Sub Doppler Cooling

Sub-Doppler Cooling

Sub-Doppler laser cooling was experimentally discovered [1] and was first explained in terms of optical pumping among the ground states of a multi-level atom [2]. It could be observed only when these sublevels had different light shifts. In those first experiments, both the optical pumping and the different light shifts arose from polarization gradients that were unavoidable in a light field that had at least three non-coplanar k-vectors (3-D).

Such polarization gradients can also be produced in 1-D or 2-D, and in preliminary experiments, sub-Doppler laser cooling was accidentally discovered even in their absence [3]. Optical pumping is still required because this is the entropic or irreversible process needed to compress the phase-space volume of a sample of atoms. But in the absence of a polarization gradient, it is mediated by a small magnetic field along an axis different from the quantization axis defined by the pure and uniform polarization of the light field.

In either case, the energy description fits the criteria for the name Sisyphus cooling because atoms climb more ``light shift hills" than they descend. the combination of various light and magnetic fields results in in an optical molasses that mixes the atomic states, and so it is simply another form of ``confused atom molasses".

[1] P. Lett et al., Phys. Rev. Lett. 61, 169 (1988).
[2] C. Cohen-Tannoudji and W. Phillips, Physics Today, 43, 33, October 1990.
[3] B. Sheehy et al., Phys. Rev. Lett. 64, 858 (1990).