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 the reduced 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 (Lett et al, 1988). 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.

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)
• Lett, P. D., R. N. Watts, C. I. Westbrook, W. D. Phillips, P. L. Gould, and H. J. Metcalf, 1988, ‘‘Observation of atoms laser cooled below the Doppler limit,’’ Phys. Rev. Lett. 61, 169.