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

turning points, they will have little residual kinetic energy. After the state
transfer the atoms will fall onto the optical sheet which holds them against
gravity. Second, by transferring the atoms near the sheet they will acquire
little energy due to free fall. The state transfer is accomplished optically with
the depopulation beam (see Sec. 3.2.1.7).
The depopulation beam is tuned near the |F = 2〉 → |F ′ = 1〉 transition
frequency. Correspondingly, its function is to transfer atoms from the |F =
2,mF = 2〉 state into the excited |F ′ = 1,mF ′ = 1〉 state. Once excited, the
atoms quickly decay (τ ∼ 26 ns). Calculation of the decay branching ratio is
discussed in Sec. 2.6 and the results are summarized in Fig. 2.12. As indicated
in this figure we can optically pump 42% of the atoms into the |F = 1,mF = 0〉
state in a single cycle. Experimentally we do somewhat better because any
atom which decays into the F = 2 manifold is reexcited by the depopulation
beam and has another chance to decay into the F = 1 manifold. We find close
to 1/2 of the atoms decay into the desired |F = 1,mF = 0〉 state.
Ensuring that the atoms are transfered into the magnetically decoupled
state near the repulsive optical sheet is a simple matter. One only needs to
form a second optical sheet from the depopulation beam and place it slightly
above the repulsive sheet (see Fig.4.2). Transferring the atoms preferentially at
their turning points is also accomplished in a straightforward manner. In this
geometry, one would initially place the two sheets below the magnetic trap
beyond the turning point of even the most energetic atom and then slowly
sweep the pair of sheets vertically towards the center of the magnetic trap. If
136