On-Chip Detection of Electronuclear Transitions in the $^{155,157}$Gd Multilevel Spin System

International audience; The properties of rare-earth elements diluted in nonmagnetic crystals make them promising candidates 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 electromagnetic mode of a superconducting resonator. We report sensitive detection of forbidden transitions of electronuclear states from the minority species of 155Gd and 157Gd isotopes, which opens the possibility of connecting quantum states with very different spin projections. Cavity perturbation measurements seen in the reflected signal allow the detection of about 7.6×107 spins and the measurement of phase memory loss rate and spin-photon coupling strength.