Quantum trajectory pictures of laser cooling

Abstract

We have applied the method of single atom trajectories to study the mechanism behind some cooling schemes in laser cooling. In several cases we recognize the cooling mechanism as being due to a "Sisyphus" process, where the atoms move in a spatially varying light shift potential and are optically pumped towards the most light shifted states. In other cases we identify a "Sisyphus" process in time, where the light shift is constant and the force on the atom alternates between positive and negative. This process is interrupted by quantum jumps at random instants and in each case we depict the mechanism leading to a cooling force on the atom. In the special case of sub-Doppler laser cooling in a strong magnetic field we obtain 12 jump operators and identify the jump operators responsible for the cooling. The versatility of the single atom trajectory method allows it to be applied to any cooling process and is therefore a very valuable tool in unraveling the physical mechanisms behind cooling processes.