Our magnetic trap is an Ioffe-Pritchard type trap, created by a Z-shaped current together with a uniform bias magnetic field in the -y direction. The Z-shaped current can be generated by running current through leads 3 and 1, as labeled in the picture below. To get a deeper trap, a bias field in the -x direction is also necessary. Unlike the trap created by the U current, this Z-trap has a nonzero magnetic field even at the bottom of the trap, which prevents the atoms from undergoing Majorana spin flips and being lost from the trap.
Before transferring atoms to the Z-trap, we need to cool atoms down to about 50 μK to ensure a good transfer efficiency. This cooling is done by switching off the MOT magnetic field and using the MOT beams as a 3D corkscrew optical molasses. The MOT beams are off for 1 ms during the magnetic field switching and their detuning is increased by 4Γ for the optical molasses phase. Since the molasses cooling is very sensitive to any stray magnetic field, during this cooling stage the magnetic field in the vicinity of the atom cloud is minimized using three pairs of external compensation coils.
Atoms in the MOT distribute equally among five magnetic sublevels of the F=2 state, but the magnetic trap is designed only for the m=2 sublevel. Thus, we apply a 100 μs optical pumping pulse, which pumps all of the atoms into the m=2 sublevel, increasing the number of atoms we can trap by a factor of five.
After carefully matching up the parameters of the MOT and the Z-trap, including their relative positions, trap depths, and temperatures, about 108 atoms can be transferred into the Z-trap. The picture above shows a shadow image of the atoms in the Z-trap. The Z-trap has an initial gradient of 60G/cm in the vertical direction and is located 2 mm below the mirror (about 3.3 mm below the center of the Z wire). 20 ms after the Z-trap is turned on, the bias magnetic field is ramped up from 22G to 40G in 120 ms, which brings the Z-trap location up by 1.2 mm. This procedure significantly compresses the atom cloud and increases the collision rate by at least a factor of 4.
Now, we are only one step away from making a BEC!
|Step 2: Secondary MOT||Step 4: Evaporative Cooling|
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