Field and temperature dependent cavity coupling for highly sensitive on-chip spin detection.*

Probing spin-photon interaction in single molecule magnets using Electron Spin Resonance is of great interest due to possible application as a molecular quantum memory\footnote{M. Blencowe,\textbf{Nature} 468, 2010}. Here we will present a method to tune the coupling of a Nb superconducting cavity\footnote{N. Groll et al, \textbf{PRB} 81, 2010} operating at $\sim$20GHz using losses induced by temperature and field. The effects are studied on a cavity empty first empty, then loaded with the molecular magnet V$_{15}$. This system has shown Rabi oscillations as well as spin-orbit dependence of the coherence time\footnote{M. Martens et al, arxiv:1505.03177}. From the zero-field temperature dependence of the resonance frequency of the empty cavity we have observed that thermally induced losses have the effect of decreasing the resonance frequency, while reaching critical coupling at a well-defined temperature. Loading the cavity shifts the critical coupling parameters which are tunable by a magnetic field in plane and/or perpendicular to the cavity. V$_{15}$ spectroscopy at critical coupling will be presented.