Recoil Limit
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: a lower limit for the temperature which can be reached with some laser cooling techniques
DOI: 10.61835/1af Cite the article: BibTex plain textHTML Link to this page LinkedIn
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
$${T_{{\rm{recoil}}}} = \frac{{{{\left( {h/\lambda } \right)}^2}}}{{2\;m\;{k_{\rm{B}}}}}$$where <$\lambda$> 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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Bibliography
[1] | A. Aspect et al., “Laser cooling below the one-photon recoil energy by velocity-selective coherent population trapping”, Phys. Rev. Lett. 61 (7), 826 (1988); https://doi.org/10.1103/PhysRevLett.61.826 |
[2] | M. Kasevich and S. Chu, “Laser cooling below a photon recoil with three-level atoms”, Phys. Rev. Lett. 69 (12), 1741 (1992); https://doi.org/10.1103/PhysRevLett.69.1741 |
[3] | 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); https://doi.org/10.1103/PhysRevLett.82.1116 |
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2020-10-25
Does photon recoil means that a particle emits a photon?
The author's answer:
Not necessarily. A recoil also occurs when a photon is absorbed or scattered.