One of the questions one asks when trapping atoms in a magneto-optic trap (MOT) is “What shall we do with the atoms?” You often have an idea before you do the trapping — it’s not like we’re trophy hunters, trapping just to have something on the wall. Trapping in and of itself hasn’t been the goal for quite some time now, at least in experimental labs; one wants to do some kind of experiment with the atoms. Some of the time that can be done in the trap, but quite often it involves moving the atoms somewhere else. Sometimes you actually wanted an atomic beam of some sort, instead of a collection of atoms just sitting there, suspended in space — the trapping environment involves bright, near-resonant laser light and magnetic fields and those could be undesirable. The atom beam gets you away from this, and if you look at the beam from a perpendicular direction, the Doppler shift is very small. Perhaps you want low-speed collisions, and tuning the speed of the beam allows you to do your experiment. There are also a number of atom-optics experiments that can be done, e.g. sending the atoms through transmission gratings comprising an interferometer. The problem could also be the relatively high vapor pressure of the gas in your vapor cell giving you excessive background signals, or collisions with that background vapor could be the problem, limiting the trap lifetime. So you need to move the atoms, transporting them to a region that is better-suited for the experiment you are doing.
When I was at TRIUMF, the problem was the background and trap lifetime. We were trapping radioactive atoms, and the idea was that when an atom decayed, the beta would go one way and the atom would recoil, and each could be detected. But a vapor-cell MOT captures only the small percentage of atoms
stupid enough moving slowly enough to get trapped, leaving the majority of the zipping around in the cell or sticking to the walls (or worse, attaching themselves to detectors). Not only did this mean they would be swamping the signal from the trapped atoms, the signals would be coming from different directions and originating from different points.
About the time we started fretting about this problem (you have to trap them first before you worry about the next step, and nobody had trapped these isotopes before) we got a visit from Zheng-Tian Lu, then at JILA/NIST, and he had come up with an ingenious method of generating a low-velocity atomic beam and shared the details with us (the paper was in the pipeline but had not yet been published at the time)
A typical vapor-cell MOT uses three beams along the cartesian axes, and it’s possible to do this by retroreflecting each of these beams — the vapor is dilute, so with decent mirrors there isn’t a large drop in intensity (any imbalances will push the trap slightly off-center as the effect of the magnetic field compensates). You get the proper polarization of the beams by placing a quarter-wave plate in front of the retroreflection mirror (this changes the circular light to linear and then back to circular of opposite helicity; if you started with linear it would circularize it and change it back to linear, perpendicular to the original. Ah the fun you can have with waveplates)