Magnetometry by using NV center, a localized coherent spin consisting of a pair of nitrogen atom and atomic void, in diamond is now regarded as a promising technique due to its potential for high sensitivity and miniaturization. The primary application for NV magnetometry was realized by optically detected magnetic resonance (ODMR). There have been plenty of reports on demonstration of magnetic sensing by using ODMR1. Towards simpler and more reliable implementation of NV magnetometry, photocurrent-detected magnetic resonance2 (PDMR) is one of the key technologies to bypass the bulky confocal optics that are necessary for ODMR-based photoluminescence detection. In a PDMR scheme, meanwhile, we use photocurrent to read out the NV spin state, which necessitates laser irradiation only for photoexcitation and spin initialization. The key for high sensitivity of PDMR-based magnetometry3 is low noise to spin-dependent signal ratio. For this sake, it is essential to mitigate the unwanted spin-independent photocurrent while maximizing the PDMR signal. In this study, we employed two different laser power and time4 in the pulse sequence to improve the signal to noise ratio5. As a result, the magnetic sensitivity as high as 13 nT/Hz0.5 was achieved.
Magnetometry by using NV center, a localized coherent spin consisting of a pair of nitrogen atom and atomic void, in diamond is now regarded as a promising technique due to its potential for high sensitivity and miniaturization. The primary application for NV magnetometry was realized by optically detected magnetic resonance (ODMR). There have been plenty of reports on demonstration of magnetic sensing by using ODMR1. Towards simpler and more reliable implementation of NV magnetometry, photocurrent-detected magnetic resonance2 (PDMR) is one of the key technologies to bypass the bulky confocal optics that are necessary for ODMR-based photoluminescence detection. In a PDMR scheme, meanwhile, we use photocurrent to read out the NV spin state, which necessitates laser irradiation only for photoexcitation and spin initialization. The key for high sensitivity of PDMR-based magnetometry3 is low noise to spin-dependent signal ratio. For this sake, it is essential to mitigate the unwanted spin-independent photocurrent while maximizing the PDMR signal. In this study, we employed two different laser power and time4 in the pulse sequence to improve the signal to noise ratio5. As a result, the magnetic sensitivity as high as 13 nT/Hz0.5 was achieved.
The nitrogen-vacancy (NV) centers in diamond have the advantages of stable triaxial structure, ultra-long electron spin coherence time and simple optical readout at room temperature. A nitrogen atom in the diamond crystal replaces a carbon atom and a vacancy is generated at the adjacent position, forming a point defect in the C3v space group structure. Its ground state and excited state are both spin triplet states. It is the key to achieving efficient preparation of optical initial state and extracting NV color center’s information in the researches of highly sensitive sensing magnetic detection, temperature detection, biological imaging, quantum computing, etc. However, there was no systematic study on relevant parameters of laser for high-concentration NV color center’s samples in previous experimental studies. Based on a high concentration diamond NV ensemble, we use pulsed optical detection magnetic resonance (ODMR) technology to systematically study the relationship among laser initial polarization time, information reading time and laser power, and the influence of laser incident polarization angle on the accuracy of sensing information. The effects of various laser parameters on the NV1 peak of ODMR on the [111] axis of the NVs of diamond are also investigated. The contrast of ODMR increases firstly with a sigmoid function and then decreases with an e-exponential function as the information reading time increases. The incident polarization angle of the laser is sinusoidal, with a period of 90°. According to the above experimental results, we finally choose the appropriate experimental parameters at 45.8 W/cm2 (300 μs of polarization, 700 ns, reading time, laser incident angle is 220°) for ODMR test. Compared with previous experimental parameters (polarization time was 50 us, read the time of 3000 ns, laser incident angle was 250°), the experimental results show that the contrast of ODMR increases from 2.1% to 4.6%, and the typical magnetic sensitivity is improved from 21.6 nT/Hz1/2 to 5.6 nT/Hz1/2. The optimization of the optical control of NVs in solid diamond is realized. The above results provide an effective support for the detection of high-sensitivity manipulation sensing based on high-concentration NV ensemble.
While photoelectric detection of magnetic resonance (PDMR) can be applied to miniaturize nitrogen-vacancy (NV) center-based quantum sensors, real demonstration of PDMR-based magnetic field sensing remains as a distinctive challenge. To tackle this challenge, in this article, we fabricate diamond samples with electrodes and microwave antenna on the surface, and realize PDMR by detecting photocurrent in picoampere range via various lock-in amplifying modes. We obtain a theoretical and experimental sensitivity 397 nT/Hz and 921 nT/Hz of magnetic field detection in DC-10 Hz range with a laser intensity and microwave frequency modulation mode, respectively, and demonstrate for the first time, a real-time tracking of alternating magnetic field with a standard deviation of 1.5 uT. Furthermore, we investigate systematically the dependence of the PDMR contrast, linewidth and the sensitivity on the laser and microwave power, and find a perfect agreement with a master equation-based theory. Thus, our results represent a critical step forward in transitioning PDMR from a spectroscopic technique to a practical sensing modality.
Quantum sensing using nitrogen-vacancy (NV) centers in diamond has emerged as a powerful platform for detecting ultra-low magnetic fields under ambient conditions. Owing to their long spin coherence times, optical addressability, and compatibility with aqueous environments, it has found widespread applications in biosensing and bio-imaging. This review presents the fundamental principles and recent advances in NV-based quantum magnetometry for biosensing applications, with a particular focus on measurements in aqueous medium and at cellular and molecular length scales. We discuss the underlying spin physics of NV centers and highlight two primary detection modalities: optically detected magnetic resonance (ODMR) and T1 relaxometry-based sensing and how these approaches aids in the detection of both static magnetic fields and dynamic magnetic noise arising from biological processes. The review explores key application areas, including nanoscale nuclear magnetic resonance (NMR), monitoring of neural activity, detection of abnormal or rogue cells using NV-based platforms etc. In addition, strategies for enhancing sensitivity, such as surface functionalization of nanodiamonds, femtosecond (fs) laser-written photonic structures, and integration with microfluidic and lab-on-chip systems have also been discussed in depth. We also address the critical challenges, including surface-induced decoherence, charge-state instability, and signal-to-noise limitations in biofluids associated with NV-based biosensing applications. Finally, we outline future prospects, highlighting how NV-based magnetic biosensing provides a promising pathway for translating quantum sensing technologies into practical biomedical applications.
Electrical detection of nitrogen-vacancy (N-V) centers in diamond is advantageous for developing and integrating quantum information processing devices and quantum sensors and has the potential to achieve a higher collection efficiency than that of optical techniques. However, the mechanism for the electrical detection of N-V spins is not fully understood. In this study, we observe positive contrast in photocurrent detected magnetic resonance (PDMR). Note that negative PDMR contrast is usually observed. To discuss the sign of the PDMR contrast, we numerically analyze the dynamics of photocarrier generation by N-V centers using a seven-level rate model. It is found that the sign of the PDMR contrast depends on the difference in the photocurrent generated from the excited states and the metastable state of N-V centers. Furthermore, we demonstrate ac magnetic field sensing using spin coherence with the PDMR technique. ac magnetic field measurement with the PDMR technique is still challenging because the noise from a fluctuating magnetic environment is greater than the measured signal. Here, we introduce noise suppression using a phase-cycling-based noise-canceling technique. We demonstrate electrically detected ac magnetic field sensing with a sensitivity of 29 nT ${\mathrm{Hz}}^{\ensuremath{-}1/2}$. Finally, we discuss sensitivity enhancement based on the proposed model.
Nitrogen-vacancy (NV) centers in diamond have found broad applications owing to optical spin initialization and readout as well as long spin coherence time at room temperature.1 Recently, we have reported the construction of a pulsed optically detected magnetic resonance (ODMR) system based on double quantum (DQ) Ramsey magnetometry with lock-in (LI) detection towards DC magnetic field sensitivity sufficient for magnetoencephalography.2 The LI detection of fluorescence allowed reduction of the noise floor, removal of the signal offset, and real-time monitoring of the magnetic field. Beyond the CW-ODMR, where spectral broadening and contrast reduction are induced by high-power excitation, the DQ-Ramsey is expected to have highersensitivity, as it allows extension of spin dephasing time T2 * and increase of contrast and fluorescence rates.2 In practice, however, the magnetic sensitivity depends also on how the optical excitation is applied. In this study, we report the excitation power stabilization by reducing the phase noise and the effect of the excitation power on the fluorescence saturation behavior.
The core issue for the implementation of NV center qubit technology is a sensitive readout of the NV spin state. We present here a detailed theoretical and experimental study of NV center photoionization processes, used as a basis for the design of a dual-beam photoelectric method for the detection of NV magnetic resonances (PDMR). This scheme, based on NV one-photon ionization, is significantly more efficient than the previously reported single-beam excitation scheme. We demonstrate this technique on small ensembles of $\ensuremath{\sim}10$ shallow NVs implanted in electronic grade diamond (a relevant material for quantum technology), on which we achieve a cw magnetic resonance contrast of 9%---three times enhanced compared to previous work. The dual-beam PDMR scheme allows independent control of the photoionization rate and spin magnetic resonance contrast. Under a similar excitation, we obtain a significantly higher photocurrent, and thus an improved signal-to-noise ratio, compared to single-beam PDMR. Finally, this scheme is predicted to enhance magnetic resonance contrast in the case of samples with a high proportion of substitutional nitrogen defects, and could therefore enable the photoelectric readout of single NV spins.