Because of the dipole force, atoms can be trapped by an electric field that has a local maximum of some point in space. Could such fields be produced by some clever arrangement of electric charges? For any system of fixed charges, the answer is no. Yet an electric field with a local maximum can be achieved in a dynamic system. In particular, because light is made up of rapidly oscillating electric and magnetic fields, a focused laser beam can produce an alternating electric field with a local maximum. When the field interacts with an atom, it alters the distribution of electrons around the atom, thereby inducing an electric dipole moment. The atom will thus be attracted to the local maximum in the field, just as the charged particle was drawn toward the rod.
The fact that the electric field changes rapidly does not present a problem. As the field changes polarity, the dipole moment of the atom also switches around. As long as the field changes at a rate slower than the natural oscillation frequencies of the atom, the dipole moment remains aligned with the field. The atom therefore continues to move toward the local maximum. As a result, this dipole force can be used to confine atoms. In 1968 Vladilen S. Letokhov first proposed that atoms could be trapped in a light beam using the dipole force, and 10 years later Arthur Ashkin of AT&T Bell Laboratories suggested a more practical trap based on focused laser beams.
Although the dipole-force trap is elegant in conception, it had practical problems. To minimize the scattering force, the light must be tuned well below the frequency at which the atoms readily absorb photons. At those large de tunings , the trapping forces are so feeble that atoms as cold as 0.01 kelvin cannot be held in the trap. Even when colder atoms were placed in the trap, they would boil out of the trap in a matter of a few thousandths of a second as a result of the ever present photon scattering. In addition, the task of injecting atoms into the trap seemed daunting because the volume of the trap would only be 0.001 cubic millimeter. For these reasons, the challenges to optical trapping seemed formidable.
Then, in 1985 , a scheme for a workable optical trap became apparent after atoms were laser cooled in all dimensions and to much lower temperatures than the stopped atomic beams. The laser-cooling idea was first proposed in 1975 by Theodor Hansch and Arthur Schawlow of Stanford University. In the same year, a similar scheme for cooling trapped ions with lasers was proposed by David J. Wineland and Hans G. Dehmelt of the University of Washington. The researchers predicted that an atom could be cooled if it is irradiated from two sides by laser light at a frequency slightly lower than the frequency needed for maximum absorption. If the atom moves in a direction oppo- sing one of the light beams, the light, from the atom's perspective, increases in frequency. The light that has been shifted up in frequency is then likely to be absorbed by the atom. The light that the atom absorbs exerts a scattering force that slows the atom down.
How does the atom interact with the light traveling in the same direction? The atom is less likely to absorb the light because the light, again from the atom's perspective, has been shifted down in frequency. The net effect of both of the beams is that a scattering force is generated, opposing the motion of the atom. The beauty of this idea is that an atom mOving in the opposite direction will also experience a scattering force dragging it toward zero velocity. By surrounding the atom with three sets of counterpropagating beams along three mutually perpendicular axes, the atom can be cooled in all three dimensions.