Long-range interactions between cold Rydberg atoms, which are used in many important applications, can be enhanced using Forster resonances between collective many-body states controlled by an external electric field. Here we report on the first experimental observation of highly-resolved radio-frequency-assisted Forster resonances in a few cold Rb Rydberg atoms. We also observed radio-frequency-induced Forster resonances which cannot be tuned by a dc electric field. They imply an efficient transition from van der Waals to resonant dipole-dipole interaction due to Floquet sidebands of Rydberg levels appearing in the rf-field. This method can be applied to enhance the interactions of almost arbitrary Rydberg atoms with large principal quantum numbers.
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