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Need three ingredients to understand Doppler cooling:
1. Three elementary processes of atom-light interaction:
2. Momentum conservation
v typically a few cm/s
3. Doppler shift
Construct velocity-dependent dissipative force on atom with two red detuned laser beams:
Red detuned
absorption preferentially from laser beam opposing velocity (Doppler shift towards resonance)
slows atom down towards v=0 if de-excitation mainly due to isotropic spontaneous emission (low intensity limit)
(=natural line width)
Force
energy dissipation
Ultimate limit for Doppler cooling
Atom cannot be cooled to v=0 because of random recoil from spontaneous emission (random walk in momentum space with increments )
heating rate associated with recoil
Landmark energy: Recoil energy
ER corresponds to temperature
(
for Na)
Note: Omitted discussion of so-called Sub-doppler cooling
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Birger Bergersen
1998-12-12