Methods of laser cooling involve the transfer of momentum from light to atoms or ions. The recoil associated with the emission or absorption of a single photon by an atom typically leads to a velocity change of the order of a few cm/s. These discrete velocity changes result in a limit to the reachable temperature of a cloud of atoms or ions in an optical trap.
The recoil limit can be defined as 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). However, 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. This shows that the recoil limit is not a fundamental limit which applies to all laser cooling methods.
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