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Highly sensitive compact quantum sensor with NV center in angle-shaped diamond

Yuta Kainuma, Yuji Hatano, Takayuki Shibata, Akimichi Nakazono, Hiromitsu Kato, Onoda Shinobu, Ohshima Takeshi, Hatano Mutsuko, Takayuki Iwasaki

Published Nov 17, 2023• 0 citations
OpenAlex

Abstract

Magnetometers using ensemble nitrogen-vacancy (NV) centers are promising because of their ability to measure magnetic fields in a variety of environments. To achieve high magnetic sensitivity, high collection efficiency of the fluorescence from the NV centers is essential. Due to the high refractive index of diamond, some of the photons from NVs diffuse to the sides of the diamond substrate[1]. In this study, we propose a method to improve the light collection efficiency by polishing the sides of the 12C-enriched chemical vapor deposition (CVD) growth diamond at an angle of 30 degrees and investigating the magnetic sensitivity.

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Magnetometers using ensemble nitrogen-vacancy (NV) centers are promising because of their ability to measure magnetic fields in a variety of environments. To achieve high magnetic sensitivity, high collection efficiency of the fluorescence from the NV centers is essential. Due to the high refractive index of diamond, some of the photons from NVs diffuse to the sides of the diamond substrate[1]. In this study, we propose a method to improve the light collection efficiency by polishing the sides of the 12C-enriched chemical vapor deposition (CVD) growth diamond at an angle of 30 degrees and investigating the magnetic sensitivity.
Diamond nitrogen-vacancy (NV) center ensemble magnetometer is a novel solid quantum magnetometer with high sensitivity and high spatial resolution, which can be used at normal temperature and pressure. However, the low fluorescence collection efficiency of the magnetometer in confocal optical system hinders further improvement in sensitivity. To solve the problem, based on silica microspheres, we propose a method to enhance the excitation and collection efficiency of NV center ensemble in massive diamond. Experimental results show, compared with the NV diamond magnetometer without microspheres, the photon collection efficiency of NV diamond magnetometer coated with a layer of microspheres is increased by 110%. Thus, the sensitivity of the NV center magnetometer is increased by 4 times to 5.3μT/√Hz.
Abstract The nitrogen-vacancy (NV) center in diamond is a promising quantum system for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels. Key material requirements for NV ensembles are a high NV − concentration, a long spin coherence time and a stable charge state. However, these are interdependent and can be difficult to optimize during diamond growth and subsequent NV creation. In this work, we systematically investigate the NV center formation and properties in bulk chemical vapor deposition (CVD) diamond. The nitrogen flow during growth is varied by over four orders of magnitude, resulting in a broad range of single substitutional nitrogen concentrations of 0.2–20 parts per million. For a fixed nitrogen concentration, we optimize electron-irradiation fluences with two different accelerated electron energies, and we study defect formation via optical characterizations. We discuss a general approach to determine the optimal irradiation conditions, for which an enhanced NV concentration and an optimum of NV charge states can both be satisfied. We achieve spin–spin coherence times T 2 ranging from 45.5 to 549 μ s for CVD diamonds containing 168 to 1 parts per billion NV − centers, respectively. This study shows a pathway to engineer properties of NV-doped CVD diamonds for improved sensitivity.
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.
Ensembles of negatively-charged nitrogen-vacancy (NV-) centres in diamond constitute a promising platform for magnetometry and other sensing applications. However, the sensitivity of present NV-ensemble devices and the need for diamond material with reproducible properties have hindered progress toward many envisioned applications. The work presented here addresses these challenges by optimizing a chemical vapor deposition (CVD) growth process to produce material at scale with improved NV-ensemble properties for both bulk and widefield magnetic sensing modalities.
Quantum sensors utilize quantized energy levels, quan- tum superposition states, and eventually quantum entan- glement, which are also key elements in quantum comput- ing. Electronic and nuclear spins in semiconductors may serve as quantum sensing elements or qubits. The phos- phorus donor in silicon is an ideal qubit in terms of coherence time because of highest chemical purity of host material with isotopic 28Si enrichment and a very low den- sity of structural defects. However, it requires a cryogenic temperature to avoid ionization of the donor. In contrast in diamond, due to the wide band gap, the nitrogen-vacancy (NV) center exhibits optical initialization and readout of its spin state and a long coherence time even above room temperature, promoting its research and development as quantum sensors or qubits. Nevertheless, the syn- thesized diamond crystals with NV centers are not so per- fect as silicon. Therefore, the host material characteriza- tion is essential to improve the bulk-ensemble sensi- tivity to the magnetic field. The present study shows how to measure the spin properties of the diamond as a sens- ing host via the NV centers themselves and how to utilize such information to improve the sensor performance.
Abstract The ensemble of nitrogen-vacancy (NV) centers in diamond allows for the potential realization of the sensitive magnetometers by leveraging their excellent spin properties. However, the NV-based magnetometers are limited by their experimental magnetic field sensitivity owing to its inefficient photon collection. Moreover, they are a disadvantage to the reduced spatial resolution and excessive excitation power. To overcome these issues, we propose a ultra-sensitive diamond magnetometer based on nanocavities. The device structure can attain a high collective efficiency and enhance the photon emission intensity of the NV ensemble. This device can allow the efficient photon collection even when considering the positional distribution of the NV centers. Our theoretical analysis indicates that the minimum expected sensitivity is 60 fT/ H z . The proposed design can achieve a volume-normalized sensitivity of 0.92 aT/ H z c m − 3 along with the required power of 7 μW, both of which are superior to those of bulk diamond. The proposed approach offers a promising route towards highly sensitive and energy-efficient magnetometers.