Quantum diamond microscope for dynamic imaging of magnetic fields
Jiashen Tang, Zechuan Yin, Connor Hart, John W. Blanchard, Jner Tzern Oon, Smriti Bhalerao, Jennifer M. Schloss, Matthew J. Turner, Ronald Lee Walsworth
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.
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.
The quantum diamond microscope (QDM) is a versatile, widely used platform for magnetic field imaging, but in applications it has been limited to broadband signals. This study integrates a QDM with a narrowband measurement protocol and a fast lock-in camera, enabling imaging of rf magnetic field patterns with micrometer-scale spatial resolution, single-hertz spectral resolution, and nanotesla sensitivity for each pixel. The rf QDM simultaneously images the amplitude, frequency, and phase of narrowband fields, for potential applications in micrometer-scale NMR imaging, ac-susceptibility mapping, impedance tomography, electronic circuit analysis, and spatial eddy-current inspection.
We introduce a double quantum (DQ) 4-Ramsey measurement protocol that enables wide-field magnetic imaging using nitrogen-vacancy (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 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.
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.
Abstract High-resolution, high-sensitivity microwave (MW) magnetic field imaging is indispensable for non-destructive integrated circuit (IC) testing, radio-frequency device characterization, and spintronic research. Yet, the practical utility of these techniques is severely constrained by the pervasive challenge of isolating weak magnetic signatures from intense optical and electronic noise, which fundamentally limits both acquisition speed and detection sensitivity. Here, we overcome this barrier by introducing a wide-field imaging scheme based on an ensemble of diamond nitrogen-vacancy (NV) centers, synergistically combined with digital lock-in amplification (DLA). By exploiting digital demodulation, the DLA precisely extracts the MW‑field response at a specific modulation frequency from background noise (e.g., laser intensity fluctuations), dramatically improving the signal-to-noise ratio (SNR). Consequently, our system attains a magnetic field sensitivity of 126 nT/Hz 1/2 . Critically, the unprecedented SNR permits a pixel dwell time of under one millisecond, allowing full-field images to be acquired within seconds-more than an order of magnitude faster than state-of-the-art NV-based wide-field techniques. This combination of speed, sensitivity, and micron-scale spatial resolution (1.6 µm) paves the way for quasi-real-time, non-invasive diagnostics of dynamic MW devices and integrated circuits. MAIN TEXT
We present a technique that uses an ensemble of nitrogen-vacancy (NV) centers in diamond to image magnetic fields with high spatio-temporal resolution and sensitivity. A focused laser beam is raster-scanned using an acousto-optic deflector (AOD) and NV center fluorescence is read out with a single photodetector, enabling low-noise detection with high dynamic range. The method operates in a previously unexplored regime, quasi-continuous-wave optically detected magnetic resonance (qCW-ODMR). In this regime, NV centers experience short optical pump pulses for spin readout and repolarization -- analogous to pulsed ODMR -- while the microwave field continuously drives the spin transitions. We systematically characterize this regime and show that the spin response is governed by a tunable interplay between coherent evolution and relaxation, determined by the temporal spacing between pump laser pulses. Notably, the technique does not require precise microwave pulse control, thus simplifying experimental implementation. To demonstrate its capabilities, we image time-varying magnetic fields from a microwire with sub-millisecond temporal resolution. This approach enables flexible spatial sampling and, with our diamond, achieves nT/Hz-level per-pixel sensitivity, making it well suited for detecting weak, dynamic magnetic fields in biological and other complex systems.
In modern-day quantum metrology, quantum sensors are widely employed to detect weak magnetic fields or nanoscale signals. Quantum devices, exploiting quantum coherence, are inevitably connected to physical constants and can achieve accuracy, repeatability, and precision approaching fundamental limits. As a result, these sensors have shown utility in a wide range of research domains spanning both science and technology. A rapidly emerging quantum sensing platform employs atomic-scale defects in crystals. In particular, magnetometry using nitrogen-vacancy (NV) color centers in diamond has garnered increasing interest. NV systems possess a combination of remarkable properties, optical addressability, long coherence times, and biocompatibility. Sensors based on NV centers excel in spatial resolution and magnetic sensitivity. These diamond-based sensors promise comparable combination of high spatial resolution and magnetic sensitivity without cryogenic operation. The above properties of NV magnetometers promise increasingly integrated quantum measurement technology, as a result, they have been extensively developed with various protocols and find use in numerous applications spanning materials characterization, nuclear magnetic resonance (NMR), condensed matter physics, paleomagnetism, neuroscience and living systems biology, and industrial vector magnetometry. In this chapter, NV centers are explored for magnetic sensing in a number of contexts. In general, we introduce novel regimes for magnetic-field probes with NV ensembles. Specifically, NV centers are developed for sensitive magnetometers for applications where microwaves (MWs) are prohibitively invasive and operations need to be carried out under zero ambient magnetic field. The primary goal of our discussion is to improve the utility of these NV center-based magnetometers.