Optical molasses

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.

When laser cooling was proposed in 1975, a theoretical limit on the lowest possible temperature was predicted. 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.

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