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arXiv:2009.02371 · Physical Review Applied

N - V –Diamond Magnetic Microscopy Using a Double Quantum 4-Ramsey Protocol

Connor Hart, Jennifer M. Schloss, Matthew J. Turner, Patrick Scheidegger, Erik Bauch, Ronald Lee Walsworth

Published Apr 12, 2021• 59 citations
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Abstract

We introduce a double quantum (DQ) 4-Ramsey measurement protocol that enables wide-field magnetic imaging using nitrogen-vacancy ($\mathrm{N}$-$V$) centers in diamond, with enhanced homogeneity of the magnetic sensitivity relative to conventional single quantum (SQ) techniques. The DQ 4-Ramsey protocol employs microwave-phase alternation across four consecutive Ramsey (4-Ramsey) measurements to isolate the desired DQ magnetic signal from any residual SQ signal induced by microwave pulse errors. In a demonstration experiment employing a 1-$\ensuremath{\mu}\mathrm{m}$-thick $\mathrm{N}$-$V$ layer in a macroscopic diamond chip, the DQ 4-Ramsey protocol provides a volume-normalized dc magnetic sensitivity of ${\ensuremath{\eta}}^{V}=34\phantom{\rule{0.2em}{0ex}}\mathrm{nT}\phantom{\rule{0.1em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1/2}\phantom{\rule{0.1em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{3/2}$ across a $125\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}\ifmmode\times\else\texttimes\fi{}125\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$ field of view, with about $5\ifmmode\times\else\texttimes\fi{}$ less spatial variation in sensitivity across the field of view compared to a SQ measurement. The improved robustness and magnetic sensitivity homogeneity of the DQ 4-Ramsey protocol enables imaging of dynamic broadband magnetic sources such as integrated circuits and electrically active cells.

DOI
10.1103/physrevapplied.15.044020

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We introduce a double quantum (DQ) 4-Ramsey measurement protocol that enables wide-field magnetic imaging using nitrogen-vacancy (N\mathrm{N}N-VVV) centers in diamond, with enhanced homogeneity of the magnetic sensitivity relative to conventional single quantum (SQ) techniques. The DQ 4-Ramsey protocol employs microwave-phase alternation across four consecutive Ramsey (4-Ramsey) measurements to isolate the desired DQ magnetic signal from any residual SQ signal induced by microwave pulse errors. In a demonstration experiment employing a 1-\ensuremathμm\ensuremath{\mu}\mathrm{m}\ensuremathμm-thick N\mathrm{N}N-VVV layer in a macroscopic diamond chip, the DQ 4-Ramsey protocol provides a volume-normalized dc magnetic sensitivity of \ensuremathηV=34nTHz\ensuremath−1/2\ensuremathμm3/2{\ensuremath{\eta}}^{V}=34\phantom{\rule{0.2em}{0ex}}\mathrm{nT}\phantom{\rule{0.1em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1/2}\phantom{\rule{0.1em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{3/2}\ensuremathηV=34nTHz\ensuremath−1/2\ensuremathμm3/2 across a 125\ensuremathμm\ifmmode×\else\texttimes\fi125\ensuremathμm125\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}\ifmmode\times\else\texttimes\fi{}125\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}125\ensuremathμm\ifmmode×\else\texttimes\fi125\ensuremathμm field of view, with about 5\ifmmode×\else\texttimes\fi5\ifmmode\times\else\texttimes\fi{}5\ifmmode×\else\texttimes\fi less spatial variation in sensitivity across the field of view compared to a SQ measurement. The improved robustness and magnetic sensitivity homogeneity of the DQ 4-Ramsey protocol enables imaging of dynamic broadband magnetic sources such as integrated circuits and electrically active cells.
Wide-field imaging of magnetic signals using ensembles of nitrogen-vacancy (NV) centers in diamond has garnered increasing interest due to its combination of micron-scale resolution, millimeter-scale field of view, and compatibility with diverse samples from across the physical and life sciences. Recently, wide-field NV magnetic imaging based on the Ramsey protocol has achieved uniform and enhanced sensitivity compared to conventional measurements. Here, we integrate the Ramsey-based protocol with spin-bath driving to extend the NV spin dephasing time and improve magnetic sensitivity. We also employ a high-speed camera to enable dynamic wide-field magnetic imaging. We benchmark the utility of this quantum diamond microscope (QDM) by imaging magnetic fields produced from a fabricated wire phantom. Over a 270 × 270 μm2 field of view, a median per-pixel magnetic sensitivity of 4.1(1) nT /Hz is realized with a spatial resolution ≲ 10 μm and sub-millisecond temporal resolution. Importantly, the spatial magnetic noise floor can be reduced to the picotesla scale by time-averaging and signal modulation, which enables imaging of a magnetic-field pattern with a peak-to-peak amplitude difference of about 300 pT. Finally, we discuss potential new applications of this dynamic QDM in studying biomineralization and electrically active cells.
Quantum sensing with solid-state spins offers the promise of high spatial resolution, bandwidth, and dynamic range at sensitivities comparable to more mature quantum sensing technologies, such as atomic vapor cells and superconducting devices. However, despite comparable theoretical sensitivity limits, the performance of bulk solid-state quantum sensors has so far lagged behind these more mature alternatives. A recent review [Barry et al., Rev. Mod. Phys. 92, 015004 (2020)] suggests several paths to improve performance of magnetometers employing nitrogen-vacancy defects in diamond, the most-studied solid-state quantum sensing platform. Implementing several suggested techniques, we demonstrate the most sensitive nitrogen-vacancy-based bulk magnetometer reported to date. Our approach combines tailored diamond growth to achieve low strain and long intrinsic dephasing times, the use of double-quantum Ramsey and Hahn-echo magnetometry sequences for broadband and narrowband magnetometry, respectively, and P1 driving to further extend dephasing time. Notably, the device does not include a flux concentrator, preserving the fixed response of the nitrogen vacancies to magnetic field. The magnetometer realizes a minimum sensitivity of 460fTs1/2460\phantom{\rule{0.2em}{0ex}}\mathrm{fT}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{1/2}460fTs1/2 in a broadband dc-sensitive mode and 210fTs1/2210\phantom{\rule{0.2em}{0ex}}\mathrm{fT}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{1/2}210fTs1/2 in a narrowband ac mode. We describe the experimental setup in detail and highlight potential paths for future improvement.
High-fidelity quantum operation of qubits plays an important role in magnetometry based on nitrogen-vacancy (NV) centers in diamonds. However, the nontrivial spin-spin coupling of the NV center decreases signal contrast and sensitivity. Here, we overcome this limitation by exploiting the amplitude modulation of microwaves, which allows us to perfectly detect magnetic signals at low fields. Compared with the traditional double-quantum sensing protocol, the full contrast of the detection signal was recovered, and the sensitivity was enhanced three times in the experiment. Our method is applicable to a wide range of sensing tasks, such as temperature, strain, and electric field.
We present a novel method for full vector magnetometry using nitrogen-vacancy (NV) centers. In contrast to conventional optically detected magnetic resonance techniques, our method employs two distinct broadband microwave pulses and measures them after transmission through the NV sensor medium, thus capturing the line splitting of the ground state triplet due to the Zeeman effect. Two orthogonally polarized microwave pulses allow resolving all magnetic field components independently by reading out differently oriented NV centers. Simulated data is analyzed using deep neural networks, whose efficacy we expect to translate very well to experiments. Our method yields sensitivities between 5 pT/Hz5~\mathrm{pT}/\sqrt{\mathrm{Hz}}5 pT/Hz​ and 100 pT/Hz100~\mathrm{pT}/\sqrt{\mathrm{Hz}}100 pT/Hz​ across different magnetic field vector components, while achieving approximately nT\mathrm{nT}nT accuracy at a signal-to-noise (SNR) ratio of 70 dB70~\mathrm{dB}70 dB. By being capable of accurately measuring magnetic fields down to 25 μT25~\mathrm{μT}25 μT, the need for a bias field beyond Earth's magnetic field is eliminated.
Shallow nitrogen-vacancy (NV) centers are promising candidates for high-precision sensing applications; these defects, when positioned a few nanometers below the surface, provide an atomic-scale resolution along with substantial sensitivity. However, the dangling bonds and impurities on the diamond surface result in a complex environment which reduces the sensitivity and is unique to each shallow NV center. To avoid the environment's detrimental effect, we apply feedback-based quantum optimal control. We first show how a direct search can improve the initialization and readout process. In a second step, we optimize microwave pulses for pulsed optically detected magnetic resonance (ODMR) and Ramsey measurements. Throughout the sensitivity optimizations, we focus on robustness against errors in the control field amplitude. This feature not only protects the protocols' sensitivity from drifts but also enlarges the sensing volume. The resulting ODMR measurements produce sensitivities below $1\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{T}$ ${\mathrm{Hz}}^{\ensuremath{-}\frac{1}{2}}$ for an 83% decrease in control power, increasing the robustness by approximately one third. The optimized Ramsey measurements produce sensitivities below 100 nT ${\mathrm{Hz}}^{\ensuremath{-}\frac{1}{2}}$ giving a twofold sensitivity improvement. Being on par with typical sensitivities obtained via single NV magnetometry, the complementing robustness of the presented optimization strategy may provide an advantage for other NV-based applications.
Abstract Quantum sensing based on nitrogen-vacancy centers in diamond has shown excellent properties. Combined with the imaging technique, it shows exciting practicability. Here, we demonstrate the simultaneously imaging technique of magnetic field and temperature using a wide-field quantum diamond microscope. We describe the operating principles of the diamond microscope and report its sensitivity (magnetic field ${\sim}1.8~\mu \mbox{T/Hz}^{1/2}$ ∼ 1.8 μ T/Hz 1 / 2 and temperature ${\sim}0.4~\mbox{K/Hz}^{1/2}$ ∼ 0.4 K/Hz 1 / 2 ), spatial resolution (1.3 μm), and field of view ( $400 \times 300~\mu \mbox{m}^{2}$ 400 × 300 μ m 2 ). Finally, we use the microscope to obtain images of an integrated cell heater and a PCB, demonstrating its ability in the application of magnetic field and temperature simultaneously imaging at wide-field.