Hybrid quantum circuits interfacing rare earth spin ensembles with microwave resonators are a
promising approach for application as coherent quantum memory and frequency converter. In this
thesis hybrid circuits based on Er and Nd ions doped into Y2SiO and YAlO crystals are
investigated by optical and on-chip microwave spectroscopy. Coherent strong coupling between
the microwave resonator and spin ensemble as well as a multimode memory for weak coherent
microwave pulses are demonstrated.
