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Extension of Spin Dephasing Time of Millimeter Scale Ensemble NV centers by Suppressing Magnetic and Strain Field Inhomogeneities

Ikuya Fujisaki, Li Zehan, Yuji Hatano, T. Sekiguchi, 直太 関口, Chkara Shinei, Masashi Miyakawa, Takeshi Taniguchi, Takeyuki Teraji, Abe Hiroshi, Onoda Shinobu, Ohshima Takeshi, Takeyuki Iwasaki, Hatano Mutsuko

Published Oct 22, 2024• 0 citations
OpenAlex

Abstract

Quantum magnetic sensors based on nitrogen-vacancy (NV) centers in diamond are expected to be applicable to biological and chemical applications However, their DC magnetic field sensitivity is still insufficient for demanding applications such as magnetoencephalography requiring extraordinary sensitivity. The spin dephasing time 𝑇2∗ is a key parameter to improve the sensitivity, but 𝑇2∗ of typical millimeter-scale NV centers is limited by magnetic field inhomogeneity, strain field inhomogeneity, and electronic spin bath. In this work, we developed (i) a magnet system suppressing both the longitudinal magnetic field and transverse strain field inhomogeneities and (ii) a microwave (MW) and radio-frequency (RF) application system for high-efficiency spin-bath driving (SBD) suitable for a diamond (111) sample.

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Solid-state quantum sensors offer the promising prospect of high-sensitivity sensing across various environments, including at room temperature and atmospheric pressure. Highly sensitive solid-state quantum sensors require long ensemble dephasing times (T2*) in mm-scale sample areas. This requirement demands a highly uniform bias magnetic field across all qubits, which has traditionally required large magnetic field sources such as Helmholtz coils or a pair of magnets. Meanwhile, to expand the range of applications of quantum sensors, portable sensors are required, but it is difficult to carry these large magnets. Balancing the miniaturization of the quantum sensor with high sensitivity is a significant challenge. In this study, we achieve a long ensemble dephasing time for nitrogen-vacancy (NV) centers in a large sample volume using a single, compact, and optimized magnet assembly. By using a magnet design derived from an inverse problem-based analysis, we reduced the magnet assembly volume to 0.23% of the Helmholtz coil volume. We achieve an ensemble dephasing time of 5.32 μs for the entire NV center ensemble across the sample thickness, comparable to that obtained with a standard Helmholtz coil setup. This dephasing is limited by strain-induced dephasing across the diamond sample thickness, with a dephasing rate of 188 kHz, which represents the state-of-the-art diamond quality for bulk quantum sensors. Our work paves the way for the miniaturization of high-sensitivity quantum sensors, enabling new sensor designs and the construction of compact, highly sensitive quantum sensor arrays.

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Ensembles of nitrogen-vacancy (NV) centers in diamond are versatile quantum sensors with broad applications in the physical and life sciences. The concentration of neutral substitutional nitrogen ( [ N s 0 ]) strongly influences NV electronic spin coherence times, sensitivity, and optimal sensing strategies. Diamonds with [ N s 0 ] ∼ 1–10 ppm are a focus of recent material engineering efforts, with higher concentrations being favorable for continuous-wave optically detected magnetic resonance (CW-ODMR) and lower concentrations expected to benefit pulsed magnetometry techniques through extended NV spin coherence times and improved sensing duty cycles. In this work, we synthesize and characterize low- [ N s 0 ] ( ∼ 0.8 ppm), NV-enriched diamond material, engineered through low-strain chemical vapor deposition (CVD) growth on high-quality substrates, 12 C isotopic purification, and controlled electron irradiation and annealing. Our results demonstrate good strain homogeneity in diamonds grown on CVD substrates and spin-bath-limited NV dephasing times. By measuring NV spin and charge properties across a wide range of optical NV excitation intensity, we provide direct comparisons of photon-shot-noise-limited magnetic field sensitivity between the current low- [ N s 0 ] and previously studied higher- [ N s 0 ] ( ∼ 14 ppm) NV-diamond sensors. We show that low- [ N s 0 ] diamond can outperform higher- [ N s 0 ] diamond at moderate and low optical NV excitation intensity. Our results provide practical benchmarks and guidance for selecting NV-diamond sensors tailored to specific experimental constraints and sensing requirements.

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Quantum magnetic sensors based on nitrogen-vacancy (NV) centers in diamond are expected to be applicable to biological and chemical applications However, their DC magnetic field sensitivity is still insufficient for demanding applications such as magnetoencephalography requiring extraordinary sensitivity. The spin dephasing time 𝑇2∗ is a key parameter to improve the sensitivity, but 𝑇2∗ of typical millimeter-scale NV centers is limited by magnetic field inhomogeneity, strain field inhomogeneity, and electronic spin bath. In this work, we developed (i) a magnet system suppressing both the longitudinal magnetic field and transverse strain field inhomogeneities and (ii) a microwave (MW) and radio-frequency (RF) application system for high-efficiency spin-bath driving (SBD) suitable for a diamond (111) sample.
Solid-state quantum sensors offer the promising prospect of high-sensitivity sensing across various environments, including at room temperature and atmospheric pressure. Highly sensitive solid-state quantum sensors require long ensemble dephasing times (T2*) in mm-scale sample areas. This requirement demands a highly uniform bias magnetic field across all qubits, which has traditionally required large magnetic field sources such as Helmholtz coils or a pair of magnets. Meanwhile, to expand the range of applications of quantum sensors, portable sensors are required, but it is difficult to carry these large magnets. Balancing the miniaturization of the quantum sensor with high sensitivity is a significant challenge. In this study, we achieve a long ensemble dephasing time for nitrogen-vacancy (NV) centers in a large sample volume using a single, compact, and optimized magnet assembly. By using a magnet design derived from an inverse problem-based analysis, we reduced the magnet assembly volume to 0.23% of the Helmholtz coil volume. We achieve an ensemble dephasing time of 5.32 μs for the entire NV center ensemble across the sample thickness, comparable to that obtained with a standard Helmholtz coil setup. This dephasing is limited by strain-induced dephasing across the diamond sample thickness, with a dephasing rate of 188 kHz, which represents the state-of-the-art diamond quality for bulk quantum sensors. Our work paves the way for the miniaturization of high-sensitivity quantum sensors, enabling new sensor designs and the construction of compact, highly sensitive quantum sensor arrays.
Abstract Nitrogen-vacancy (NV) center ensembles in diamond are one of the most promising solid-state quantum platforms for various sensing applications. Achieving ultimate sensitivity requires simultaneously long spin dephasing times ( T2∗{T}_{2}^{* }T2∗​ T 2 * ) and high NV center concentrations. In this work, we propose a systematic measurement approach to quantify the electron spin dephasing in NV center ensembles and analyze the contributions of various sources to the dephasing time, including NV-NV interactions, strain and electric field distributions, 13 C nuclear spins, and P1 electron spins. Our method is validated using a series of high-performance diamond samples, providing a comprehensive understanding of dephasing mechanisms and revealing correlations between NV concentration and different dephasing sources. Building on these insights, we outline strategies to further enhance the achievable sensitivity for DC magnetic field measurements.
Nitrogen-vacancy (NV) centers in diamond are a leading competitor for quantum sensing. The resolution of DC magnetic field sensing is limited by dephasing of the NV electron spin, with the main dephasing source being the thermal and quantum noise from the 13C^{13}\mathrm{C}13C nuclear spin bath. In this work, we propose an effective method to extend the dephasing time of a single NV electron spin by suppressing the nuclear spin noise. On the one hand, we improve the technique of bath driving to accommodate nuclear spin bath decoupling, and extend the dephasing time from 2.6 to 10.2 $\textmu{}\mathrm{s}$ at a magnetic field of 455 G. On the other hand, our method enables decoupling the quantum noise by inserting $\ensuremath{\pi}$ rotations around the ZZZ axis of the nuclear spin bath into the free evolution. By suppressing both the thermal and quantum noise, the dephasing time is extended by more than four times at 100 G. Our results provide a new route to explore strategies of suppressing the thermal noise and quantum noise in quantum information processing, and can be applied to other solid-state defects.
Negatively charged nitrogen vacancy (NV−) centres in diamond crystals are promising colour centres for high-sensitivity quantum sensors. A long dephasing time (T2* > 10 μs) is essential for achieving increased sensitivity and higher uniformity of T2* in millimetre-scale diamond is strongly desired for femto-tesla weak magnetic field detection. High uniformity of T2* for NV− centres is achieved herein. The median value of T2*, <T2*>, in the 12C-enriched high-pressure, high-temperature (HPHT) grown diamond with a nitrogen concentration of 1.3 ± 0.4 ppm is 4.5 μs. The variance of T2* is only 10% over a millimetre-scale region (1.1 × 1.1 mm2) within the 0.4 mm thick {111} growth sector. <T2*> is ~2/3 times the value limited by the dipole-dipole interaction from the electron-spin bath of nitrogen impurities, suggesting that the residual strain gradient in the HPHT diamond crystal partially limits T2*. Reducing the strain gradient in diamond crystals provide a pathway to achievement of high sensitivity magnetometry using NV quantum sensing. Negatively charged nitrogen vacancy centres in diamond crystals are promising for achieving high-sensitivity quantum sensors. Here, a long dephasing time with high uniformity is achieved in a millimetre-scale area along the {111} plane using 12C enriched high-pressure high-temperature diamond crystals.
The nitrogen-vacancy (NV) center in diamond is extensively studied for a next-generation high-sensitivity magnetometer. The DC magnetic sensitivity can be improved by extending the inhomogeneous spin dephasing time through suppressing the effects of inhomogeneous fluctuations or by increasing the number of effective NV centers through expanding the sensor volume. It has been demonstrated that strain-induced dephasing can be suppressed by double quantum (DQ) technique with two-tone microwaves (MW), and spin bath-induced dephasing can be suppressed by spin bath drive (SBD) technique with multi-tone radio-frequency (RF) to drive the P1 centers. To combine these T2* -extension techniques with a bulk diamond, it is necessary to apply MW and RF uniformly to the sample. A coplanar waveguide (CPW) circuit has been reported to be useful for uniform microwave irradiation to a bulk sample. In this study, we reports the newly designed and fabricated a cross-aligned two-layer CPW circuit.
Ensembles of nitrogen-vacancy (NV) centers in diamond are versatile quantum sensors with broad applications in the physical and life sciences. The concentration of neutral substitutional nitrogen ( [ N s 0 ]) strongly influences NV electronic spin coherence times, sensitivity, and optimal sensing strategies. Diamonds with [ N s 0 ] ∼ 1–10 ppm are a focus of recent material engineering efforts, with higher concentrations being favorable for continuous-wave optically detected magnetic resonance (CW-ODMR) and lower concentrations expected to benefit pulsed magnetometry techniques through extended NV spin coherence times and improved sensing duty cycles. In this work, we synthesize and characterize low- [ N s 0 ] ( ∼ 0.8 ppm), NV-enriched diamond material, engineered through low-strain chemical vapor deposition (CVD) growth on high-quality substrates, 12 C isotopic purification, and controlled electron irradiation and annealing. Our results demonstrate good strain homogeneity in diamonds grown on CVD substrates and spin-bath-limited NV dephasing times. By measuring NV spin and charge properties across a wide range of optical NV excitation intensity, we provide direct comparisons of photon-shot-noise-limited magnetic field sensitivity between the current low- [ N s 0 ] and previously studied higher- [ N s 0 ] ( ∼ 14 ppm) NV-diamond sensors. We show that low- [ N s 0 ] diamond can outperform higher- [ N s 0 ] diamond at moderate and low optical NV excitation intensity. Our results provide practical benchmarks and guidance for selecting NV-diamond sensors tailored to specific experimental constraints and sensing requirements.