Talk:Optical molasses

This could stand to be improved a bit. In particular, this part:
 * 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. [bolding mine]

That isn't describing how optical molasses works, it's describing how Sisyphus cooling works. Sisyphus cooling is a way to use optical molasses to cool below the doppler limit. Basically, it's a way to get extra cooling out of your molasses. But there's a more basic level of cooling in optical molasses. You tune the lasers so the frequency is a little below the resonant frequency of the atoms. Then the atoms will only be resonant with the laser beam that they're moving towards the source of the beam, because there's a blue shift. (There's beams coming in from six different directions, so at least one of them will be blue shifted.) Because the laser beam is hitting the atom head-on and scattering at a random angle, on average it slows the atom down. (Of course you can still have the other laser beams scatter off the atom, but because they're further from resonance they don't have as much of an effect.) It's that effect that is the optical molasses. See this journal article:
 * S. Chu, L. Hollberg, J.E. Bjorkholm, A. Cable, A. Ashkin, Three-dimensional viscous confinement and cooling of atoms by resonance radiation pressure, Phys. Rev. Lett. 55 (1985) 48.

The discovery of subdoppler cooling and explanation based on the Sisyphus effect came later, see these articles:
 * P.D. Lett, R.N. Watts, C.I. Westbrook, W.D. Phillips, P.L. Gould, H.J. Metcalf, Observation of atoms laser cooled below the Doppler limit, Phys. Rev. Lett. 61 (1988) 169.
 * J. Dalibard, C. Cohen-Tannoudji, Laser cooling below the Doppler limit by polarization gradients: simple theoretical models, J. Opt. Soc. Am. B 6 (1989) 2023.
 * P.J. Ungar, D.S. Weiss, E. Riis, S. Chu, Optical molasses and multilevel atoms: theory, J. Opt. Soc. Am. B 6 (1989) 2058.

It should also probably be mentioned that optical molasses is really a kind of optical lattice, namely one that's tuned to the right frequency to produce the cooling effect described above. They called it molasses when they could slow the atoms but not enough to trap them in the individual potential wells, but with Sisyphus cooling they are able to cool the atoms so much that they fall into the potential wells and basically just oscillate around these fixed positions. (Of course once you have your atoms trapped you can always reduce the amplitude of your lattice to lower the barrier between wells and let them hop around again.)

Anyway, if someone wants to revise the article with these points in mind, feel free. If not, I'll try to get to it myself when I have a bit more free time. -- Tim314 (talk) 10:52, 9 January 2009 (UTC)