When they enter the top chamber, the atoms are recaptured by another MOT. This secondary MOT is located underneath a gold mirror attached upside-down to a copper structure in the center of the top chamber. Two MOT beams, hitting at 45° with respect to the mirror surface, are brought into the chamber from both sides. After bouncing off the mirror, the beams counter-propagate with respect to each other. These two beams and their reflections, together with another counter-propagating pair of beams parallel to the mirror surface, form the optical geometry of the secondary MOT. The magnetic field of the secondary MOT is first created by two external coils, which provide a magnetic field gradient of 10 G/cm with a large capture volume. This external secondary MOT can capture up to 109 atoms after 15 s loading. The size of the captured atom cloud is around 6 mm.

In order to achieve a much higher atomic density, the MOT is compressed after the atoms are collected. The compression is done by switching off the external MOT coils and turning on a 45 A current through a U-shaped copper wire right above the mirror. At the same time, an uniform magnetic field along the -y direction is applied. We refer to this MOT as U-MOT. To better understand the U-MOT field, let us first take a look at the combined magnetic field created by a wire current in x direction and a uniform B field in -y direction as shown in the figure below.

The combination of these two fields gives rise to a 2D quadropole magnetic field with the same topology as that created by two coils. In order to get a 3D trap, the wire is bent into a U shape. The magnetic fields created by the current flowing through two legs provide the trapping potential in the third dimension. The picture below shows our real U-wire structure. With a 45 A current going through leads 2 and 1 and a 20 G magnetic field in -y direction, a U-MOT with a gradient of about 50 G/cm is created. The transfer between the external MOT and the U-MOT takes 70ms. As a result, the atom cloud is compressed to about 1.5 mm. No significant atom loss is observed during the procedure.

With a large, dense atom cloud, we are one step closer to getting a BEC. The next step is to transfer atoms to a pure magnetic trap so that we can cool them further.

Step 1: Primary MOT Step 3: Magnetic Trap

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