Experiments to Control Atom Number and Phase-Space Density in ...

ω-Δ
∆
ω-Δ
|e>
|g>
(a) v=0 (b)
ω-Δ-kv
v>0
∆
|e>
ω-Δ+kv
Figure 2.14: 1D optical molasses consist of a pair of counter propagating beams, detuned
below the atomic resonance frequency, impinging on the atom. (a) If the atom is at rest,
both beams are equally detuned from resonance, and the number of photons scattered
from each beam is on average the same. (b) If the atom is in motion, the Doppler effect
shifts the beam opposing the atomic motion closer to resonance and the atom scatters
photons preferentially from this beam, effectively slowing the atom.
from the right hand side than from the beam coming from the left hand side. Thus the
velocity of the atom is decreased.
More quantitatively, the overall force on the atom in optical molasses is deter-
mined by the scattering forces caused by the counter-propagating beams. In the one
dimensional situation described above, the net molasses force on the atoms is thus given
by
Fmolasses = Fscatt(ω −ω0 −kv)−Fscatt(ω −ω0 +kv). (2.30)
For small velocities this simplifies to
|g>
Fmolasses = αv, (2.31)
2 I −2∆Γ
where α = 4k Isat (1+(2∆/Γ) 2 ) 2, and the force on the atom is proportional to the velocity
of the atom [22, 35].
The lowest temperature possible in a true two-level system cooled by optical
molasses is determined by the Doppler temperature TD = Γ
2kB
[22, 35]. In a multi-level
atom it is possible to beat the Doppler limit in the optical molasses configuration due
to the Sisyphus cooling effect. This effect has been observed in the heavier alkali atoms.
For lithium, the lowest temperatures reported in an optical molasses are around the
Doppler temperature.
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