Observation of sub-doppler cooling via swept adiabatic transfer in Rb

Norcia et al (arXiv:1707.01944) recently demonstrated a novel cooling mechanism on narrow-linewidth optical transitions~in Sr. Counter-propagating laser beams are swept in frequency in a sawtooth manner to cause adiabatic Landau-Zener~transfers between an atom's ground state and excited state. ~Doppler shifts impose a time-ordering of the interactions with each laser beam such that the associated~photon recoils oppose the atom's motion, leading to cooling. Here we expand the idea to artificial two-photon transitions in Rb.~We are able to cool atoms from 42 uK to 6 uK over several sweeps, and we achieve a moderate amount of phase space~density compression. Because the dynamics of the adiabatic transfer are crucial for understanding the cooling rate, we~developed a generic model for assessing the feasibility of two-photon Landau-Zener transitions in the presence of free~space scattering, and propose a dimensionless quality factor~to quantify the maximum efficiency of the process. Both~the experiment and theoretical modeling may find potential applications in cooling molecules or other systems that lack~high-quality cycling transitions.