Laser (Doppler) cooling

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xts

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In several papers, diagrams, etc. about atomic experiments I saw pairs of lasers beaming in opposite directions.

Naive question: why two lasers are used instead of one and a mirror?
Or even further: why six lasers are used (2 in each axis) instead of just one and bunch of mirrors?
 
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Andy Resnick

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I think standing waves (one laser) are used to make an optical lattice.

For cooling experiments, I think multiple sources are needed to either cool or to probe the cooled state. Since doppler shifting only occurs in the direction of motion (the atoms are usually from a beam), orthogonal illumination is not needed.

But I don't do these experiments, so I'm not that aware of the apparatus details.

Edit: I spoke too soon- looking through Metcalf and van der Straten's "Laser cooling and trapping", multiple beam setups are often used (optical molasses), and furthermore, by adjusting the polarization one can apparently cool below the Doppler limit.
 
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Redbelly98

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Mirrors are used in place of multiple lasers. (Though maybe not for the probe beam, if there is one and it has a different wavelength than the main cooling beams.)
 

xts

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Mirrors are used in place of multiple lasers.
Thanks!
As I read about several experiments, they mentioned dual lasers and I couldn't understand why - I suspected some fundamental reason behind it (coherence spoiling the cooling effect somehow), which I didn't understand.
So I see that if they use dual laser it is only due to practical issues (like ability to individually tune each beam, e.g. in order to deflect atoms)
 

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