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Multipoint Lock-in Detection for Diamond Nitrogen-Vacancy Magnetometry Using DDS-Based Frequency-Shift Keying

Qidi Hu, Luheng Cheng, Yushan Liu, Xinyi Zhu, Yu Peng Tian, Nanyang Xu

Published Dec 21, 2023• 3 citations
Publisher

Abstract

In recent years, the nitrogen-vacancy (NV) center in diamonds has been demonstrated to be a high-performance multiphysics sensor, where a lock-in amplifier (LIA) is often adopted to monitor photoluminescence changes around the resonance. It is rather complex when multiple resonant points are utilized to realize a vector or temperature-magnetic joint sensing. In this article, we present a novel scheme to realize multipoint lock-in detection with only a single-channel device. This method is based on a direct digital synthesizer (DDS) and frequency-shift keying (FSK) technique, which is capable of freely hopping frequencies with a maximum of 1.4 GHz bandwidth and encoding an unlimited number of resonant points during the sensing process. We demonstrate this method in experiments and show it would be generally useful in quantum multi-frequency excitation applications, especially in the portable and highly mobile cases.

DOI
10.3390/mi15010014

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In recent years, the nitrogen-vacancy (NV) center in diamonds has been demonstrated to be a high-performance multiphysics sensor, where a lock-in amplifier (LIA) is often adopted to monitor photoluminescence changes around the resonance. It is rather complex when multiple resonant points are utilized to realize a vector or temperature-magnetic joint sensing. In this article, we present a novel scheme to realize multipoint lock-in detection with only a single-channel device. This method is based on a direct digital synthesizer (DDS) and frequency-shift keying (FSK) technique, which is capable of freely hopping frequencies with a maximum of 1.4 GHz bandwidth and encoding an unlimited number of resonant points during the sensing process. We demonstrate this method in experiments and show it would be generally useful in quantum multi-frequency excitation applications, especially in the portable and highly mobile cases.
Nitrogen vacancy (NV) center magnetometers offer theoretical advantages such as high sensitivity, wide dynamic range, and vector capability; however, achieving all these performance metrics simultaneously in a single device remains a significant challenge. Our study presents an NV-based vector magnetometry system optimized for high-sensitivity, wide-range magnetic field measurements to overcome this issue. Our approach utilizes a frequency-locking technique combined with frequency hopping, enabling robust tracking of resonance frequencies over a wide magnetic field range via a high-speed direct digital synthesizer. We achieve real-time vector field reconstruction with high temporal resolution by continuously monitoring the resonance frequency. The proposed system demonstrates significant improvements in dynamic range, sensitivity, and tracking speed, positioning it as an essential tool for a variety of quantum sensing applications.
We demonstrate a robust, scale-factor-free vector magnetometer, which uses a closed-loop frequency-locking scheme to simultaneously track Zeeman-split resonance pairs of nitrogen-vacancy (NV) centers in diamond. This technique offers a three-orders-of-magnitude increase in dynamic range compared to open-loop methodologies; is robust against fluctuations in temperature, resonance linewidth, and contrast; and allows for simultaneous interrogation of multiple transition frequencies. By directly detecting the resonance frequencies of NV centers oriented along each of the diamond's four tetrahedral crystallographic axes, we perform full vector reconstruction of an applied magnetic field.
One of the promising solid-state quantum sensors is nitrogen-vacancy (NV) centers in diamond which can maintain quantum state at room temperature and under atmospheric pressure condition. Further improvement of sensitivity of the sensor is needed to detect the magnetic field from the brain. Noise reduction and simplified system are key techniques especially for the practical use of highly sensitive DC magnetic sensor. In this study, we devised a digital filtering detection method without synchronization between detector and modulator which is a similar noise reduction effect as a lock-in detection technique. We implemented the detection method for the quantum diamond sensor system with the large sensor volume [1]. The Lock-in detection technique is a typical high sensitivity technique for DC magnetic measurement. There are several different implementations of lock-in detection technique for the quantum diamond sensor including microwave modulation, magnetic field modulation, and laser modulation. However, it is a challenging task to obtain high magnetic sensitivity with a compact sensor module because these lock-in detection techniques require a lock-in amplifier or synchronization system. We devised a digital filtering detection method which is an alternative method to the lock-in detection technique. Fluorescence from the NV center is modulated by frequency modulated microwave during optical detected magnetic resonance (ODMR) measurement. The fluorescence is analyzed by digital signal processing which achieves a similar noise reduction effect as a lock-in detection technique. Our method relies on a fact that ODMR converts frequency modulation to amplitude modulation. Thus, our method is applicable not only to the NV center system but also another color center system. This work was supported by MEXT Q-LEAP, CAO PRISM, Japan. [1] Y. Masuyama et al., Rev. Sci. Instrum. 89, 125007 (2018).
Magnetometers based on the nitrogen-vacancy defect centers in diamonds are currently an active research topic due to their high sensitivity and relative ease of use compared to other similar solutions. Despite already being used by numerous research institutions, their widespread adoption is still hindered by high cost and complexity of the required associated equipment. This paper presents a hardware-accelerated measurement system implemented in FPGA chip, as well as the supporting analog electronics and software application. The developed system is capable of ODMR spectrum measurement, lock-in detection and multipoint resonance frequency tracking.
Introduction Nitrogen-vacancy (NV) centers in diamond are promising solid-state quantum sensors. The sensor can potentially monitor the real-time magnetic field at room-temperature toward the brain-machine interface. One of the biggest challenges is to implement a highly sensitive sensor in a compact system. There are two major ways to achieve it. An analog signal processing method with a lock-in amplifier is fast to obtain the signal, but it increases the size of the sensor system. The digital signal processing method with the Fourier transform is simple, but it needs more computational resources. Reducing computational resources with the simple system is an important task for monitoring a real-time magnetic field. In this study, we demonstrate an alternative method of digital signal processing with less computational resources than the Fourier transform. The computational time of the method to compute the signal from n points of data is O(n), whereas the time with Fourier transform is O(n log n). Method and Result We confirm our method with a large detection volume of the ensemble NV centers. The microwave and optical systems used in our experiment are shown in Fig. 1. The diamond sample containing NV centers was placed between the microwave resonator and a compound parabolic concentrator1. We used a coplanar waveguide resonator (CWR) with a wide center electrode for strong and uniform microwave irradiation2. The power of microwave with frequency modulation is 25 dBm before the CWR. The 300 mW laser beam to the diamond was focused with a beam diameter of about 30 μm. The fluorescence from the NV centers was collected through the compound parabolic concentrator and detected using a photodiode. The detected signal of the photodiode is analyzed by digital signal processing. The method obtains the result using a digital filter that effectively becomes a sinc filter using the orthogonality of trigonometric functions by multiplication of the acquired data by a trigonometric function. Figures 2 shows continuous-wave optical detected magnetic resonance (CW-ODMR) spectrum using the digital signal processing with a different modulation frequency of the microwave. The maximum slope and the noise of the signal become smaller with increasing the modulation frequency for different reasons. The low transition rate between quantum states of the NV center reduces the maximum slope for high modulation frequency. While the noise becomes smaller for high modulation frequency. We evaluate the maximum signal slope (Fig. 3) and noise level of the spectrum (Fig. 4). We found the reduction of the noise is greater than the reduction of the maximum slope. This work was supported by MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118067395.
Modern sensing applications increasingly require the simultaneous measurement of multiple physical parameters with high sensitivity. However, traditional sensors often suffer from cross-sensitivity to different variables, making it difficult to separate overlapping effects, such as those caused by temperature and magnetic fields, in complex environments. This challenge highlights the need for robust multiparametric sensors capable of independently and accurately monitoring diverse conditions in real time. In this context, nitrogen-vacancy (NV) centers in diamond present a promising solution. These atomic-scale defects can be optically initialized, manipulated, and read out via laser excitation using optically detected magnetic resonance (ODMR), and they can operate under ambient and harsh conditions. In this work, we demonstrate a simultaneous and internally cross-validated sensing platform capable of detecting both magnetic fields and temperature with a sensitivity of around 500 nT/Hz and 10 mK/Hz, respectively. Our method integrates ODMR, zero-phonon line spectral shifts, and linewidth broadening, each providing independent spectral signatures sensitive to distinct physical effects. This approach enhances measurement precision, enables internal consistency checks, and decouples intertwined phenomena such as temperature-induced variations in magnetization. The demonstrated technique significantly expands the functional capabilities of NV-based sensors, advancing their potential for high-precision metrology in complex environments.