Controlling the Inhomogeneous Broadening and Impedance Matching of a Spin Ensemble

We control the spectral distribution of a spin ensemble by applying a magnetic field gradient using an anti-Helmholtz coil inside a dilution refrigerator, and demonstrate impedance matching between the ensemble and a transmission line, achieving -50 dB absorption of incident radiation. This represents a first step toward a spin-ensemble-based quantum memory for itinerant microwave photons. We further model the spectral distribution under the applied gradient to predict the spin-resonator response, and use the device to systematically tune the weak-to-strong coupling transition in both continuous-wave and time-domain pulsed measurements.