Tailoring Rydberg interactions via F\"orster resonances: state   combinations, hopping and angular dependence
  F\"orster resonances provide a highly flexible tool to tune both the strength and the angular shape of interactions between two Rydberg atoms. We give a detailed explanation about how F\"orster resonances can be found by searching through a large range of possible quantum number combinations. We apply our search method to $SS$, $SD$ and $DD$ pair states of $^{87}$Rb with principal quantum numbers from 30 to 100, taking into account the fine structure splitting of the Rydberg states. We find various strong resonances between atoms with a large difference in principal quantum numbers. We quantify the strength of these resonances by introducing a figure of merit $\tilde C_3$ which is independent of the magnetic quantum numbers and geometry to classify the resonances by interaction strength. We further predict to what extent excitation exchange is possible on different resonances and point out limitations of the coherent hopping process. Finally, we discuss the angular dependence of the dipole-dipole interaction and its tunability near resonances. 
Asaf Paris-Mandoki, Hannes Gorniaczyk, Christoph Tresp, Ivan   Mirgorodskiy, Sebastian Hofferberth
physics.atom-ph