Researchers have been pushing atoms around with laser beams for many years, cooling them to record low temperatures and capturing them in confined spaces. Most such efforts require many photons. Then the coupling energy of the atom to the field can exceed the atom's kinetic energy, and the atom can become ensnared. The photons in these experiments played a double role, not only trapping the atom but giving information about its position. Getting this result required the Caltech team to build an exceptionally small cavity that is strongly coupled to the atom.
Yellow path indicates the trajectory reconstructed from the passage of an atom through the cavity.
The figure shows a continuous laser beam shining through the cavity rather than a single photon bounding around inside of it. That's because the lifetime of a photon in the cavity is so much briefer than the time for an atom to transit the cavity that one needs a steady stream of single photons to exert on the atom an average dipole force equivalent to that of a single photon. The experiment by the Max Planck group was similar, except that an atomic fountain directed the atoms upward into the cavity. In the cavity used by this group, the quality factor Q was about ten times higher, meaning that it held each photon longer, but the coupling of the atom to the cavity field was seven times lower. Because of the weaker coupling, diffusion plays a much larger role in the atom's motion than does the dipole force of the photon.