High-Efficiency, High-Fidelity Charge Initialization of Shallow Nitrogen Vacancy Centers in Diamond

Nitrogen vacancy (NV) centers in diamond exhibit long spin coherence times, optical initialization, and optical spin readout under ambient conditions, making them excellent quantum sensors. However, the conventional scheme for charge state initialization based on off-resonant green excitation results in significant state preparation errors, typically around 30%. One method for improving charge state initialization fidelity is to use multicolor excitation, which has been demonstrated to achieve a near-unity preparation fidelity for bulk NV centers by using a few milliseconds of near-infrared (5 mW) and green (10 {\mu}W) excitation. The translation of such schemes to NV centers near the diamond surface with higher efficiency optical pumping would enable myriad tasks in nanoscale sensing. Here, we demonstrate a protocol for efficient charge initialization of shallow NV centers between 5 nm and 15 nm from the diamond surface. By carefully studying the charge dynamics of shallow NV centers, we identify a region of parameter space that allows for near-unity (95%) charge initialization within 300 {\mu}s of near-infrared (1 mW) and green (10 {\mu}W) excitation. The time to 90% charge initialization can be as fast as 10 {\mu}s for 4 mW of near-infrared and 39 {\mu}W of green illumination. This fast, efficient charge initialization protocol will enable nanoscale sensing applications where state preparation errors currently prohibit scaling, such as measuring higher-order multi-point correlators.