Excitonlike exchange in two-photon transitions of pairs of cold Rb Rydberg atoms

We have observed an excitonlike exchange in two-photon microwave transitions between pairs of cold Rb Rydberg atoms, specifically, transitions in which a ns(1/2)n(1/2) pair undergoes the transition to the np(1/2)nP(3/2) and n(3/2) np(1/2) states. This transition occurs due to the excitonlike ns(1/2) np(j) <-> np(j)ns(1/2) exchange in the intermediate states, and the process can be thought of as a Forster resonant energy transfer between Floquet, or dressed, states. In addition, the measurements provide clear evidence of the importance of the three-dimensional nature of the dipole-dipole interaction.