  Dynamic nuclear polarization (DNP) is an attractive method for initializing
nuclear spins that are strongly coupled to optically active electron spins
because it functions at room temperature and does not require strong magnetic
fields. In this Letter, we demonstrate that DNP, with near-unity polarization
efficiency, can be generally realized in weakly coupled hybrid registers, and
furthermore that the nuclear spin polarization can be completely reversed with
only sub-Gauss magnetic field variations. This mechanism offers new avenues for
DNP-based sensors and radio-frequency free control of nuclear qubits.
