When a group of atoms is super-cooled to temperatures near absolute zero, they form a Bose–Einstein condensate, a state of matter where quantum effects can be observed in the macroscopic system. In this state, the trajectories of the atoms can be manipulated using light.
If a super-cooled atomic gas is placed in a standing light 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 a photon in the other beam receiving a net momentum of