  We present cavity QED experiments with an Er:YSO crystal magnetically coupled
to a 3D cylindrical sapphire loaded copper resonator. Such waveguide cavities
are promising for the realization of a superconducting quantum processor. Here,
we demonstrate the coherent integration of a rare-earth spin ensemble with the
3D architecture. The collective coupling strength of the Er$^{3+}$ spins to the
3D cavity is 21 MHz. The cylindrical sapphire loaded resonator allowed us to
explore the anisotropic collective coupling between the rare-earth doped
crystal and the cavity. This work shows the potential of spin doped solids in
3D quantum circuits for application as microwave quantum memories as well as
for prospective microwave to optical interfaces.
