LINEAR MOMENTUM CONSERVATION IN COHERENT POPULATION TRAPPING - A CASE-STUDY FOR A QUANTUM FILTERING PROCESS

We discuss the question of linear momentum conservation when an atom coupled to a laser field enters into a state which is not an eigenstate of the linear momentum. Such a situation happens in the recently demonstrated laser cooling of atoms by velocity selective coherent population trapping. We show that this process can be understood as a filtering of the atomic state by the laser field taken as a classical measuring apparatus. In a different approach, the laser field can be taken as a quantized system; then the filtering process leads to an "entangled state" of the atom and the field.