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Filtered Boxcar-Averaging Detection of Diamond NV Ramsey-based Magnetometry for Enhancing DC Field Sensitivity

Y. Araki, T. Sekiguchi, Y. Hatano, C. Shinei, M. Miyakawa, T. Taniguchi, Onoda Shinobu, T. Shibata, T. Teraji, Ohshima Takeshi, Hatano Mutsuko, T. Iwasaki

Published Jul 28, 2025• 0 citations
OpenAlex

Abstract

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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Nitrogen-vacancy (NV) centers in diamond have found broad applications owing to optical spin initialization and readout as well as long spin coherence time at room temperature.1 Recently, we have reported the construction of a pulsed optically detected magnetic resonance (ODMR) system based on double quantum (DQ) Ramsey magnetometry with lock-in (LI) detection towards DC magnetic field sensitivity sufficient for magnetoencephalography.2 The LI detection of fluorescence allowed reduction of the noise floor, removal of the signal offset, and real-time monitoring of the magnetic field. Beyond the CW-ODMR, where spectral broadening and contrast reduction are induced by high-power excitation, the DQ-Ramsey is expected to have highersensitivity, as it allows extension of spin dephasing time T2 * and increase of contrast and fluorescence rates.2 In practice, however, the magnetic sensitivity depends also on how the optical excitation is applied. In this study, we report the excitation power stabilization by reducing the phase noise and the effect of the excitation power on the fluorescence saturation behavior.

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Nitrogen-vacancy (NV) color centers in diamond are promising solid-state quantum sensors working at room temperature. The number of NV centers limits the magnetic sensitivity of the quantum magnetometer. Increasing the number of NV centers is a challenging task due to technical issues such as microwave power density, detection noise.Here, we developed the sensor system with a large detection volume which contains a lot of NV centers. The microwave circuit enhances microwave power density. Digital filter with frequency modulation of microwave allows us to detect the magnetic field with low noise. Consequently, we obtained a DC magnetic field sensitivity of 980 pT/√Hz.This work was supported by MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118067395.

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Diamond quantum sensors utilizing nitrogen-vacancy (NV) centers in diamond can measure a tiny magnetic-field variation at room temperature with a high linearity and high spatial resolution. Because of these advantages, they have been actively studied for application to measure a biomagnetic field. In biomagnetic sensing applications, it is effective to measure at multiple points simultaneously with high sensitivity. In this study, we developed a highly sensitive diamond quantum sensor that can simultaneously measure the magnetic field at two points at the millimeter interval. We used two diamonds that were split from one HPHT-grown diamond and placed them at the 3-mm interval. A laser-induced fluorescence from the two diamonds were detected by a single photodiode. We applied different bias magnetic fields to each diamond, and thus the magnetic resonances at these diamonds had different resonance frequencies. By applying microwave fields whose frequencies were modulated at different modulation frequencies, the magnetic fields at the two diamonds can be obtained independently after demodulation. The noise spectral densities in the magnetic field measurement at the two diamonds’ locations. Since the measurement was performed outside a magnetic shield, a severe low-frequency environmental noise from railways and power line (50 Hz) noise peaks were observed. We evaluated the field sensitivities at the two diamonds to be 22 pT/√Hz in 25-300 Hz range with neglecting the power-line noise peaks and the environment noise peaks. The capability of simultaneously measuring a magnetic field at two points with the such high sensitivities will facilitate the biomagnetic sensing applications.

•Mar 19, 2025•0 citations
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.
Nitrogen-vacancy (NV) centers in diamond have found broad applications owing to optical spin initialization and readout as well as long spin coherence time at room temperature.1 Recently, we have reported the construction of a pulsed optically detected magnetic resonance (ODMR) system based on double quantum (DQ) Ramsey magnetometry with lock-in (LI) detection towards DC magnetic field sensitivity sufficient for magnetoencephalography.2 The LI detection of fluorescence allowed reduction of the noise floor, removal of the signal offset, and real-time monitoring of the magnetic field. Beyond the CW-ODMR, where spectral broadening and contrast reduction are induced by high-power excitation, the DQ-Ramsey is expected to have highersensitivity, as it allows extension of spin dephasing time T2 * and increase of contrast and fluorescence rates.2 In practice, however, the magnetic sensitivity depends also on how the optical excitation is applied. In this study, we report the excitation power stabilization by reducing the phase noise and the effect of the excitation power on the fluorescence saturation behavior.
In this study, we developed a diamond quantum magnetometer based on Ramsey interferometry with a short sensor-to-sample distance. Conventional biomagnetic sensors with ensemble nitrogen-vacancy centers using continuous-wave optically detected magnetic resonance and Ramsey methods typically rely on watt-level lasers to achieve high sensitivity, resulting in thermal issues. In contrast, by employing the light-trapping diamond waveguide technique in a high-pressure and high-temperature diamond sample treated with electron beam irradiation, we obtained a high photon conversion efficiency of 9.5%, enabling us to simultaneously achieve a high sensitivity of 2.93(7) pT/Hz^1/2 in the 100-400 Hz frequency range and a minimal temperature increase of only approximately 13 K at a low laser power of 210 mW. Using a dry phantom designed to mimic magnetoencephalography signals, we measured a weak magnetic field of 77.7(2) pT without signal averaging at a sensor-to-sample distance of 2.5 mm. This short-distance measurement prevents severe spatial signal attenuation, yielding a high signal-to-noise ratio. The development here is crucial for practical biomagnetic applications based on Ramsey interferometry.
T2T_2T2​-limited dc Quantum Magnetometry via Flux Modulation
High-sensitivity magnetometry is of critical importance to the fields of biomagnetism and geomagnetism. However, the magnetometry for the low-frequency signal detection meets the challenge of sensitivity improvement, due to multiple types of low-frequency noise sources. In particular, for the solid-state spin quantum magnetometry, the sensitivity of low frequency magnetic field has been limited by short T2∗T_2^*T2∗​. Here, we demonstrate a T2T_2T2​-limited dc quantum magnetometry based on the nitrogen-vacancy centers in diamond. The magnetometry, combining the flux modulation and the spin-echo protocol, promotes the sensitivity from being limited by T2∗T_2^*T2∗​ to T2T_2T2​ of orders of magnitude longer. The sensitivity of the dc magnetometry of 32 pT/Hz1/2\rm pT/Hz^{1/2}pT/Hz1/2 has been achieved, overwhelmingly improved by 100 folds over the Ramsey-type method result of 4.6 nT/Hz1/2\rm nT/Hz^{1/2}nT/Hz1/2. Further enhancement of the sensitivity have been systematically analyzed, although challenging but plenty of room is achievable. Our result sheds light on realization of room temperature dc quantum magnetomerty with femtotesla-sensitivity in the future.
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.
Nitrogen-vacancy (NV) color centers in diamond are promising solid-state quantum sensors working at room temperature. The number of NV centers limits the magnetic sensitivity of the quantum magnetometer. Increasing the number of NV centers is a challenging task due to technical issues such as microwave power density, detection noise.Here, we developed the sensor system with a large detection volume which contains a lot of NV centers. The microwave circuit enhances microwave power density. Digital filter with frequency modulation of microwave allows us to detect the magnetic field with low noise. Consequently, we obtained a DC magnetic field sensitivity of 980 pT/√Hz.This work was supported by MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118067395.
Diamond quantum sensors utilizing nitrogen-vacancy (NV) centers in diamond can measure a tiny magnetic-field variation at room temperature with a high linearity and high spatial resolution. Because of these advantages, they have been actively studied for application to measure a biomagnetic field. In biomagnetic sensing applications, it is effective to measure at multiple points simultaneously with high sensitivity. In this study, we developed a highly sensitive diamond quantum sensor that can simultaneously measure the magnetic field at two points at the millimeter interval. We used two diamonds that were split from one HPHT-grown diamond and placed them at the 3-mm interval. A laser-induced fluorescence from the two diamonds were detected by a single photodiode. We applied different bias magnetic fields to each diamond, and thus the magnetic resonances at these diamonds had different resonance frequencies. By applying microwave fields whose frequencies were modulated at different modulation frequencies, the magnetic fields at the two diamonds can be obtained independently after demodulation. The noise spectral densities in the magnetic field measurement at the two diamonds’ locations. Since the measurement was performed outside a magnetic shield, a severe low-frequency environmental noise from railways and power line (50 Hz) noise peaks were observed. We evaluated the field sensitivities at the two diamonds to be 22 pT/√Hz in 25-300 Hz range with neglecting the power-line noise peaks and the environment noise peaks. The capability of simultaneously measuring a magnetic field at two points with the such high sensitivities will facilitate the biomagnetic sensing applications.