The recoil limit is the lowest temperature reachable with laser cooling methods which involve a permanent interaction of the cooled atoms with light. It is given by the equation
where λ is the wavelength of the light and m is the mass of the atoms. At this temperature, the thermal energy equals the energy of an atom with a momentum equal to the photon momentum. Typical values for the recoil limit of atoms are of the order of 1 μK.
The recoil limit can be approached (although not fully reached) with polarization gradient cooling (→ Sisyphus cooling). Temperatures below the recoil limit have been achieved with velocity-selective coherent population trapping, where atoms become trapped in an electronic state where they do no longer interact with light.
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|||H. Katori et al., “Magneto-optical trapping and cooling of strontium atoms down to the photon recoil temperature”, Phys. Rev. Lett. 82 (6), 1116 (1999)|
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