The previous MOT video showed the atoms squirting out of the side when the trapping field was turned off. In this video things are more balanced, and you can see the atoms remaining in the beam overlap region, and fluorescing quite brightly. The trap is cycled on and off and you can see the trap “grab” the atoms and pull them back to the center; when the trapping field is left off it takes several seconds for the cloud to dim as atoms diffuse out, and that’s a qualitative sign that the atoms in the molasses are pretty cold. Probably tens of microKelvin.

“Mr. Hands” is pointing out the trap axis at the beginning, as a cue to the person adjusting the trimming magnetic field. This kind of adjustment can be very laborious, as there are several parameters which need to be optimized, and they aren’t independent of each other. Beam alignment, magnetic field and beam intensity all need to be optimized, but all exert forces which can be offset by one of the the parameters, e.g. a slight imbalance in intensity can be offset by a small magnetic field, and the small amount of swirling of the atoms when the trap is turned off is likely an indication that this is the case.

However, at this level of adjustment, the atoms are the best indicators. An optical power meter or a magnetic field probe aren’t going to yield the precision necessary — they can only get you close. At this point you just have to wander around phase space, checking that you aren’t merely at a locally optimum signal. The true test comes when you can actually measure the temperature of the atoms, by imaging them in time-of-flight and seeing how much the cloud has expanded.