However, atomic physicists must first learn how to place a few neutral atoms into such traps and control their tendencies to escape or randomly change their quantum states during the manipulation. In the 30 October PRL , German researchers report that they successfully transferred an exactly known number of cesium atoms from a trap that uses lasers and magnetic fields into a calmer snare that uses only a single laser beam. The technique holds promise as a source of one well-behaved atom at a time. Both traps are popular tools for atomic physicists. The first, called a magneto-optical trap, or MOT, relies on radiation pressure from several laser beams to confine atoms within a magnetic field.
Atoms absorb photons from the lasers and then re-emit photons in random directions.
This process slows the atoms until they settle in a cold knot at the center of the magnetic field. By contrast, the second trap uses a single laser. In this so-called optical dipole trap, the oscillating electric field of the laser polarizes neutral atoms and attracts them, just as a charged plastic rod attracts neutral hairs or paper shavings. The attraction is highest at the sharp focal point of the laser beam. An optical trap is more fragile than a MOT, but atoms linger more quietly there because the laser is tuned to a frequency at which the atoms do not absorb photons.