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Conference on Precision Electromagnetic Measurements

Diamond Quantum Magnetic Sensor toward a Precision Comparison of Currents with a Current Comparator

Hidekazu Muramatsu, Yuta Kainuma, Yuji Hatano, Yasutaka Amagai, Hiromitsu Kato, Norihiko Sakamoto, Tatsuji Yamada, Chiharu Urano, Nobu‐Hisa Kaneko, Hiroshi Abe, Shinobu Onoda, Takeshi Ohshima, Mutsuko Hatano, Takayuki Iwasaki

Published Jul 8, 2024• 3 citations
Publisher

Abstract

In this paper, a diamond nitrogen-vacancy (NV) magnetic sensor toward a precision comparison of currents with a current comparator is described. A diamond quantum sensor can sensitively detect magnetic fields based on the Zeeman effect of the electron spin state of NV centers, which may be used as a detector in a current comparator. The compact diamond quantum sensor head has been developed, being capable of positioning in an air gap formed in a ring-shaped magnetic core. The sensor exhibited the DC magnetic noise floor of less than $100\,{\text{pT}}/\sqrt {{\text{Hz}}} $. The obtained magnetic noise floor is less than equivalent current noise level required for a current comparator.

DOI
10.1109/cpem61406.2024.10646090

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In this paper, a diamond nitrogen-vacancy (NV) magnetic sensor toward a precision comparison of currents with a current comparator is described. A diamond quantum sensor can sensitively detect magnetic fields based on the Zeeman effect of the electron spin state of NV centers, which may be used as a detector in a current comparator. The compact diamond quantum sensor head has been developed, being capable of positioning in an air gap formed in a ring-shaped magnetic core. The sensor exhibited the DC magnetic noise floor of less than 100 pT/Hz100\,{\text{pT}}/\sqrt {{\text{Hz}}} 100pT/Hz​. The obtained magnetic noise floor is less than equivalent current noise level required for a current comparator.
Precision measurements of electric current flow in motors and power converters is indispensable for efficient digital control by monitoring their energy use. Nitrogen-vacancy (NV) centers in a crystalline diamond are promising candidates for sensitive magnetometry and electric current measurements. In a diamond quantum sensor with NV centers, the Zeeman splitting of the electron spin state of the NV center can be utilized to detect magnetic field. This is because the splitting of the spectral line is proportional to the external magnetic field strength, and the arbitrary constant of the gyromagnetic ratio is independent of both frequency and magnetic field. Then, the Zeeman splitting can be measured with optically detected magnetic resonance (ODMR) spectroscopy.
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.
The recent decade has witnessed exciting developments in quantum science and technology. Quantum sensing is a rapidly growing branch, which exploits the fragility of quantum systems to detect small external signals. The extreme control at the single-atom level and the remarkably long coherence time of engineered quantum systems enable unprecedented sensitivity and precision, beating the performance of classical techniques. The nitrogen-vacancy (NV) center in diamond is one of the most promising quantum sensors. Its unique advantages have attracted considerable attention, including a long spin coherence time throughout a broad temperature range, nanoscale resolution limited by the sensor-sample distance, convenient optical control of its spin states and remarkably versatile sensing capabilities. Its properties are well studied under a bias magnetic field oriented along its quantization axis. However, its sensing capabilities in the presence of an off-axis magnetic field are much less explored. My research aims to unlock the full potential of NV quantum sensor by developing new sensing modalities under a magnetic field acting perpendicular to the NV axis, and furthermore, integrating them into a scanning probe microscope. In this dissertation, I will report our progress toward building a scanning NV multimodal quantum imaging platform, which will open up exciting opportunities in measuring nanoscale phenomena. The first chapter provides an introduction to the background of quantum sensing with NV centers, followed by a discussion on its basic properties and established sensing techniques. The second and third chapters are dedicated to two experiments performed under a perpendicular magnetic field where traditional NV magnetometry based on the Zeeman effect fails. In the second chapter, I will introduce a new sensing strategy that uses the entanglement between the electron and nuclear spins to restore the magnetic field sensitivity under this unfavored condition. This allows us to sense small changes in the magnetic field angle relative to the NV axis, and to detect anisotropic magnetic noise in the local environment. In the third chapter, I will present our latest development of scanning NV electrometry, where a single NV at the apex of a diamond scanning probe images external alternating (AC) and direct (DC) electric fields with sub-100 nm resolution. In the last chapter, I will discuss our efforts on four experiments that were left unfinished. I will present the results we have achieved, analyze the challenges that we faced and propose potential solutions to overcome them.
The research focuses on the solid-state spin system of diamond NV (Negatively Charged Nitrogen Vacancy, NV) color centers, which exhibit excellent optical readout, polarization properties, and millisecond-scale coherent time. This makes it an important means to achieve quantum information processing and quantum precision measurement. Additionally, its ultrahigh sensitivity to magnetic fields provides a potential path for diamond’s high-precision current measurement. This paper investigates a reliable method in the field of direct current measurement using quantum sensing technology based on diamond NV color centers. It involves measuring the change in the optically detected magnetic resonance spectrum based on a fixed microwave frequency to achieve magnetic field measurement. Simultaneously, by utilizing the spatial distribution of magnetic sensor heads, it achieves current inversion. Finally, an experimental platform was constructed for validation. Preliminary results indicate that the linear fit error for direct current amplitude is less than 10^-7, confirming the feasibility of this technological approach and providing a potential application for diamond in measuring currents in power systems.
Magnetic sensors are typically much more sensitive to oscillating (AC) magnetic fields than static (DC) fields, due to the presence of more noise at lower frequency, typically scaling with a 1/f1/f1/f dependence. For quantum magnetometers, this characteristic noise is reflected in the ensemble dephasing time T2∗T_2^*T2∗​, the relevant sensing time for a DC field, being much lower than the spin coherence time T2T_2T2​, which determines the sensitivity to AC fields. Here, we demonstrate measurement of DC magnetic fields using a physically rotating ensemble of nitrogen-vacancy centres at a precision limited by the electron spin coherence time, T2T_2T2​. We rotate the host diamond with a period comparable to T2T_2T2​, such that the angle between the NV axis and the magnetic field to be detected changes as a function of time, upconverting the static magnetic field to an oscillating field in the physically rotating frame. Using spin-echo interferometry, we observe an order of magnitude improvement in DC magnetometer response compared to a conventional Ramsey experiments. Enhancements to our scheme could realise DC sensitivities equivalent to demonstrated AC magnetic field sensitivities with NV centres.
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.