General Overview
There are three states in this picture, a dark state, a bright state and an excited state. These ground states are actually super positions of the actual states. We apply blue detuned light so that the bright state lies higher in energy than the dark state. The process happens as follows
- The atom is in a dark state
- It couples to the bright state via non adiabatic coupling
- Once in the bright state the atom can transition to the excited state and this is most likely to happen at the top of a hill
- After a small time in the excited state it decays and most likely enters the dark state
This is a nice illustration of the process. Notice the blue detuning is important, if it were red detuned the bright state would lie below the dark state. Then the transition from dark state to bright state would happen at a maxima and would be pumped to a valley causing heating.
We also need to be careful because there are two types of “Gray Molasses” one which is just gray molasses and the other which is -Enhanced gray molasses. In the first blue detuned light is used still and we still have a dark state but
References
[Sub Doppler Cooling J. Dalibard and C. Cohen-Tannoudji](Laser cooling below the Doppler limit by polarization gradients: simple theoretical models)