| Summary: | The Rydberg excitation blockade has been at the heart of an impressive array of recent achievements; however, state-mixing interactions can compromise its efficiency. When ultracold atoms are excited to Rydberg states near Förster resonance, up to ∼50% of atoms can be found in dipole coupled product states within tens of ns after excitation. There has been disagreement in the literature regarding the mechanism by which this mixing occurs. We use state-selective field ionization spectroscopy to measure, on a shot-by-shot basis, the distribution of Rydberg states populated during narrow-band laser excitation. Our method allows us to both determine the number of additional Rydberg excitations added by each mixing event, and to quantify the extent to which state mixing “breaks” the blockade. For excitation of ultracold rubidium atoms to nD_{5/2} states, we find that the mixing is consistent with a three-body process, except near exact Förster resonance.
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