Longitudinal coupling between a molecular spin ensemble and a superconducting resonator

Hybrid quantum architectures coupling electronic spin ensembles to superconducting circuits have advanced remarkably in the past decade. So far, however, they rely on transverse coupling schemes, tuning the spin transition in close proximity to the circuit’s frequency. Here, we demonstrate longitudinal coupling between spins in a microcrystal of Cr7Ni and the kinetic inductance of a granular aluminum superconducting microwave resonator. Remarkably, this enables the measurement of the crystal’s magnetization independent of the detuning between the spin and resonator mode. Separate resonators fabricated from niobium on the same chip allow us to excite and measure the relaxation of the spins more than 2 GHz detuned from the readout resonator.