  Electron spins and photons are complementary quantum-mechanical objects that
can be used to carry, manipulate and transform quantum information. To combine
these resources, it is desirable to achieve the coherent coupling of a single
spin to photons stored in a superconducting resonator. Using a circuit design
based on a nanoscale spin-valve, we coherently hybridize the individual spin
and charge states of a double quantum dot while preserving spin coherence. This
scheme allows us to achieve spin-photon coupling up to the MHz range at the
single spin level. The cooperativity is found to reach 2.3, and the spin
coherence time is about 60ns. We thereby demonstrate a mesoscopic device
suitable for non-destructive spin read-out and distant spin coupling.
