Modulation of the Rabi Frequency of a Solid State Qubit by a Proximal Soft Ferromagnet*

We report the local modulation of microwave-driven Rabi oscillations of an NV center in a diamond scanning tip depending on its location relative to a proximal soft magnetic nanowire. We found up to a 2.34x increase in the NV center's Rabi frequency at 83 nm NV-sample separation, occurring over the entire 20 dBm power range tested. These results are supported by additional micromagnetic simulations, which suggest that the local alteration of the applied magnetic field is the result of the nanowire's stray field oscillating at the applied frequency either in or out of phase with the applied field. This results in constructive or destructive interference depending on the relative position of the qubit and the orientation of the applied field, leading to the dual enhancement and suppression effects we see. These measurements provide a basis for external-field free, wide bandwidth modulation of microwave fields, which could see use in many applications, including quantum computing, quantum sensing, spintronics, and the engineering of nanoscale inductors.