  Electron spins hold great promise for quantum computation due to their long
coherence times. An approach to realize interactions between distant
spin-qubits is to use photons as carriers of quantum information. We
demonstrate strong coupling between single microwave photons in a NbTiN high
impedance cavity and a three-electron spin-qubit in a GaAs triple quantum dot.
We resolve the vacuum Rabi mode splitting with a coupling strength of
$g/2\pi\simeq31$ MHz and a qubit decoherence of $\gamma_2/2\pi\simeq 20$ MHz.
We can tune the decoherence electrostatically and obtain a minimal
$\gamma_2/2\pi\simeq 10$ MHz for $g/2\pi\simeq 23$ MHz. The dependence of the
qubit-photon coupling strength on the tunable electric dipole moment of the
qubit is measured directly using the ac Stark effect. Our demonstration of
strong spin-photon interaction is an important step towards coherent
long-distance coupling of spin-qubits.
