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

temperature TB of atoms loaded into the magnetic trap, typical values used
during these experiments were NB ≈ 5 × 10 7 atoms and TB ≈ 40µK. The
number and temperature of atoms loaded into the magnetic trap was varied
by controlling the MOT beam detuning.
The presence of the repump beam during the MOT, optical molasses,
and optical pumping stages ensures that the magnetically trapped atoms are
in the 5 2 S1/2(F = 2) hyperfine manifold. This manifold contains two mag-
netically trappable states: |F = 2,mF = 2〉 and |F = 2,mF = 1〉. The
purpose of the optical pumping stage is to populate the |F = 2,mF = 2〉 state
preferentially over the |F = 2,mF = 1〉 state. While helpful, this process
is not 100% efficient and so there is a distribution of the two states in the
magnetic trap. Later in this section we will calculate the amount of phase
space compression achieved by the single-photon cooling process. To do this,
we must find the phase space density of the atoms in the magnetic trap. This
calculation requires knowledge of the number of magnetically trapped atoms
in the |F = 2,mF = 2〉 state. Unfortunately, our imaging method does not
distinguish between atoms in the |F = 2,mF = 2〉 and |F = 2,mF = 1〉 states,
so the atoms in these two states must be separated before imaging to get the
number of interest. We achieved this separation by setting the magnetic field
gradient to a value capable of levitating atoms in the |F = 2,mF = 2〉 state
against gravity, but not atoms in the |F = 2,mF = 1〉 state. Figure 4.15
shows the number of atoms remaining in the magnetic trap as a function of
the current in the quadrupole coils. As seen in this figure, below ≈ 8 A a sud-
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