Detecting electric-field-tuned atom–atom level mixing in a cold 85Rb Rydberg gas by two-unequal-frequency photons

We use two-unequal-frequency microwave photons as sensitive probes to study the nsns to the np(n + m - 1)p Rydberg pair state transition in a cold 85Rb gas, where n and m are integers. The principal quantum number n dependence of the energy differences between ns(n + 1)s and np3/2np3/2 as well as ns(n + 2)s and np1/2(n + 1)p1/2 has been measured and the results are consistent with the calculated values. Additionally, we demonstrate that the pair state Stark shift is the sum of the Stark shifts of both isolated Rydberg atoms. Finally, we tune the dipole--dipole mixing probability by applying an electric field to tune the energy difference between the dipole--dipole coupled states, ns(n + 2)s and np1/2(n + 1)p1/2. The results reported here are essential steps towards electric-field-controlled chemical reactions [1], quantum gates, molecular autoionization and few-body correlations.