# Real-Time Charge Initialization of Diamond Nitrogen-Vacancy Centers for Enhanced Spin Readout

## TL;DR

This paper introduces a real-time control technique to deterministically initialize the charge state of diamond NV centers at room temperature, significantly improving spin readout fidelity and speed for quantum sensing applications.

## Contribution

The authors develop a real-time control method that achieves near-perfect charge initialization fidelity, enabling faster and more accurate NV center spin readout compared to traditional steady-state methods.

## Key findings

- Charge initialization fidelity reaches 99.4%
- Experiment speed increases by a factor of two
- Achieves a magnetic sensitivity of 1.3 nT/Hz^{1/2}

## Abstract

A common impediment to qubit performance is imperfect state initialization. In the case of the diamond nitrogen-vacancy (NV) center, the initialization fidelity is limited by fluctuations in the defect's charge state during optical pumping. Here, we use real-time control to deterministically initialize the NV center's charge state at room temperature. We demonstrate a maximum charge initialization fidelity of 99.4$\pm$0.1% and present a quantitative model of the initialization process that allows for systems-level optimization of the spin-readout signal-to-noise ratio. Even accounting for the overhead associated with the initialization sequence, increasing the charge initialization fidelity from the steady-state value of 75% near to unity allows for a factor-of-two speedup in experiments while maintaining the same signal-to-noise-ratio. In combination with high-fidelity readout based on spin-to-charge conversion, real-time initialization enables a factor-of-20 speedup over traditional methods, resulting in an ac magnetic sensitivity of 1.3 nT/Hz$^{1/2}$ for our single NV-center spin. The real-time control method is immediately beneficial for quantum sensing applications with NV centers as well as probing charge-dependent physics, and it will facilitate protocols for quantum feedback control over multi-qubit systems.