Robust Magnetometry with Single NV Centers via Two-step Optimization

Abstract

Shallow Nitrogen-Vacancy (NV) centers are promising candidates for high-precision sensing applications; these defects, when positioned a few nanometers below the surface, provide an atomic-scale resolution along with substantial sensitivity. However, the dangling bonds and impurities on the diamond surface result in a complex environment which reduces the sensitivity and is unique to each shallow NV center. To avoid the environment's detrimental effect, we apply feedback-based quantum optimal control. We first show how a direct search can improve the initialization/readout process. In a second step, we optimize microwave pulses for pulsed Optically Detected Magnetic Resonance (ODMR) and Ramsey measurements. Throughout the sensitivity optimizations, we focus on robustness against errors in the control field amplitude. This feature not only protects the protocols' sensitivity from drifts but also enlarges the sensing volume. The resulting ODMR measurements produce sensitivities below 1$\mu$T\,Hz${}^{-\frac{1}{2}}$ for an 83\% decrease in control power, increasing the robustness by approximately one third. The optimized Ramsey measurements produce sensitivities below 100\,nT\,Hz${}^{-\frac{1}{2}}$ giving a two-fold sensitivity improvement. Being on par with typical sensitivities obtained via single NV magnetometry, the complementing robustness of the presented optimization strategy may provide an advantage for other NV-based applications.

16 pages, 11 figures

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• Applications of model-aware reinforcement learning in Bayesian quantum metrology
• Quantum optimal control in quantum technologies. Strategic report on current status, visions and goals for research in Europe
• Bichromatic microwave manipulation of the NV center nuclear spin using transition not detectable via optically detected magnetic resonance
• Rapid transform optimisation strategy for decoherence-protected quantum register in diamond
• Efficiency of optimal control for noisy spin qubits in diamond