Why can't Doppler cooling reach zero velocity despite random recoil being a random walk?

[Niles]

Hi

I keep reading that an atom cannot be cooled to v=0 because of random recoil from spontaneous emission, i.e. a random walk in momentum space.

If it is a random walk, then - on average - we don't have any net motion, i.e. we stay at the initial position. This also holds in momentum space. But if we stay at the same position (on average), then why aren't we able to go to v=0?


Niles.

 

[zhermes]

The average velocity can become arbitrarily close to 0, but the instantaneous velocity will not... for the above reasons. That's taking poisson noise into account, but there are also the quantum effects which prevent it.