Just chillin' – molecules that steady quantum computers: "If the initial photons all arrive from a specific direction – along a laser beam – but the emitted photons leave the atom in random directions, the net result is that the atom travels more slowly towards the laser. Atoms or molecules travel more sluggishly at low temperatures, and so because the atom moves more slowly towards the laser, it is lower in temperature in this direction.
To approach absolute zero requires several thousand cycles of photon absorption and emission. There's a risk that during one cycle the atom might fall into a 'dark state' that does not absorb the photons in the narrow frequency range emitted by the laser. It is for this reason that molecular laser cooling is difficult, says David DeMille of Yale University. Atoms have relatively simple sets of energy states making it easy to avoid a dark state. But molecules have more subtle energy states – the bonds between their atoms can stretch and squash into a range of vibrational and rotational states, some of which will be dark."