Coupling of a locally implanted rare-earth ion ensemble to a   superconducting micro-resonator
  We demonstrate the coupling of rare-earth ions locally implanted in a substrate (Gd$^{3+}$ in Al$_{2}$O$_{3}$) to a superconducting NbN lumped-element micro-resonator. The hybrid device is fabricated by a controlled ion implantation of rare-earth ions in well-defined micron-sized areas, aligned to lithographically defined micro-resonators. The technique does not degrade the internal quality factor of the resonators which remain above $10^{5}$. Using microwave absorption spectroscopy we observe electron-spin resonances in good agreement with numerical modelling and extract corresponding coupling rates of the order of $1$ MHz and spin linewidths of $50 - 65$ MHz. 
I.Wisby, S. E. de Graaf, R. Gwilliam, A. Adamyan, S. Kubatkin, P. J.   Meeson, A. Ya. Tzalenchuk and T. Lindstr\"om
cond-mat.mes-hall cond-mat.supr-con quant-ph