High-efficiency, high-fidelity charge initialization of shallow nitrogen-vacancy centers in diamond

Nitrogen-vacancy (N-V) 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 N-V centers by using a few milliseconds of near-infrared (NIR) (5-mW) and green (10-μW) excitation. The translation of such schemes to N-V centers near the diamond surface with higher-efficiency optical pumping would enable new applications in nanoscale sensing. Here, we demonstrate a protocol for efficient charge initialization of shallow N-V centers between 5 nm and 15 nm from the diamond surface. By carefully studying the charge dynamics of shallow N-V centers, we identify a region of parameter space that allows for near-unity (95%) charge initialization within 300μs of NIR (905-nm, 1-mW) and green (520-nm, 10-μW) excitation. The time to 90% charge initialization can be as fast as 10μs for 4 mW of NIR and 39μW of green illumination. This fast, efficient charge initialization protocol will especially benefit nanoscale sensing applications in which state-preparation errors currently prohibit scaling, such as measuring higher-order multipoint correlators.