By laser cooling methods beyond the two-level approximations of atoms, temperature below this limit can be achieved.
Optical pumping between the sublevels that make up an atomic state introduces a new mechanism for achieving ultra-low temperatures.
The essential feature of sub-Doppler cooling is the non-adiabaticity of the moving atoms to the light field.
[2] Various methods have been used independently or combined in an experimental sequence to achieve sub-Doppler cooling.
A specific mechanism within polarization gradient cooling is Sisyphus cooling, where atoms climb "potential hills" created by the interaction of their internal energy states with spatially varying light fields.
For example, an optical molasses time-of-flight technique was used to cool sodium (Doppler limit
In order to solve this problem, the other re-pumping light would be incident on the system to repopulate the atoms to restart the Doppler cooling process.
[4] The Doppler cooling limit is set by balancing the heating from the momentum kicks.
Thus at low velocities, spontaneous emission would leave the atom with a residual momentum around
cannot be well described by the Fokker Planck equation, and this sets an intuitive lower limit on the temperature.
[2] Furthermore, polarization gradient cooling depends on the ability to localize atoms to a scale of
Due to the uncertainty principle, this localization also imposes a minimum momentum spread
These theories are tested in the analytical and numerical calculations in [5] with a one-dimensional polarization gradient molasses.
It was shown that in the limit of large detuning, the velocity distribution depends only on a dimensionless parameter, the light shift of the ground state divided by the recoil energy.