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This is something I've been wondering for a while. I've read about the experiment with NASA to reach unprecedented cold temperatures in space because of the microgravity environment not being such a problem as supposed to how it is on Earth. On the other hand, I've read that gravity can be compensated for in a laser cooling scheme with magnetism. Is gravity the only limitation to that at this point to achieve temperatures in the picokelvin range and lower, or is it factors related to the uncertainty principle? Is gravity a limitation to any laser cooling setup?

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    $\begingroup$ A link would be nice... $\endgroup$ – Rococo Apr 28 '16 at 14:17
  • $\begingroup$ Microgravity could be useful for doing interference experiments with already cooled atoms. Perhaps this is what you are talking about? $\endgroup$ – Asaf Apr 28 '16 at 14:20
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The temperature limit for laser cooling is not related to gravity but to the always-present momentum kick during absoprtion/emission of photons.

Ultracold atom experiments typically use laser cooling at an initial stage and afterwards evaporative cooling is used to reach the lowest temperatures. In evaporative cooling the most energetic atoms are discarded to decrease the temperature so one trades atoms for lower temperature. The practical limit of evaporative cooling is the initial number of atoms.

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Gravity fluctuations will always cause vibrations in atoms and molecules limiting the lowest temperature obtainable. Closer to the mass source, the stronger the gravity field. As stated by Asaf earlier, evaporative cooling will lower the temperature only so far. Adding a magnetic field may temporarily increase temperature by increasing vibrations in the sample, therefore discharging additional lower bound/ joined photons, but that process may ultimately allow a lower temperature to be obtained.

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