  Plasmonic nanoparticles hold great promise as photon handling elements and as
channels for coherent transfer of energy and information in future all-optical
computing devices. Coherent energy oscillations between two spatially separated
plasmonic entities via a virtual middle state exemplify electron-based
population transfer, but their realization requires precise nanoscale
positioning of heterogeneous particles. Here, we show the assembly and optical
analysis of a triple particle system consisting of two gold nanoparticles with
an inter-spaced silver island. We observe strong plasmonic coupling between the
spatially separated gold particles mediated by the connecting silver particle
with almost no dissipation of energy. As the excitation energy of the silver
island exceeds that of the gold particles, only quasi-occupation of the silver
transfer channel is possible. We describe this effect both with exact classical
electrodynamic modeling and qualitative quantum-mechanical calculations. We
identify the formation of strong hot spots between all particles as the main
mechanism for the loss-less coupling and thus coherent ultra-fast energy
transfer between the remote partners. Our findings could prove useful for
quantum gate operations, but also for classical charge and information transfer
processes.
