Polarization of P1 electron spin bath via repetitive Hartmann-Hahn transfer

Diamond hosts a hybrid defect spin network of nitrogen-vacancy (NV) centers coupled to a denser substitutional nitrogen (P1) bath. While NV centers are well-established quantum sensors, initializing the P1 spin ensemble is essential for harnessing it as a resource in quantum technologies. Here, we implement repeated polarization transfer from an optically initialized NV ensemble to the P1 ensemble. The accumulated P1 polarization, measured via subsequent NV spin-locking, saturates with an increasing number of transfer cycles. The saturation level also depends on the Rabi frequency, consistent with modulation of the effective disorder. Finally, we employ this method to initialize, coherently control, and characterize the relaxation dynamics of the polarized P1 ensemble. Our approach enables precise probing and control of P1 bath polarization in the NV-P1 spin network system, advancing its utility as a controllable quantum resource.