New Technique for High-Precision Measurement of Atomic-Scale Magnetic Fields

Scientists at MIT have devised a new technique for measuring atomic-scale magnetic fields with excellent precision, not just up and down but even sideways.

The innovative tool could be helpful in an extensive range of applications, such as characterization of new magnetic materials, mapping of electrical impulses within a firing neuron, and investigation of exotic quantum physical phenomena.

The new technique has been reported in the Physical Review Letters journal on March 15^th, 2019, in a paper by graduate student Yi-Xiang Liu, former graduate student Ashok Ajoy, and professor of nuclear science and engineering Paola Cappellaro.

The tool has been developed based on a platform already created to investigate magnetic fields with excellent precision, by making use of tiny defects in diamond, known as nitrogen-vacancy (NV) centers. These defects include two adjacent spots in the orderly lattice of carbon atoms in diamond at which carbon atoms are missing. A nitrogen atom replaces one spot and the other one is left empty, leaving behind missing bonds in the structure, with electrons that are highly sensitive to even small variations in their environment, whether they are magnetic, electrical, or light-based.

Although earlier uses of single NV centers for the detection of magnetic fields have been highly precise, they could measure only the variations along a single dimension, which was aligned with the sensor axis. However, for certain applications like mapping out the connections between neurons by evaluating the accurate direction of each firing impulse, it would be helpful to measure the sideways component of the magnetic field also.

Fundamentally, the new technique overcomes this drawback by using a secondary oscillator offered by the nuclear spin of the nitrogen atom. The sideways component of the field to be evaluated prods the secondary oscillator’s orientation. The sideways component knocks it slightly off-axis, thereby inducing a type of wobble that seems to be a periodic fluctuation of the field aligned with the sensor. This turns the perpendicular component into a wave pattern superimposed on the primary, static magnetic field measurement. This can be later mathematically converted back to establish the sideways component’s magnitude.

According to Liu, the technique offers as much precision in the second dimension as in the first dimension, even if only a single sensor is used, thereby retaining its nanoscale spatial resolution. The researchers read out the results by using an optical confocal microscope that exploits a unique property of the NV centers: Upon being exposed to green light, the centers emit a red glow, or fluorescence, the intensity of which is dependent on their exact spin state. The NV centers have the ability to function as qubits—the quantum-computing equivalent of the bits used in ordinary computing.

“We can tell the spin state from the fluorescence,” explained Liu. “If it’s dark,” producing less fluorescence, “that’s a ‘one’ state, and if it’s bright, that’s a ‘zero’ state,” she stated. “If the fluorescence is some number in between then the spin state is somewhere in between ‘zero’ and ‘one’.”

The direction of a magnetic field, and not its strength, is denoted by the needle of a simple magnetic compass. Certain prevalent devices for evaluating magnetic fields can perform the opposite—precisely evaluating the strength of the field along a single direction, but they do not denote anything about the overall orientation of that field. The new detector system can precisely denote that directional information.

According to Liu, in this new type of “compass, we can tell where it’s pointing from the brightness of the fluorescence,” as well as the variations in that brightness. The overall, steady brightness level denotes the primary field; in contrast, the wobble induced by knocking the magnetic field off-axis appears as a regular, wave-like variation of that brightness, which can then be accurately measured.

According to Liu, an attractive application for this method would be to place the diamond NV centers in contact with a neuron. When the cell triggers another cell by firing its action potential, the system should be in a position to detect not just the intensity of its signal but also its direction, thereby helping to map out the connections and observe which cells trigger which others. Likewise, while testing new magnetic materials that could be suitable for data storage or other applications, the new system must enable a thorough measurement of the orientation and magnitude of magnetic fields in the material.

In contrast to other systems that need extremely low temperatures to function, this new magnetic sensor system can operate well at ambient temperatures, stated Liu, rendering it possible to test biological samples without destroying them.

The technology for this new method is already at hand.

You can do it now, but you need to first take some time to calibrate the system.

Yi-Xiang Liu, Graduate Student, MIT

For the time being, the system can only measure the total perpendicular component of the magnetic field, and not its precise orientation. “Now, we only extract the total transverse component; we can’t pinpoint the direction,” stated Liu. However, it is possible to add that third-dimensional component by introducing an added, static magnetic field as a reference point. “As long as we can calibrate that reference field,” she stated, it would be feasible to obtain the complete three-dimensional information related to the orientation of the field, and “there are many ways to do that.”

This is high quality research. ... They obtain a sensitivity to transverse magnetic fields on par with the DC sensitivity for parallel fields, which is impressive and encouraging for practical applications.

Amit Finkler, Senior Scientist in Chemical Physics, Weizmann Institute, Israel

Finkler was not involved in this study.

As the authors humbly write in the manuscript, this is indeed the first step toward vector nanoscale magnetometry. It remains to be seen whether their technique can indeed be applied to actual samples, such as molecules or condensed matter systems.

Amit Finkler, Senior Scientist in Chemical Physics, Weizmann Institute, Israel

However, he added that “The bottom line is that as a potential user/implementer of this technique, I am highly impressed and moreover encouraged to adopt and apply this scheme in my experimental setups.”

Although this study was particularly targeted at evaluating magnetic fields, according to the researchers, it is possible to use the same fundamental methodology to evaluate other properties of molecules, such as electric fields, pressure, rotation, and other characteristics. The National Science Foundation and the U.S. Army Research Office supported this study.