The new tool could be useful in applications such as mapping the electrical impulses inside a firing neuron, 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.
Essentially, 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.
The method provides as much precision in this second dimension as in the first dimension, Yi-Xiang Liu, who worked on the project, explains, while still using a single sensor. It therefore retains its nanoscale spatial resolution.
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 needle of a simple magnetic compass tells the direction of a magnetic field, but not its strength. Some existing devices for measuring magnetic fields can do the opposite, measuring the field’s strength precisely along one direction, but they tell nothing about the overall orientation of that field. That directional information is what the new detector system can n provide.
In this new kind of “compass,” Liu says, “we can tell where it’s pointing from the brightness of the fluorescence,” and the variations in that brightness. The primary field is indicated by the overall, steady brightness level, whereas the wobble introduced by knocking the magnetic field off-axis shows up as a regular, wave-like variation of that brightness, which can then be measured precisely.
Unlike some other systems that require extremely low temperatures to operate, this new magnetic sensor system can work well at ordinary room temperature, Liu says, making it feasible to test biological samples without damaging them.
For now, the system only provides a measurement of the total perpendicular component of the magnetic field, not its exact orientation. “Now, we only extract the total transverse component; we can’t pinpoint the direction,” Liu says. But adding that third dimensional component could be done by introducing an additional static magnetic field as a reference point. “As long as we can calibrate that reference field,” she says, it would be possible to get the full three-dimensional information about the field’s orientation, and “there are many ways to do that.”
While this research was specifically aimed at measuring magnetic fields, the researchers say the same basic methodology could be used to measure other properties of molecules including rotation, pressure, electric fields, and other characteristics.
