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Photon-mediated interactions between distant artificial atoms

Photon-mediated interactions between atoms are of fundamental importance in quantum optics, quantum simulations and quantum information processing. The exchange of real and virtual photons between atoms gives rise to non-trivial interactions the strength of which decreases rapidly with distance in three dimensions. Here we study much stronger photon mediated interactions using two superconducting qubits in an open onedimensional transmission line. Making use of the unique possibility to tune these qubits by more than a quarter of their transition frequency we observe both coherent exchange interactions at an effective separation of $3λ/4$ and the creation of super- and sub-radiant states at a separation of one photon wavelength $λ$. This system is highly suitable for exploring collective atom/photon interactions and applications in quantum communication technology.