Magneto-Optical Traps (MOTs)
With the addition of a magnetic field gradient, the velocity reduction of Doppler cooling can be used to help confine and trap neutral atoms.
In a weak inhomogeneous magnetic field, the internal energy levels of the atoms are split into their Zeeman components. By using circularly polarized light to Doppler cool the atoms in the presence of this magnetic field, these atoms can also feel a spatially-dependent force that pushes them towards the zero of the magnetic field. Using a quadrupole magnetic field and retroreflected, circularly polarized laser beams in a 3-D geometry enables cooling and trapping in all three dimensions.
This type of magneto-optical trap, or MOT, was first demonstrated with the trapping and cooling of alkali atoms, but the method can be easily extended to atomic states that have broader cooling linewidths by increasing the applied magnetic field gradient.