Saturation of wave-packet interferences: Direct observation of spin precession in potassium atoms

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Abstract
A sequence of two identical ultrashort laser pulses creates quantum interferences in an atom. In the weak-field regime, the observed beats between interferences arising from various transitions are not a conclusive signature of wave-packet creation. On the contrary, in the saturation regime, the appearance of slow modulation is. We report on the observation of such modulations in atomic potassium in this field regime. The results are explained within the bright-state–dark-state framework. The wave packet corresponds to the precession of the electronic spin and orbital angular momentum.