Decoherence of Atomic Gases in Largely Detuned Laser Fields

In experiments with atomic Bose-Einstein condensates [1,2] the trap which is used to isolate the atoms from the environment plays an important role for the physical behaviour of the system. In a magnetic trap usually only atoms with a specific magnetic hyperfine quantum number are stored. An optical trap [3] can be used to confine atoms in all Zeeman sublevels. However, in order to preserve the atomic coherence spontaneous emission of photons must be avoided. A large detuning ∆ of the laser beams providing the trap field reduces the excitation probability for atoms and thus reduces the number of spontaneously emitted photons. At the same time, a relatively strong intensity of the laser beams, and correspondingly a large Rabi frequency Ω(x), ensures that the laser beams’ effect is still large enough to produce a strong potential for the atoms.