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arXiv:2310.18959 · Physical Review Applied

Wavelet-based Ramsey magnetometry enhancement of a single N - V center in diamond

Ekrem Taha Güldeste, Ceyhun Bulutay

Published Oct 11, 2024• 3 citations
arXivHTMLPDF

Abstract

Nitrogen-vacancy ($\mathrm{N}$-$V$) centers in diamond constitute a solid-state nanosensing paradigm. Specifically for high-precision magnetometry, the so-called Ramsey interferometry is the prevalent choice where the sensing signal is extracted from time-resolved spin-state-dependent photoluminescence (PL) data. Its sensitivity is ultimately limited by the photon shot noise, which cannot be sufficiently removed by averaging or frequency filtering. Here, we propose Ramsey dc magnetometry of a single $\mathrm{N}$-$V$ center enhanced by a wavelet-denoising scheme specifically tailored to suppress photon shot noise. It simply operates as a classical postprocessing applied on a collected PL time series. Our implementation is based on a method that we named template margin thresholding, which enables not only frequency but also time-dependent denoising. Thanks to this multiresolutional approach, we can enhance the PL both in near-resonant and on-resonant frequency bands and hence surpass conventional filters in photon-shot-noise suppression. We computationally benchmark that its dc magnetic field sensing signal-to-noise-ratio improvement over the raw PL data is around an order of magnitude.

DOI
10.1103/physrevapplied.22.044030

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Nitrogen-vacancy (N\mathrm{N}N-VVV) centers in diamond constitute a solid-state nanosensing paradigm. Specifically for high-precision magnetometry, the so-called Ramsey interferometry is the prevalent choice where the sensing signal is extracted from time-resolved spin-state-dependent photoluminescence (PL) data. Its sensitivity is ultimately limited by the photon shot noise, which cannot be sufficiently removed by averaging or frequency filtering. Here, we propose Ramsey dc magnetometry of a single N\mathrm{N}N-VVV center enhanced by a wavelet-denoising scheme specifically tailored to suppress photon shot noise. It simply operates as a classical postprocessing applied on a collected PL time series. Our implementation is based on a method that we named template margin thresholding, which enables not only frequency but also time-dependent denoising. Thanks to this multiresolutional approach, we can enhance the PL both in near-resonant and on-resonant frequency bands and hence surpass conventional filters in photon-shot-noise suppression. We computationally benchmark that its dc magnetic field sensing signal-to-noise-ratio improvement over the raw PL data is around an order of magnitude.
We demonstrate an absolute magnetometer based on quantum beats in the ground state of nitrogen-vacancy centers in diamond. We show that, by eliminating the dependence of spin evolution on the zero-field splitting D, the magnetometer is immune to temperature fluctuation and strain inhomogeneity. We apply this technique to measure low-frequency magnetic field noise by using a single nitrogen-vacancy center located within 500 nm of the surface of an isotopically pure (99.99% 12C) diamond. The photon-shot-noise limited sensitivity achieves 38 nT/sqrt[Hz] for 4.45 s acquisition time, a factor of sqrt[2] better than the implementation which uses only two spin levels. For long acquisition times (>10 s), we realize up to a factor of 15 improvement in magnetic sensitivity, which demonstrates the robustness of our technique against thermal drifts. Applying our technique to nitrogen-vacancy center ensembles, we eliminate dephasing from longitudinal strain inhomogeneity, resulting in a factor of 2.3 improvement in sensitivity.
The nitrogen-vacancy (NV) center in diamond has demonstrated a potential of biomagnetic sensing.1 Realization of magnetoencepharlography (MEG) using NV centers requires further improvement of magnetic sensitivity at a target frequency ranges of 0 to 200 Hz. Based on the Ramsey magnetometry, which can in principle reach the standard quantum limit. So far, a sensitivity of 460 fT/VHZ has been reported above 2 kHz.2 However, a sensitivity better than 1 pT/vHz in the DC region below 200 Hz has not yet been realized. To enhance the sensitivity for MEG, it is necessary to increase the signal response to magnetic field variations (Slope) while suppressing the noise in such a low-frequency range. The slope is increased with the Ramsey contrast, the free precession time, and the number of detected fluorescence photons. On the other hand, "intensity" noises caused by the laser and acousto-optic modulator may be suppressed by lock-in (LI) detection. However, the conventional LI detection extracts only the principal component at its demodulation frequency, so a part of the transient fluorescence signal is also filtered out. In this study, we focus on enhancing the contrast. Our core idea is to acquire higher-order harmonic signals by introducing boxcar averaging.
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We demonstrate quantum sensing of dc magnetic fields that exceeds the sensitivity of conventional ${T}_{2}^{\ensuremath{\ast}}$-limited dc magnetometry by more than an order of magnitude. We use nitrogen-vacancy centers in a diamond rotating at periods comparable to the spin coherence time, and characterize the dependence of magnetic sensitivity on measurement time and rotation speed. Our method up-converts only the dc field of interest and preserves the quantum coherence of the sensor. These results definitively improve the sensitivity of a quantum magnetometer to dc fields, demonstrating that sensitivity below the ${T}_{2}^{\ensuremath{\ast}}$ limit is possible and can be applied to any diamond magnetometer where ${T}_{2}\ensuremath{\gg}{T}_{2}^{\ensuremath{\ast}}$ to yield an order-of-magnitude sensitivity improvement.