Laser trapping is the application of laser cooling. Trapping takes place in a MOT, a magneto-optical trap, or sometimes called a ZOT, Zeeman-optical trap. ZOT is in reference to the Zeeman Effect, the energy level splitting that results from exposing an atom to an external magnetic field. First, an electron has an orbital angular momentum, l, and a spin angular momentum, s. Moving charges are essentially current and where there's current, there's a magnetic field. The interaction of internal fields cause l and s to couple. That is, they lock together in quantized configurations, producing quantized momentum. This is the fine atomic structure that is viewable through spectroscopy. Exposing the atom to a weak external magnetic field produces the Zeeman Effect. This resolves the splitting of the fine structure into hyperfine structure. Ok, the physical setup of the MOT is a vacuum chamber between Helmholtz coils, which provide a uniform and measureable magnetic field. Outside of the MOT there are six mirrors along the x, y, and z axes. A tuned laser is reflected off these mirrors so that they all intersect at one point in the chamber.

So you pump the air out of the chamber and release some gas into it. We use rubidium, but you can do it easily with sodium and a few others. Oh, oh, the Zeeman Effect does it's thing and splits the energy levels. A single beam is sent through a series of beam splitters and 1/4 wave plates to produce six circularly polarized lasers (each axis has oppositely polarized lasers) which do the cooling. One brand of polarized light will interact with the lower energy line and the other with the higher energy line. Ok, imagine a single atom for now. It has some component velocity along one or more axes. It sees its repsective incoming laser and it does its cooling thing. Laser cooling will not bring the atoms to a halt due to the nature of laser cooling and Doppler shift. Once the atoms reaches a certain speed, the laser stops interacting with the incoming atom which may be moving rather slowly. The reason the atoms simply don't drift out of the trap is the magnetic field pushes them back into the middle by interacting with the atom's internal magnetic field. Since the atom is now moving in the other direction, the laser on the other end of the axis gets to do its thing.

The process doesn't trap all atoms in the system. It can only hold as many atoms will fit in the volume of the intersection of the lasers. The trap will only work on atoms with an initial velocity in some range. This is ok, you'll catch enough given you increase the beam diameter to your liking.

So, you've trapped some gas. Now what? Well, super-cooled gas is of interest because it can get within micro-Kelvin of absolute zero. To put it in perspective, these atoms are moving at a mere 50 m/s or so! This make it easier for chemists and physicists to study nuclear processes.

To put those results in perspective:

  • 50 m/s for Rb, gives an average temperature of 8.56 K
  • Space is about 2.73 K
  • Recently, a sample of 2000 or so Rb atoms were cooled in the nanoKelvin range! This was acheived by using laser cooling and trapping in conjuction with evaporative cooling using a dilution refrigerator
Sources: My lab notes and memory.

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