Probing Coherence in a Cold Rydberg Gas

We have used pulsed electric-field sequences to probe the coherence of dipole-dipole interactions in a MOT. Nanosecond dye lasers excite Rb atoms to the $\vert $25s$_{1/2}>$ and $\vert $33s$_{1/2}>$ states in an electric field. The field tunes the atoms so that the energy difference between $\vert $25s$_{1/2}>\vert $33s$_{1/2}>$ and $\vert $24p$_{1/2}>\vert $34p$_{3/2}>$ atom pairs is $\Delta $. For typical separations, the dipole-dipole coupling between atoms enables coherent population transfer from $\vert $25s$_{1/2}>\vert $33s$_{1/2}>$ to $\vert $24p$_{1/2}>\vert $34p$_{3/2}>$ at a rate comparable to $\Delta $.After a time T, an electric field step diabatically tunes the pair energy splitting to -$\Delta $. The atoms interact for an additional time T and the population in the $\vert $24p$_{1/2}>\vert $34p$_{3/2}>$ state is measured. The pulse sequence enhances the $\vert $24p$_{1/2}>\vert $34p$_{3/2}>$ population relative to that obtained in a time 2T at fixed detuning, +/-$\Delta $. Sequences involving multiple resonance traversals lead to further enhancement, indicating that this effect is due to the coherence of the dipole-dipole interaction rather than excitation of different sets of atoms in the ensemble. The enhancement decays as T approaches several microseconds. On this time scale, atomic motion may play a substantive role. This work is supported by NSF and AFOSR.