I'm a second year physics undergraduate and am currently taking part in a summer studentship programme where I spend a few weeks working in the cold atoms lab at my university. I've been looking into sub-doppler cooling, and one text in particular keeps referring to 'light-shifted ground states' or 'light shifted energies', but does not explain what this means. 'Light shift' also seems to be used as a noun at times. I was hoping someone might be able to explain to me what this means, as I can't find an explanation online anywhere. Thanks very much!