Techniques to generate slow cold atom beams are of great interest in a variety of applications, from atomic frequency standards and atom optics to experimental studies of cold collisions and Bose-Einstein condensation (BEC). Slow atom beams have been produced using lasers to slow hot atoms from an oven source, or by using laser-cooling techniques to extract a slow atomic beam from the background gas in a low-pressure vapor cell. This latter approach was first demonstrated by researchers at NIST/JILA, using a variation of the conventional vapor cell magneto-optical trap (MOT) they termed the "low-velocity intense source" (LVIS).

In the LVIS, a cold atom beam is formed in the "extraction column" defined by a small aperture in the center of one of the retro-optics. This aperture produces an imbalance in radiation pressure in a narrow region along one axis. Cold atoms at the center of the trap are accelerated toward the aperture by this imbalance, and thus form a slow (< 30 m/s) beam of cold atoms. The divergence of the cold atom beam is quite small, typically 40 mrad or less, and the brightness (flux into unit solid angle) can be more than an order of magnitude greater than that of slowed thermal beam sources. The slow velocities and narrow velocity distribution resulting from the LVIS are well-matched to the capture range of a standard magneto-optic trap, allowing efficient loading of a second MOT from an LVIS beam. With proper design, a very high transfer efficiency (>70 percent) of atoms through the exit aperture can be obtained while maintaining a low conductance between the source region and an adjacent chamber. This allows the use of differential pumping to obtain UHV pressures in the adjacent chamber. For this reason, this cold atom beam source is particularly well suited to studies requiring very low background pressures, such as BEC experiments or other studies requiring long trap lifetimes.

Our current work implements a simple and robust design based on a pyramidal trap geometry, and allows use of a single large diameter (20 cm) laser beam to obtain large capture rates of atoms from the cesium background vapor. As shown in photos above, four 45-degree mirrors are truncated just before the apex of the pyramid, and the 1 cm² region at the center of the incident laser beam is retro-reflected by a 1/4-wave plate with a high-reflectance gold coating on the second surface. A small (1 mm diameter) hole in this retro-optic forms an extraction column for the atoms while maintaining a low conductance between the source region and an adjacent UHV chamber. We have measured cold atom beam fluxes greater than 10⁹ atom/s with longitudinal velocities of 15 m/s. Beam divergences are less than 20 mrad (approx. 1 degree).