Output-coupling schemes
(a) Various output couplers have been employed to extract coherent beams of matter waves from a BoseEinstein condensate. (a) At MIT pulses of radio-frequency (rf) radiation are applied to transfer atoms in the condensate from a trapped magnetic sublevel (mF = 1) to an untrapped one (mF = 0). At Munich a continuous radio-frequency field is used.
(b) At Yale the condensate atoms are confined in a one-dimensional optical standing-wave potential. The potential is sufficiently shallow for there to be an appreciably probability for the atoms to tunnel downwards and out of the wells due to gravity. As the trap is aligned in the direction of gravity, the energy difference between adjacent potential wells is therefore E = mgl/2, so atoms leaving adjacent traps have a frequency that differs by w = E/ h/2p. The various waves will combine to produce a wavefunction that beats at the frequency w = mgl/2 h/2p. When the periodic potential is loaded with condensate atoms, the initial relative phase of the atoms is identical and the beating is observed as a train of pulses of atoms.
(c) In the NIST output-coupling scheme, lasers are used to coherently transfer atoms from the trapped magnetic sublevel (mF = 1) to the mF = 0 sublevel, which does not feel the influence of the magnetic trap. Stimulated Raman scattering involves the simultaneous absorption and stimulated emission of photons. The direction of the momentum transfer to the atom is determined by the orientation of the laser beams, so the beam is not restricted to the downward direction.