Electron spins are ideal two-level systems that may couple coherently with microwave photons and, under specific conditions, hybrid spin-photon states can be created. This represents a fundamental step for the transfer and the manipulation of quantum information.
We have first developed and optimized high critical temperature YBCO/sapphire superconducting coplanar resonators to investigate the coherent coupling between microwave photons and ensembles of molecular spin systems in a broad range of temperature and magnetic field . With these planar resonators we have systematically investigated different molecular spin centers, identifying the optimal parameters to couple molecular spins with microwave photons . By exploiting exchange narrowing of ESR lines of organic radicals, we achieved the strong coupling regime up to 50 K and cooperativity values up to 4300 at 2 K. Playing with the slightly different g-factors of DPPH and PyBTM radicals, we were also able to coherently couple distinct/remote spin ensembles and observe multiple anticrossing in spectroscopic experiments .
Our latest results show that it is possible to reach high cooperativity regime using diluted VOPc spin qubits  with cooperativity values at 0.5 K comparable to what reported for NV centers at lower temperature. Transmission spectroscopy measurements performed in applied magnetic field clearly show Rabi splitting of the frequency spectra taken at resonance. Overall, these results demonstrate that molecular spin centers can be successfully integrated in hybrid quantum devices.
 A. Ghirri, C. Bonizzoni, D. Gerace, S. Sanna, A. Cassinese, and M. Affronte, Appl. Phys. Lett. 106 (2015) 184101
 C. Bonizzoni, A. Ghirri, K. Bader, J. van Slageren, M. Perfetti, L. Sorace, Y. Lan, O. Fuhr, M. Ruben, and M. Affronte, Dalton Trans. 45 (2016) 16596
 A. Ghirri, C. Bonizzoni, F. Troiani,N. Buccheri, L. Beverina, A. Cassinese, and M. Affronte, Phys. Rev. A 93 (2016) 063855
 C. Bonizzoni, A. Ghirri, M. Atzori, L. Sorace, R. Sessoli, M. Affronte, submitted (2017)