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Magneto Optical Trap

 

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At left is a simplified diagram of the lithium magneto-optical trap (MOT). The ~8 inch vacuum chamber is kept at low pressures (<10^-7 T). An effusive oven at ~450C and apertures generate an atomic beam. A slowing beam and three pairs of counter-propagating trapping beams slow, cool and trap ~10^7 atoms at the zero of a radially increasing magnetic field. (The anti-Helmholtz-like current-carrying coils on the top and bottom of the chamber aren’t shown.) The resulting cold atoms likely have a temperature less than one millikelvin and the trap has a lifetime of ~300 ms (see below). We will load the cold atoms into a high power focused laser beam (dipole trap) for single-photon cooling.

The plot at right shows the fluorescence from atoms in the MOT. When the slowing beam is blocked (red) the loading rate decreases to almost zero and the atoms escape the trap due to collisions with the atomic beam, background gas and collisions with each other. The decay time is estimated at ~305 ms. When the slowing beam is unblocked (blue) the trap loads in ~326 ms. The significant noise in the trapped-atom fluorescence at steady-state is due in part to a laser lock that needs improvement.

Bethel lithium magneto-optical trap apparatus