# Optical molasses

Optical Molasses Schematic

Optical molasses is a laser cooling technique that can cool down neutral atoms to temperatures colder 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. Known as the Doppler Limit, $T_d= \hbar \Gamma / {2 k_b}$, 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, $\Gamma$, is the natural line-width of the atomic transition, $\hbar$, is Planck's constant and, $k_b$, 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. 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. Counterpropagating beams of circularly polarized light cause a standing wave, where the light polarization depends on the spatial location. The AC Stark Shift of atoms in different magnetic sub-levels is also spatially dependent. The basic idea is that atoms moving with a velocity climb a polarization gradient hill, thereby losing their velocity. At the top of the hill, atoms are resonant with the other molasses beams, absorb a photon and decay into a lower energy magnetic sub-level, thereby having shed some of their velocity.[citation needed]

## References

• Cooling of gases by laser radiation, T.W. Hänsch, and A.L. Schawlow, Optics Communications, 13, 68 (1975).
• Laser cooling below the Doppler limit by polarization gradients: simple theoretical models, Jean Dalibard and Claude Cohen-Tannoudji JOSA B, Vol. 6, Issue 11, pp. 2023- (1989)