Manipulation of atoms by optical field
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When cold atoms or a Bose-Einstein Condensate (BEC) of an atomic gas is placed in a light standing wave produced by two counter-propagating lasers of certain frequency, the atoms are diffracted in different order. A cold atom absorbs a photon from one of the laser beams and emits in the other beam receiving a net momentum of 
in the direction of the photon. Here k is the magnitude of the wave vector of the laser. This is called two-photon recoil process. In such situation, a BEC cloud sitting at the center of a trap is split into two identical clouds. The clouds then travel in opposite directions with a velocity, 
in the direction of the photon.
