Researchers at MIT have developed a new way to measure magnetic fields at the atomic scale with great accuracy, not only from top to bottom, but also from the side. The new tool could be useful in applications as diverse as the mapping of electrical pulses in a trigger neuron, the characterization of new magnetic materials, and the detection of exotic quantum physical phenomena.
The new approach is described today in the journal Letters of physical examination Yi-Xiang Liu, Graduate Student, Ashok Ajoy, Former Graduate Student, and Paola Cappellaro, Professor of Nuclear Science and Engineering.
The technique relies on a platform already developed to probe magnetic fields with great precision, exploiting tiny defects in diamond centers called empty nitrogen. These defects consist of two adjacent locations in the ordered array of carbon atoms of the diamond where carbon atoms are missing; one of them is replaced by one nitrogen atom and the other is left empty. This leaves missing bonds in the structure, with electrons extremely sensitive to minute variations in their environment, be they electrical, magnetic or light-based.
Previous uses of single NV centers to detect magnetic fields were extremely accurate, but only measured these variations in one dimension, aligned with the sensor axis. But for some applications, such as mapping the connections between neurons by measuring the exact direction of each trigger pulse, it would also be useful to measure the lateral component of the magnetic field.
The new method basically solves this problem by using a secondary oscillator provided by the nuclear spin of the nitrogen atom. The lateral component of the field to be measured changes the orientation of the secondary oscillator. By striking it slightly off-axis, the lateral component induces a kind of wobbling that appears as a periodic fluctuation of the field aligned with the sensor, thus transforming this perpendicular component into a superimposed wave pattern on the primary, static field magnetic. This can then be mathematically reconverted to determine the magnitude of the lateral component.
The method provides as much precision in this second dimension as in the first dimension, explains Liu, while using a single sensor, thus retaining its spatial resolution at the nanoscale. To read the results, researchers use a confocal optical microscope that exploits a particular property of NV centers: when they are exposed to green light, they emit a red glow, or fluorescence, whose intensity depends on the exact rotation state of their spin. . These NV centers can function as qubits, the equivalent in quantum computation of bits used in ordinary computing.
"We can distinguish the spin state of fluorescence," says Liu. "It's dark," producing less fluorescence ", it's a" one "state and, if it's bright, a" zero "" , she says. "If the fluorescence is a number between the two then the spin state is somewhere between" zero "and" one "."
The needle of a simple magnetic compass indicates the direction of a magnetic field, but not its strength. Some existing magnetic field measurement devices can do the opposite, by measuring the intensity of the field accurately in one direction, but they give no indication of the overall orientation of that field. This directional information is what the new detection system can provide.
In this new type of "compass", explains Liu, "we can say where it points depending on the brightness of the fluorescence" and variations in this brightness. The main field is indicated by the constant global brightness level, while the oscillation introduced by offsetting the magnetic field appears as a regular variation of this brightness, similar to a wave, which can then be measured accurately.
An interesting application for this technique would be to put the NV diamond centers in contact with a neuron, explains Liu. When the cell triggers its action potential to trigger another cell, the system must be able to detect not only the intensity of its signal, but also its direction, thus helping to map the connections and to see which cells are triggering. what others. Similarly, when testing new magnetic materials that may be suitable for storing data or other applications, the new system should allow a detailed measurement of the magnitude and orientation of the magnetic fields in the material.
Unlike other systems that require extremely low temperatures to operate, this new magnetic sensor system can work well at normal room temperature, says Liu, making it possible to test biological samples without damaging them.
The technology for this new approach is already available. "You can do it now, but first you have to take some time to calibrate the system," says Liu.
For now, the system provides only a measure of the total perpendicular component of the magnetic field, and not its exact orientation. "Now we are only extracting the total transverse component, we can not determine the direction," says Liu. But the addition of this third dimensional component could be achieved by introducing an added static magnetic field as a reference point. "As long as we can calibrate this reference field," she says, it would be possible to get all three-dimensional information about the field orientation and "there are many ways to do it".
Amit Finkler, a researcher in chemical physics at the Weizmann Institute in Israel, who did not participate in this work, said: "This is a high quality research. They get a sensitivity to transverse magnetic fields equal to that of parallel currents, which is impressive and encouraging for practical applications. "
Finkler adds: "As the authors write humbly in the manuscript, this is the first step towards nanometric-scale vector magnetometry, and it remains to be seen whether their technique can actually be applied to real samples, such as only molecules or systems of condensed matter. " However, he says, "the end result is that as a potential user / implementer of this technique, I am very impressed and, furthermore, encouraged to adopt and apply this scheme in my experimental setups. . "
A new tool simultaneously detects magnetic fields in different directions
Yi-Xiang Liu et al, Nanoscale Direct Current Vector Magnetometry via Ancilla-assisted Ascending Frequency Conversion, Letters of physical examination (2019). DOI: 10.1103 / PhysRevLett.122.100501, dx.doi.org/10.1103/PhysRevLett.122.100501