Thesis (pdf) - Swinburne University of Technology

Chapter 2: Theoretical Background
be pumped into a low-lying energetic state. Figure 2.3 depicts this concept.
It can be seen as the atom is moving uphill in the potential, and close to the
peak, is falling back into a valley where it starts a new climb. Because of the
similarity to the fate of the greek tragic hero Sisyphos, the cooling mechanism
was dubbed “Sisyphus cooling”.
E
λ/2
m = - 1/2
F
m = + 1/2
F
Figure 2.3: Sub-Doppler cooling: the atom continuously “moves uphill” as
transitions from the energetically higher to lower mF -state are driven.
The ratio of this cooling and the Doppler cooling can be shown to be about
2|∆|/Γ [Met99]. For larger detunings ∆ lower temperatures can be reached,
but this cooling mechanism has a smaller capture range and only works with
sufficiently cold, Doppler cooled atoms. The lowest limit here is given by the
momentum of the last emitted photon ¯hk
Trec = Erec/kB = (¯hk)2
2mkB
with m being the mass of the atom. For 87 Rb this recoil limit is 349 nK.
27
(2.35)