Density and T1 of surface and bulk spins in diamond in high magnetic field gradients

Superconducting resonators and solid-state qubits such as N-$V$ centers in diamond suffer from intrinsic dissipation, which limits their coherence times. This is typically attributed to the presence of paramagnetic impurities acting as fluctuating two-level systems. In this study of surface and bulk spins in diamond, ultrasensitive magnetic force microscopy at millikelvin temperatures reveals that a high magnetic field gradient suppresses spin diffusion, enhancing relaxation times of surface spins. The technique offers a valuable tool for characterizing dilute spin systems, which could yield insight on how to reduce dissipation in qubits and other nanodevices.