrically neutral particles (a "shallow" magnetic trap).

  2. Cooling trapped atoms by photons.

    Photons carry momentum - in collisions with atoms (absorption processes) they can exchange it. The "mass" of photons (determined by their momentum) is small in comparison with atom mass - in that way, in every collision, atoms change their momnetum by a small quantity - like a rubin ball in a sea of the photons2 .

    3. melasa

    Cooling, ex. "rubin" atom of Rb in "molassa" of the photons (here polistyrene balls).

  3. Cooling by evaporation.

    Laser cooling alone allows to obtain temperatures in micro Kelvin range, a bit to high to obtain a condensation. The needed temperature jump can be obtained by a forced evaporation of the warmest atoms (by a radio frequency, which cause transitions between magnetic sublevels, split by the  Zeeman's effect ).

    It is amazing how the forced evaporation (even by a hot air stream from the hair dryer) lowers quickly the temperature.

    Cooling without evaporation

    Cooling with evaporation

    Forced cooling (with warm air)

    Comparison of cooling speeds.

1 In regional dialect of  Trentino , Italy, supposingly from German word - bock.

2 An ddditional conditions is the absorption of photons by atoms only during frontal collisions - the frequency of the laser is tuned to longer waves (a red shift) than the exact frequency of absorbed quanta by atom.