Robust diamond-defect magnetometry
In diamond’s otherwise perfect crystal lattice, point defects known as nitrogen–vacancy (NV) centers can be introduced. Consisting of a nitrogen atom (blue) adjacent to a vacant site, as shown in the figure, an NV center serves as a tiny magnetic field sensor. That’s because an external magnetic field splits the spin state of the electrons in the dangling bonds into long-lived Zeeman sublevels. By measuring the resonances between those states, researchers can probe magnetic signals in single cells or picoliter-sized samples. (See Physics Today, May 2018, page 21 , and the article by Lilian Childress, Ronald Walsworth, and Mikhail Lukin, Physics Today, October 2014, page 38 .)
Interest in NV-center magnetometry has been expanding to include more macroscopic applications as well, such as magnetic navigation (particularly useful in military settings where GPS isn’t available), geological surveys, and archaeological-site mapping (see Physics Today, March 2014, page 24 ). Now Danielle Braje and her colleagues at MIT Lincoln Laboratory have taken an important step toward creating an NV magnetometer that’s suitable for use in those real-world conditions.
A practical magnetic field sensor needs to provide fast and continuous measurements of the field vector over a wide range of values, and it should be insensitive to changes in temperature and other parameters that would otherwise necessitate frequent recalibrations. Braje and colleagues achieved those goals by designing feedback circuits that lock the frequency of a microwave drive field to each of the spin resonances in an ensemble of NV centers. Changing the magnetic field changes the resonance frequencies, and the microwave drive frequencies follow along.
Because an NV center occupies two adjacent sites on diamond’s tetrahedral lattice, the defect can have four possible orientations, each of which responds differently to a given field vector. By tracking the resonances of all four orientations, the researchers can reconstruct the field for magnitudes up to at least 11 mT. For comparison, the magnitude of Earth’s magnetic field at ground level ranges from 25 µT to 65 µT. Previous NV magnetometry schemes are limited to ranges of just a few microteslas. (H. Clevenson et al., Appl. Phys. Lett., in press. Image by James Hedberg, CC BY-NC-SA 3.0 .)