Component 1 (below) is our rubidium reservoir, which provides Rb atoms in vapor form. When Rb atoms enter Component 2, they are cooled and trapped by a magneto-optic trap (MOT). Our primary MOT is a vapor-cell pyramidal MOT. A pyramid-shaped mirror is afixed, facing downwards, inside Component 2 as shown in the figure inset below. The mirror is made of four glass plates coated with a layer of pure aluminum.
When a large-diameter laser beam is sent up into the pyramid, it reflects off the four walls (its helicity changes to the opposite after each reflection), creating six counter-propagating beams for the MOT. The inset in the above figure shows a 2-D slice of how the pyramidal mirror generates four counter-propagating beams for the MOT. In the perpendicular plane, the setup has the same geometry. A quadropole magnetic field, centered where the six beams meet, is produced by two 100-turn coils outside the chamber with opposite current running through. Thus a cloud of about 109 atoms and with a temperature of about 200 μK is collected under the pyramidal mirror.
There is a 1 mm aperture at the apex of the pyramidal mirror. Since no light is reflected by the vertices of the pyramid, when the atom cloud is directly below the aperture there is no light reflected directly downward onto it. Due to the radiation pressure imbalance, the atoms (already cooled by the pyramidal MOT) are pushed upwards through the aperture as a cold beam. They are launched into the top (secondary) chamber, passing through a gate valve and a differential pumping stage along the way.
The differential pumping stage, located between the primary and secondary chambers, consists of two ion pumps and a Ti sublimation pump. This stage is very important for maintaining ultra-high-vacuum (UHV), below 2x10-11 Torr, inside the top chamber, while still allowing the lower chamber to be kept at a much higher pressure so that enough atoms can be collected.
|Setup||Step 2: Secondary MOT|
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