Coherent dipole-dipole coupling between two single Rydberg atoms at an electrically-tuned Forster resonance

Resonant energy transfers, the non-radiative redistribution of an electronic excitation between two particles coupled by the dipole-dipole interaction, lie at the heart of a variety of phenomena(1), notably photosynthesis. In 1948, Forster established the theory of fluorescence resonant energy transfer (FRET) between broadband, nearly-resonant donors and acceptors(2). The 1/R-6 scaling of the energy transfer rate, where R is the distance between particles, enabled widespread use of FRET as a 'spectroscopic ruler' for determining nanometric distances in biomolecules(3). The underlying mechanism is a coherent dipolar coupling between particles, as recognized in the early days of quantum mechanics(4), but this coherence has not been directly observed so far. Here we study, spectroscopically and in the time domain, the coherent, dipolar-induced exchange of excitations between two Rydberg atoms separated by up to 15 mu m, and brought into resonance by applying an electric field. Coherent oscillation of the system between two degenerate pair states then occurs at a frequency scaling as 1/R-3, the hallmark of resonant dipole-dipole interactions(5). Our results not only demonstrate, at the fundamental level of two atoms, the basic mechanism underlying FRET, but also open exciting prospects for active tuning of strong, coherent interactions in quantum many-body systems.