Near-Unity Charge State Initialization of NV Centers in Diamond via Multi-Color Excitation

Nitrogen-vacancy (NV) centers in diamond have long spin coherence times and optical readout in ambient conditions, making them excellent quantum sensors. However, NV centers have significant state preparation error owing to charge dynamics under off-resonant excitation. Specifically, charge cycling under green illumination results in state preparation and measurement errors reaching up to 30%. One method for reducing this error is to use multicolor excitation to change the relative ionization and recombination rates [1]. However, previous demonstrations of this technique for bulk NV centers required relatively high excitation powers over a long period of time. In this work, through carefully calibrated excitation, we show that this multi-color initialization approach is also efficient for surface NV centers (5-15 nm deep) and that the relevant photophysics is preserved when the state-of-art diamond surface preparation is used [2]. From a meticulous examination of the experimental parameters, we identify a region within the parameter space where simultaneous near-infrared (905 nm) and green (520 nm) illumination of moderate powers can be used to achieve near-unity NV charge initialization within 10 μs while retaining its spin polarization. This work opens the door to high-fidelity initialization in multiplexed measurements and may enable quantifying many-point correlation functions.<br/><br/>[1] D. A. Hopper, R. R. Grote, A. L. Exarhos, and L. C. Bassett, Physical Review B 94, 241201 (2016)<br/>[2] S. Sangtawesin, B. L. Dwyer, S. Srinivasan, J. J. Allred, L. V. Rodgers, K. De Greve, A. Stacey, N. Dontschuk, K. M. O’Donnell, D. Hu, et al., Physical Review X 9, 031052 (2019)