Ultracold 87Rb beam

Currently, the cold atomic beam apparatus we have constructed is capable of producing a continuous beam of rubidium atoms with an unprecedented total flux of 3.2 x 1012 (1.0 x 1012) atoms/sec at 116 (20) m/sec. This flux is more than an order of magnitude above the absolute best of previous cold atomic sources and opens a new regime in the production of ultracold atomic samples.

A picture of the existing experimental set-up is shown above.  Our source of thermal rubidium atoms originates from a rubidium candlestick atomic beam source, which provides an intense, well-collimated beam with a mean velocity of 400 m/s.  To utilize this source of hot thermal atoms, the beam is first transversely collimated and cooled to remove as much transverse velocity spread as possible, thus increasing the overall flux.  Subsequently, a Zeeman-slower decelerates and longitudinally cools the atomic distribution, and also provides us with a means to tune the final beam velocity.

The initial transverse cooling of the atomic beam employs a novel atomic collimator, which has a large transverse capture velocity and acts to uniformly decelerate the atoms over its whole length. In the collimator, the 15 cm of transverse collimation is accomplished by two pairs of nearly plane parallel 6" x 1" mirrors (one pair for each transverse axis), with two 1 cm wide cooling laser beams coupled in on one side at small angles relative to the mirrors' normals.  By using multiple reflections down the length of this cavity, we may effectively apply a cooling force throughout the entire region without the need for excessively large optics or unreasonable laser power.

By design, as the atomic beam propagates down the collimator, it becomes more and more collimated by the laser radiation pressure. The wavefronts of the collimating laser fields are designed to precisely match the deflection/collimation angle of the atomic beam.  In this way, it is possible for the laser beams to remain orthogonal to the trajectory of the atoms which are then uniformly accelerated radially inward with the maximum spontaneous force and which, therefore, follow an approximately circular arc. 

By matching the transverse cooling structure design to the divergence characteristics of the candlestick, we demonstrate that we are able to utilize a full 50% of the atoms emitted from the source for deceleration. As a final consideration, and for fine pointing of the atomic beam, we also employ a 2" region of optical molasses at the end of the collimator, increasing the total cooling region's length to 8 inches.

With the atomic beam transversely cooled and collimated, we next cool and decelerate the atomic beam longitudinally by employing a one meter long Zeeman-slower, which uses a precisely sculpted magnetic field profile to compensate for Doppler shifts as the atomic velocity decreases. The field also allows the final velocity of the beam to be controlled to any value between 50-150 m/s.   

For more information see:

Christopher Slowe, Laurent Vernac, Lene Vestergaard Hau, "A High Flux Source of Cold Rubidium," Rev. Sci. Instrum. 76, 103101 (2005).