  The properties of rare-earth elements diluted in non-magnetic crystals make
them a promising candidate for a quantum memory due to their limited Hilbert
space. The control and readout of the qubit states require a highly sensitive
measurement and large coupling of the spin ensemble with the electro-magnetic
mode of a superconducting resonator. We report sensitive detection of forbidden
transitions of electronuclear states from the minority species of $^{155}$Gd
and $^{157}$Gd isotopes which opens the possibility of connecting quantum
states with very different spin projections. Cavity perturbation measurements
seen in the reflected signal allows the detection of about 7.6$\times10^{7}$
spins and the measurement of spin decoherence rate and spin-photon coupling
strength.
