Optical molasses

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 constant and, ${\displaystyle k_{b}}$, is the Boltzmann 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.

In 1985, Chu et al. directed a thermal beam of sodium atoms through an optical molasses region formed by counter-propagating light from a frequency-doubled Neodymium:YAlG laser. They measured the average temperature by quickly shutting off the beams and measuring the fluorescence of the released atoms. They measured an average temperature of ${\textstyle 240_{-60}^{+200}\mu K}$, near the Doppler cooling limit of sodium.[3]

The best explanation of the phenomenon of optical molasses is based on the principle of polarization gradient cooling.[4] 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.

• Gray molasses