Subnanometre Resolution in Three-Dimensional Magnetic Resonance Imaging of Individual Dark Spins
Schematic of the NV-MRI technique depicting an NV centre in diamond situated in a confocal laser spot with nearby dark spins. A scanning magnetic tip is placed within 100 nm of the diamond surface. Applied microwave (MW) and radiofrequency (RF) signals allow for independent coherent control of the NV spin and dark spins. By scanning the magnetic tip, non-resonant dark spins (shown in black) are systematically brought into resonance with the RF signal (resonant spins shown in blue) and are measured via optically detected magnetic resonance of the NV sensor. [Figure reprinted by permission from Macmillan Publishers Ltd: M. S. Grinolds, M. Warner, K. De Greve, Y. Dovzhenko, L. Thiel, R. L. Walsworth, S. Hong, P. Maletinsky & A. Yacoby, "Subnanometre Resolution in Three-Dimensional Magnetic Resonance Imaging of Individual Dark Spins," Nature Nanotechnology 9, 279–284(2014) | doi:10.1038/nnano.2014.30c ©2014.]
Magnetic resonance imaging (MRI) has revolutionized biomedical science by providing non-invasive, three-dimensional biological imaging. However, spatial resolution in conventional MRI systems is limited to tens of micrometres, which is insufficient for imaging on molecular scales. In a recent article in Nature Nanotechnology, professors Ronald Walsworth and Amir Yacoby and a team of scientists from Harvard University, University of Basel, the Harvard-Smithsonian Center for Astrophysics, and the Vienna Center for Quantum Science and Technology report on developing a new MRI technique that provides subnanometre spatial resolution in three dimensions, with single electron-spin sensitivity. They demonstrate that their imaging method works under ambient conditions and can measure ubiquitous 'dark' spins, which constitute nearly all spin targets of interest. In this technique, the magnetic quantum-projection noise of dark spins is measured using a single nitrogen-vacancy (NV) magnetometer located near the surface of a diamond chip. The distribution of spins surrounding the NV magnetometer is imaged with a scanning magnetic-field gradient. To evaluate the performance of the NV-MRI technique, the scientists image the three-dimensional landscape of electronic spins at the diamond surface and achieve an unprecedented combination of resolution (0.8 nm laterally and 1.5 nm vertically) and single-spin sensitivity. Their measurements uncover electronic spins on the diamond surface that can potentially be used as resources for improved magnetic imaging. This NV-MRI technique is immediately applicable to diverse systems including imaging spin chains, readout of spin-based quantum bits, and determining the location of spin labels in biological systems.