Magnetic imaging is a powerful tool for probing biological and physical systems. However, existing techniques either have poor spatial resolution compared to optical microscopy and are hence not generally applicable to imaging of sub-cellular structure (e.g., magnetic resonance imaging [MRI]1), or entail operating conditions that preclude application to living biological samples while providing sub-micron resolution (e.g., scanning superconducting quantum interference device [SQUID] microscopy2, electron holography3, and magnetic resonance force microscopy [MRFM]4). Here we demonstrate magnetic imaging of living cells (magnetotactic bacteria) under ambient laboratory conditions and with sub-cellular spatial resolution (400 nm), using an optically-detected magnetic field imaging array consisting of a nanoscale layer of nitrogen-vacancy (NV) colour centres implanted at the surface of a diamond chip. With the bacteria placed on the diamond surface, we optically probe the NV quantum spin states and rapidly reconstruct images of the vector components of the magnetic field created by chains of magnetic nanoparticles (magnetosomes) produced in the bacteria, and spatially correlate these magnetic field maps with optical images acquired in the same apparatus. Wide-field sCMOS acquisition allows parallel optical and magnetic imaging of multiple cells in a population with sub-micron resolution and >100 micron field-of-view. Scanning electron microscope (SEM) images of the bacteria confirm that the correlated optical and magnetic images can be used to locate and characterize the magnetosomes in each bacterium. The results provide a new capability for imaging bio-magnetic structures in living cells under ambient conditions with high spatial resolution, and will enable the mapping of a wide range of magnetic signals within cells and cellular networks5, 6.

Genomics-assisted breeding methods have been rapidly developed with novel technologies such as next-generation sequencing, genomic selection and genome-wide association study. However, phenotyping is still time consuming and is a serious bottleneck in genomics-assisted breeding. In this study, we established a high-throughput phenotyping system for sorghum plant height and its response to nitrogen availability; this system relies on the use of unmanned aerial vehicle (UAV) remote sensing with either an RGB or near-infrared, green and blue (NIR-GB) camera. We evaluated the potential of remote sensing to provide phenotype training data in a genomic prediction model. UAV remote sensing with the NIR-GB camera and the 50th percentile of digital surface model, which is an indicator of height, performed well. The correlation coefficient between plant height measured by UAV remote sensing (PHUAV) and plant height measured with a ruler (PHR) was 0.523. Because PHUAV was overestimated (probably because of the presence of taller plants on adjacent plots), the correlation coefficient between PHUAV and PHR was increased to 0.678 by using one of the two replications (that with the lower PHUAV value). Genomic prediction modeling performed well under the low-fertilization condition, probably because PHUAV overestimation was smaller under this condition due to a lower plant height. The predicted values of PHUAV and PHR were highly correlated with each other (r = 0.842). This result suggests that the genomic prediction models generated with PHUAV were almost identical and that the performance of UAV remote sensing was similar to that of traditional measurements in genomic prediction modeling. UAV remote sensing has a high potential to increase the throughput of phenotyping and decrease its cost. UAV remote sensing will be an important and indispensable tool for high-throughput genomics-assisted plant breeding.

Optically-detected magnetic resonance using Nitrogen Vacancy (NV) color centres in diamond is a leading modality for nanoscale magnetic field imaging, 1-3 as it provides single electron spin sensitivity, 4 three-dimensional resolution better than 1 nm, 5 and applicability to a wide range of physical 6-8 and biological 9 samples under ambient conditions. To date, however, NV-diamond magnetic imaging has been performed using "real space" techniques, which are either limited by optical diffraction to ≈250 nm resolution 10 or require slow, point-by-point scanning for nanoscale resolution, e.g., using an atomic force microscope, 11 magnetic tip, 5 or superresolution optical imaging. 12 Here we introduce an alternative technique of Fourier magnetic imaging using NV-diamond. In analogy with conventional magnetic resonance imaging (MRI), we employ pulsed magnetic field gradients to phase-encode spatial information on NV electronic spins in wavenumber or "k-space" 13 followed by a fast Fourier transform to yield real-space images with nanoscale resolution, wide field-of-view (FOV), and compressed sensing speed-up. 2Key advantages of NV-diamond Fourier magnetic imaging, relative to real-space imaging, are: (i) spatially multiplexed detection, 14 which enhances the signal-to-noise ratio (SNR) for typical NV centre densities; (ii) a high data acquisition rate that can be further boosted with compressed sensing; 15,16 and (iii) simultaneous acquisition of signal from all NV centres in the FOV, which allows probing of temporally correlated dynamics and provides isolation from system drift. As described below, we apply the Fourier technique using a relatively simple apparatus, and demonstrate one-dimensional imaging of individual NV centres with <5 nm resolution; two-dimensional imaging of multiple NV centres with ≈30 nm resolution; and two-dimensional imaging of nanoscale magnetic field patterns, with magnetic gradient sensitivity ~14 nT/nm/Hz 1/2 and spatial dynamic range (FOV/resolution) ~500. We also show that compressed sensing can accelerate the image acquisition time by an order-of-magnitude using sparse sampling followed by convex-optimization-based signal recovery. With further improvements, NV Fourier magnetic imaging may enable MRI with atom-scale resolution and centimeter FOV, with applications ranging from mapping the structure of individual biomolecules to functional MRI in living cells to studies of quantum phenomena in magnetic materials. Similar Fourier techniques could also be applied to NV-diamond imaging of nanoscale electric fields, temperature, and pressure, as well as to other optically-addressable solid-state spin systems.Schematic views of the Fourier magnetic microscope are shown in Figs. 1a and 1b.The diamond sample has a thin layer of NV centres at the surface created via ion implantation (see Methods). NV electronic spin states (Fig. 1c) are optically polarized with green illumination (λ=532 nm), coherently manipulated using resonant microwave fields applied by a microwave loop, and detected via sp...

Under ambient conditions, spin impurities in solid-state systems are found in thermally mixed states and are optically ''dark''; i.e., the spin states cannot be optically controlled. Nitrogen-vacancy (NV) centers in diamond are an exception in that the electronic spin states are ''bright''; i.e., they can be polarized by optical pumping, coherently manipulated with spin-resonance techniques, and read out optically, all at room temperature. Here we demonstrate a scheme to resonantly couple bright NV electronic spins to dark substitutional-nitrogen (P1) electronic spins by dressing their spin states with oscillating magnetic fields. This resonant coupling mechanism can be used to transfer spin polarization from NV spins to nearby dark spins and could be used to cool a mesoscopic bath of dark spins to near-zero temperature, thus providing a resource for quantum information and sensing, and aiding studies of quantum effects in many-body spin systems.

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