The new tool could be used for characterising new magnetic materials and probing exotic quantum physical phenomena.
The technique builds on a platform already developed to probe magnetic fields with high precision, using tiny defects in diamond called nitrogen-vacancy (NV) centres. These defects consist of two adjacent places in the diamond’s orderly lattice of carbon atoms where carbon atoms are missing; one of them is replaced by a nitrogen atom, and the other is left empty. This leaves missing bonds in the structure, with electrons that are extremely sensitive to tiny variations in their environment, be they electrical, magnetic, or light-based.
Previous uses of single NV centres to detect magnetic fields have been extremely precise but only capable of measuring those variations along a single dimension, aligned with the sensor axis. But for some applications, such as mapping out the connections between neurons by measuring the exact direction of each firing impulse, it would be useful to measure the sideways component of the magnetic field as well.
The new method solves that problem by using a secondary oscillator provided by the nitrogen atom’s nuclear spin. The sideways component of the field to be measured nudges the orientation of the secondary oscillator. By knocking it slightly off-axis, the sideways component induces a kind of wobble that appears as a periodic fluctuation of the field aligned with the sensor, thus turning that perpendicular component into a wave pattern superimposed on the primary, static magnetic field measurement. This can then be mathematically converted back to determine the magnitude of the sideways component.
In order to read out the results, the researchers use an optical confocal microscope that makes use of a special property of the NV centres: When exposed to green light, they emit a red glow, or fluorescence, whose intensity depends on their exact spin state. These NV centres can function as qubits, the quantum-computing equivalent of the bits used in ordinary computing.