Single-Photon Atomic Cooling - Raizen Lab - The University of ...

to minimize residual kinetic energy. Notice that in contrast to the potential
produced by the optical box (Fig. 4.11), atoms do not have to climb a potential
hill to reach the depopulation beam, resulting in a final population with less
residual kinetic energy.
In this iteration of the experiment, the cooling process was initiated by
adiabatically lowering the magnetic trapping potential. This is done by linearly
ramping down the current in the quadrupole coils in a time tramp, which is
on the order of 1 s. As the magnetic field gradient is reduced, the atomic
cloud expands and the turning point of each atom (in the vertical direction)
approaches the depopulation beam which is held a fixed distance below the
magnetic trap. To ensure that each atoms encounters the depopulation beam
near its classical turning point, the adiabaticity condition 〈τB〉/tramp ≪ 1 must
be satisfied, where 〈τB〉 is the average oscillation period in the magnetic trap.
We found this scheme to be advantageous over simply moving the centers of
the traps together, as in the previous two sections, because it reduced optical
trap loss due to atomic collisions with magnetically trapped atoms.
As discussed above, when the atoms encounter the depopulation beam
they are driven from the initial |F = 2,mF = 2〉 state into the 5 2 S1/2(F = 1)
manifold. Once decoupled from the magnetic trap they fall into the optical
trough where they are captured.
Once the cooling process is complete, atoms transfered into the op-
tical trough via single-photon atomic cooling are imaged using the vertical
probe beam (see Sec. 3.4.1). To remove any residual atoms, all magnetic fields
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