Optical Spin Readout of Nuclear Spins Beyond the NV Electron T₁

We investigate optical readout of nitrogen nuclear spin states coupled to dephased electron spins by optical re-pumping. The nitrogen nuclear spin intrinsic to each diamond nitrogen-vacancy (NV) center has been demonstrated as a basis for rotation sensing. The sensitivity is partially determined by the coherence time of the nuclear spins, which is limited by the NV center electron spin T1. To improve sensing protocols, decoupling electron spins from nuclear spins will allow nuclear spin coherence to persist past the electron T1. This introduces a challenge for future sensors as current protocols use the conditional rotation of electron spins to optically readout the nuclear spins, which cannot work if the electron spins have depolarized. To address this, we investigate the efficiency of repumping thermalized NV center electron spins without disturbing nuclear spins. We measure at fields near 500 G, where spin flip-flops between the nuclear and electron spins in the NV excited state are a significant source of nuclear spin dephasing. We find that while the spin flip-flops harm the nuclear spin fidelity, this sensitivity loss can be compensated by optical contrast enhancement between nuclear spin states caused by the same flip-flops.