  We report on hybrid circuit QED experiments with focused ion beam implanted
Er$^{3+}$ ions in Y$_2$SiO$_5$ coupled to an array of superconducting lumped
element microwave resonators. The Y$_2$SiO$_5$ crystal is divided into several
areas with distinct erbium doping concentrations, each coupled to a separate
resonator. The coupling strength is varied from 5 MHz to 18.7 MHz, while the
linewidth ranges between 50 MHz and 130 MHz. We confirm the paramagnetic
properties of the implanted spin ensemble by evaluating the temperature
dependence of the coupling. The efficiency of the implantation process is
analyzed and the results are compared to a bulk doped Er:Y$_2$SiO$_5$ sample.
We demonstrate the successful integration of these engineered erbium spin
ensembles with superconducting circuits.
