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Simultaneous amplitude and phase imaging of AC magnetic signal on wide field setup

Kosuke Mizuno, Hitoshi Ishiwata, Y. MASUYAMA, Takayuki Iwasaki, Mutsuko Hatano

Published Dec 17, 2019• 0 citations
OpenAlex

Abstract

The nitrogen-vacancy center in diamond is an exquisite solid-state sensor based upon quantum nature. Numerous studies have shown that dynamical decoupling, such as an XY8 sequence, provides us a high sensitivity for an oscillating magnetic field parametrized by amplitude and phase. However, there are some drawbacks. (1) A measurement using XY8 requires a careful adjustment of timings between XY8 sequences and the target signal. (2) The XY8 has an intricate dependence on both parameters, so it is not applicable in a situation that both parameters simultaneously change. (3) It is impossible to measure both parameters simultaneous, and they have to be measured severally. In this study, we present that Qdyne1-3, a quantum-classical hybrid protocol, solves them, consisted of sequential and stroboscopic measurements. The Qdyne does not require a timing adjustment before itself, has an intuitive dependency on both parameters, and let us a simultaneous measurement of both amplitude and phase. Moreover, we implemented it on a widefield of view. The large observation area of widefield modality and the high spatial resolution of NV center pave the way for a tiny-scale inspection by Eddy-current, which established sensors cannot detect.

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The nitrogen-vacancy center in diamond is an exquisite solid-state sensor based upon quantum nature. Numerous studies have shown that dynamical decoupling, such as an XY8 sequence, provides us a high sensitivity for an oscillating magnetic field parametrized by amplitude and phase. However, there are some drawbacks. (1) A measurement using XY8 requires a careful adjustment of timings between XY8 sequences and the target signal. (2) The XY8 has an intricate dependence on both parameters, so it is not applicable in a situation that both parameters simultaneously change. (3) It is impossible to measure both parameters simultaneous, and they have to be measured severally. In this study, we present that Qdyne1-3, a quantum-classical hybrid protocol, solves them, consisted of sequential and stroboscopic measurements. The Qdyne does not require a timing adjustment before itself, has an intuitive dependency on both parameters, and let us a simultaneous measurement of both amplitude and phase. Moreover, we implemented it on a widefield of view. The large observation area of widefield modality and the high spatial resolution of NV center pave the way for a tiny-scale inspection by Eddy-current, which established sensors cannot detect.
Nitrogen—Vacancy (NV) center in diamond can detect tiny magnetic field due to relative long coherence time even at room temperature. Its optical accessibility and capability of thin-film formation provide us an interesting sensing modality such as wide field imaging. We have reported phase sensitive protocol for ac magnetic field designed for wide field setup.[1] In this study we demonstrate an application of phase sensitive protocol; measuring distribution of ac magnetic field strength by phase sensing. Previous methods of multi-pulse quantum sensing such as XY8 realize quite high sensitivity of ac magnetic field. Since XY8 has dependencies on strength and phase of ac magnetic field, it is necessary to adjust ac phase to an optimal point in order to obtain a best sensitivity, and we cannot distinguish the effect of amplitude and phase. Recent studies have showed utilizing time-domain information is effective for improving frequency resolution of quantum sensing,[2,3] and we found same technique is also effective for simultaneous measurement of strength and phase. Here we propose a robust method for amplitude estimation by phase sensing. We added another signal for reference, then sensing phase difference between target signal and reference can be converted into amplitude. Conventional XY8 sensing is based upon fluorescence measurement which can be affected easily by many obstacles such as stray light, autofluorescence, and fringes of excitation laser. In the other hand, our method can be more robust because it is based on phase measurement. Nothing can change the phase except absorption of ac signal. This work was supported by MEXT Q-LEAP, CAO PRISM, Japan. [1] Mizuno et al., AIP advances 2018 [2][3] Boss, Schmitt
Spectroscopic analysis of AC magnetic signal using diamond quantum magnetometry is a promising technique for inductive imaging. Conventional dynamic decoupling like XY8 provides a high sensitivity of an oscillating magnetic signal with intricate dependence on magnitude and phase, complicating high throughput detection of each parameter. In this study, a simple measurement scheme for independent and simultaneous detection of magnitude and phase is demonstrated by a sequential measurement protocol. Wide-field imaging experiment was performed for an oscillating magnetic field with approximately [Formula: see text]-squared observation area. Single pixel phase precision was [Formula: see text] for [Formula: see text] AC magnetic signal. Our method enables potential applications including inductive inspection and impedance imaging.
Magnetometry utilizing a spin qubit in a solid state possesses high sensitivity. In particular, a magnetic sensor with a high spatial resolution can be achieved with the electron-spin states of a nitrogen-vacancy (NV) center in the diamond. In this study, we demonstrated that NV quantum sensing based on multiple-pulse decoupling sequences can sensitively measure not only the amplitude but also the phase shift of an alternating-current (AC) magnetic field. In the AC magnetometry based on decoupling sequences, the maximum phase accumulation of the NV spin due to an AC-field can be generally obtained when the π-pulse period in the sequences matches the half time period of the field and the relative phase difference between the sequences and the field is zero. By contrast, the NV quantum sensor acquires no phase accumulation if the relative phase difference is π/2. Thus, this phase-accumulation condition does not have any advantage for the magnetometry. However, we revealed that the nonphase-accumulation condition is available for detecting a very small phase shift of an AC-field from its initial phase. Measurements of the real part and the imaginary part of AC impedance require the amplitude and the phase information of an AC-field, and thus, this finding is expected to provide a guide for realizing sensitive measurement of AC impedance in the micrometer and nanometer scales.
Nitrogen vacancy (NV) centers in diamond are atom-scale defects that can be used to sense magnetic fields with high sensitivity and spatial resolution. Typically, the magnetic field is measured by averaging sequential measurements of single NV centers, or by spatial averaging over ensembles of many NV centers, which provides mean values that contain no nonlocal information about the relationship between two points separated in space or time. Here, we propose and implement a sensing modality whereby two or more NV centers are measured simultaneously, and we extract temporal and spatial correlations in their signals that would otherwise be inaccessible. We demonstrate measurements of correlated applied noise using spin-to-charge readout of two NV centers and implement a spectral reconstruction protocol for disentangling local and nonlocal noise sources.
Wide-field quantum magnetometry using nitrogen-vacancy (NV) center in diamond can be a breakthrough for a nuclear magnetic resonance (NMR) in a small volume, which is important for biological applications. Although the coherence time of the electron spin of the NV center results in a limited frequency resolution for diamond magnetometry in the range 10–100 kHz, recent studies have shown that a phase-sensitive protocol can circumvent this limit using a confocal setup. We proposed a new measurement protocol, “iQdyne,” which facilitates an improved frequency resolution of wide-field imaging, unencumbered by the coherence limit imposed by the NV center. We demonstrated wide-field magnetometry with a frequency resolution of 238 mHz and a magnetic sensitivity of 65 nT/Hz1/2, which are superior to those obtained using a conventional XY8-based technique, and showed the potential of the iQdyne protocol for the wide-field NMR imaging.
Nitrogen-vacancy centers in diamond have been shown to be capable of detecting AC magnetic fields with high sensitivity, spectral resolution, and spatial resolution. However, most studies so far have focused on the regime of time-averaged or time-correlated measurements, while little attention has been paid to the single-shot regime. Here we show that the amplitude and phase of an AC field can be retrieved from a single pair of two consecutive measurements. We demonstrate this concept by measuring a 4 MHz AC field with a per-shot amplitude and phase sensitivity of 78 nT and 63 mrad, respectively, at a temporal resolution of 320 us. We also investigate the effects and quantify the errors resulting from probe frequency detunings, as well as operating in the strong field regime. Moreover, we showcase the ability of the measurement protocol to dynamically change the probe frequency in real-time. This work advances the use of NV centers for real-time measurements of AC magnetic fields.