  Interfacing superconducting quantum processors, working in the GHz frequency
range, with optical quantum networks and atomic qubits is a challenging task
for the implementation of distributed quantum information processing as well as
for quantum communication. Using spin ensembles of rare earth ions provide an
excellent opportunity to bridge microwave and optical domains at the quantum
level. In this letter, we demonstrate magnetic coupling of Er$^{3+}$ spins
doped in Y$_{2}$SiO$_{5}$ crystal to a high-Q coplanar superconducting
resonator.
