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IEEE Sensors Journal

Toward T2-Limited DC Diamond Quantum Magnetometry via Flux Concentration and Modulation

Yijin Xie, Caijin Xie, Yunbin Zhu, Ke Jing, Yu Tong, Xi Qin, Haosen Guan, Chang‐Kui Duan, Ya Wang, Xing Rong

Published Aug 2, 2023• 8 citations
Publisher

Abstract

High-sensitivity magnetometry is of critical importance to the fields of biomagnetism and geomagnetism. However, magnetometry for dc and low-frequency signal detection faces the challenge of sensitivity improvement, resulting from the low-frequency noise induced by multiple types of noise sources. In particular, for the solid-state spin dc quantum magnetometry, the sensitivity has been limited by short dephasing time. Here, we realized the conversion of the magnetic signal from dc to ac by utilizing a flux concentration and modulation (FCM) technique so that the dynamical decoupling method can be applied to prolong the dephasing time. Hence, we have successfully demonstrated a dc magnetometer with a sensitivity of 32 pT/Hz$^{{1}/{2}}$, which is about two orders of magnitude over that of the Ramsey-type dc magnetometer. Further enhancement of the sensitivity is systematically analyzed, although is challenging but plenty of room is achievable. Our results shed light on the realization of room temperature dc quantum magnetometry with femtotesla-sensitivity in the future.

DOI
10.1109/jsen.2023.3299596

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High-sensitivity magnetometry is of critical importance to the fields of biomagnetism and geomagnetism. However, magnetometry for dc and low-frequency signal detection faces the challenge of sensitivity improvement, resulting from the low-frequency noise induced by multiple types of noise sources. In particular, for the solid-state spin dc quantum magnetometry, the sensitivity has been limited by short dephasing time. Here, we realized the conversion of the magnetic signal from dc to ac by utilizing a flux concentration and modulation (FCM) technique so that the dynamical decoupling method can be applied to prolong the dephasing time. Hence, we have successfully demonstrated a dc magnetometer with a sensitivity of 32 pT/Hz1/2^{{1}/{2}}1/2, which is about two orders of magnitude over that of the Ramsey-type dc magnetometer. Further enhancement of the sensitivity is systematically analyzed, although is challenging but plenty of room is achievable. Our results shed light on the realization of room temperature dc quantum magnetometry with femtotesla-sensitivity in the future.
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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.
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Nitrogen-vacancy (NV) diamond magnetometry provides high sensitivity and room-temperature operation, yet its performance is limited by microwave inhomogeneity, phase noise, and nonlinear response under broadband conditions. This paper proposes a fluorescence count-rate modulation (FCM) strategy integrated with synchronized microwave control and FPGA-based timing, enabling enhanced spin manipulation efficiency, broadband phase-noise suppression, and robust sensitivity improvement. FCM introduces controlled perturbations to NV spin evolution via micro-pulse fluorescence modulation, accelerating Rabi dynamics and stabilizing optical readout. Experiments demonstrate improved coherence (6.3μ s6.3 \mu \mathrm{~s}6.3μ s), increased contrast (52.5%), and enhanced sensitivity (6.9 nT/Hz\mathrm{nT} / \sqrt{ } \mathrm{Hz}nT/​Hz), outperforming intensity modulation (IM), dynamic decoupling (DD), and unmodulated schemes. The method maintains stability across 100\mathbf{1 0 0}100-hour continuous operation and distinct environmental conditions. Results indicate that FCM provides a scalable pathway toward practical high-precision broadband quantum magnetometry.
Portable quantum sensors are crucial for developing practical quantum sensing and metrology applications. Fiberized nitrogen-vacancy (NV) centers in diamonds have emerged as one of the most promising candidates for compact quantum sensors. Nevertheless, due to the difficulty of coherently controlling the ensemble spin and noise suppression in a large volume, it often faces problems such as reduced sensitivity and narrowed bandwidth in integrated lensless applications. Here, we propose a fluorescence signal treatment method for NV spin ensemble manipulation by the exponential fitting of spin polarization processes, instead of integrating the photon emission. This enables spin state readout with a high signal-to-noise ratio and applies to the pulse sensing protocols for large-volume NV spins. Based on this, we further developed a fiberized diamond-based AC magnetometer. With an XY8-N dynamical decoupling pulse sequence, we demonstrated a T2-limited sensitivity of 8 pT/Hz and T1-limited frequency resolution of 90 Hz over a wide frequency band from 100 kHz to 3 MHz. This integrated diamond sensor leverages quantum coherence to achieve enhanced sensitivity in detecting AC magnetic fields, making it suitable for implementation in a compact and portable endoscopic sensor.