# Optical molasses

Optical molasses schematic

Optical molasses is a laser cooling technique that can cool neutral atoms to temperatures lower than a magneto-optical trap (MOT). An optical molasses consists of 3 pairs of counter-propagating circularly polarized laser beams intersecting in the region where the atoms are present. The main difference between optical molasses and a MOT is the absence of magnetic field in the former. While a typical Sodium MOT can cool atoms down to 300μK, optical molasses can cool the atoms down to 40μK, an order of magnitude colder.

## History

When laser cooling was proposed in 1975, a theoretical limit on the lowest possible temperature was predicted.[1] Known as the Doppler limit, ${\displaystyle T_{d}=\hbar \Gamma /{2k_{b}}}$, this was given by the lowest possible temperature attainable considering the cooling of two-level atoms by Doppler cooling and the heating of atoms due to momentum diffusion from the scattering of laser photons. Here, ${\displaystyle \Gamma }$, is the natural line-width of the atomic transition, ${\displaystyle \hbar }$, is the reduced Planck's constant and, ${\displaystyle k_{b}}$, is Boltzmann's constant.

Experiments at the National Institute of Standards and Technology, Gaithersburg, found the temperature of cooled atoms to be well below the theoretical limit.[2] Initially, it was a surprise to theorists, until the full explanation came out.

## Theory

The best explanation of the phenomenon of optical molasses is based on the principle of polarization gradient cooling.[3] Counterpropagating beams of circularly polarized light cause a standing wave, where the light polarization is linear but the direction rotates along the direction of the beams at a very fast rate. Atoms moving in the spatially varying linear polarisation have a higher probability density of being in a state that is more susceptible to absorption of light from the beam coming head-on, rather than the beam from behind. This results in a velocity dependent damping force that is able to reduce the velocity of a cloud of atoms to near the recoil limit.

## References

1. ^ Hänsch, T.W.; Schawlow, A.L. (1975). "Cooling of gases by laser radiation". Optics Communications. 13 (1): 68–69. ISSN 0030-4018. doi:10.1016/0030-4018(75)90159-5.
2. ^ Lett, Paul D.; Watts, Richard N.; Westbrook, Christoph I.; Phillips, William D.; Gould, Phillip L.; Metcalf, Harold J. (1988). "Observation of Atoms Laser Cooled below the Doppler Limit". Physical Review Letters. 61 (2): 169–172. ISSN 0031-9007. PMID 10039050. doi:10.1103/PhysRevLett.61.169.
3. ^ Dalibard, J.; Cohen-Tannoudji, C. (November 1989). "Laser cooling below the Doppler limit by polarization gradients: simple theoretical models". JOSA B. 6 (11): 2023–2045. doi:10.1364/JOSAB.6.002023. We present two cooling mechanisms that lead to temperatures well below the Doppler limit. These mechanisms are based on laser polarization gradients and work at low laser power when the optical-pumping time between different ground-state sublevels becomes long.