  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 $3\lambda/4$ and the creation of super- and sub-radiant
states at a separation of one photon wavelength $\lambda$. This system is
highly suitable for exploring collective atom/photon interactions and
applications in quantum communication technology.
