This might be common knowledge among cold-atom enthusiasts. You have a MOT, then to observe the free expansion of the atom cloud one should shut off all em fields including the quadruple field. But if the light is switched off and the B-gradient left on, the cloud expansion is slower. I have observed it many times without understanding why it happens (note, there is no rf field here). Any publication references would be appreciated.
You have jogged my memory, since I have seen this figure before — most amusing.
I should pay more attention to the details.
Hi John..If you have no light, then how are you 'seeing' the expanding cloud? Regarding your question, the quadrupole field coils of the MOT are now acting as a magnetic trap, although about 10 times weaker than the usual magnetic trap (due to weak B fields involved). And it is obvious that the expansion in a magnetic trap would be slower than the free expansion of the cloud because a considerable fraction of the MOT atoms has quite low K.E