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

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 Gd-155 and Gd-157 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 x 10(7) spins and the measurement of phase memory loss rate and spin-photon coupling strength.