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The quantum detection method measures small magnetic fields



March 15, 2019

(Nanowerk news) Scientists at MIT have developed a new way of measuring magnetic fields at the atomic scale with great precision, not only up and down, but also sideways. The new tool can be useful in applications as diverse as mapping electrical impulses inside the firing neuron, characterizing new magnetic materials and examining exotic quantum physical phenomena.

The new approach is described today in the journal Physical Review Letters ("Vector nanometry of dc magnetometers through Ancilla-Assisted Frequency Up-Conversion") in the work of doctoral student Yi-Xiang Liu, former graduate Ashok Ajoy, professor of nuclear science and engineering Paola Cappellaro.an experimental setup used to test their magnetic sensor system using green laser light for confocal microscopyAn experimental setup used by scientists to test their magnetic sensor system using green laser light for confocal microscopy. (Image: RLE)

This technique is based on a platform already developed for testing magnetic fields with high precision, using small defects in diamond nitrogen-vacancy centers. These defects consist of two adjacent places in an ordered network of diamonds with carbon atoms lacking carbon atoms; one of them is replaced with a nitrogen atom, and the other remains empty. This leaves missing bonds in the structure, with electrons that are extremely sensitive to small changes in their surroundings, whether electric, magnetic or based on light.

Previous applications of single NV centers to detect magnetic fields were extremely precise, but were able to measure these changes along a single dimension, aligned with the sensor's axis. But for some applications, such as mapping the connections between neurons by measuring the exact direction of each firing pulse, it would also be useful to measure the lateral component of the magnetic field.

Essentially, the new method solves this problem with the help of a secondary oscillator provided by the spin of a nitrogen atom. The lateral element of the measured field shifts the orientation of the secondary oscillator. By breaking it out of the axis slightly, the side component causes some kind of vibrations that appear as periodic fluctuations of the field aligned with the sensor, thus changing the perpendicular component of the wave pattern imposed on the basic, static magnetic field measurement. This can then be mathematically transformed back to determine the size of the side component.

This method provides such high precision in the second dimension as in the first dimension, explains Liu, still using a single sensor, thus maintaining spatial resolution at the nanoscale. To read the results, scientists use a confocal optical microscope that uses the special property of NV centers: when exposed to green light, they emit a red glow or fluorescence whose intensity depends on their exact spin state. These NV centers can function as qubits, quantum-equivalent bits used in ordinary computers.

"We can determine the state of spinning from fluorescence," explains Liu. "If it is dark", producing less fluorescence, "this is the" one "state, and if it is bright, then the" zero "state – he says. "If fluorescence is a number between, then the spin state is somewhere between" zero "and" one ".

The needle of a simple magnetic compass indicates the direction of the magnetic field, but not its strength. Some existing magnetic field measurement devices can work the other way around, measuring the field strength in exactly one direction, but do not say anything about the overall orientation of the field. This directional information is what the new detector system can provide.

In this new kind of "compass," says Liu, "we can tell where the brightness of fluorescence points it," and change this brightness. The primary field is indicated by a general, constant level of brightness, while the vibrations introduced by the knocking of the magnetic field outside the axis appear as a regular, wave-like variation of this brightness, which can then be accurately measured.

An interesting application of this technique would be to contact the NV diamond centers with the neuron, says Liu. When a cell triggers its action potential to trigger another cell, the system should be able to detect not only the intensity of its signal, but also its direction, thus helping to map the connections and see which cells trigger which others. Similarly, in testing new magnetic materials that may be suitable for data storage or other applications, the new system should allow detailed measurement of the magnitude and orientation of magnetic fields in the material.

Unlike some other systems that require extremely low temperatures for operation, this new magnetic sensor system can work well at normal room temperatures, says Liu, which allows testing of biological samples without damaging them.

The technology of this new approach is now available. "You can do it now, but first you have to spend some time calibrating the system," says Liu.

For now, the system only provides a measurement of the complete perpendicular magnetic field component, not its exact orientation. "Now we only extract the entire transverse component; we can not point the direction – says Liu. However, adding that the third dimension component can be made by adding an added static magnetic field as a reference point. "As long as we can calibrate this reference field," he says, it will be possible to get the full three-dimensional information about the orientation of the field and "there are many ways to do it".

Amit Finkler, a senior scientist in chemical physics at the Isizmann Institute in Israel, who was not involved in this work, says: "These are high-quality research. … They gain sensitivity to transverse magnetic fields as well as DC sensitivity for parallel fields, which is impressive and encourages practical applications. "

Finkler adds: "As the authors humbly write in the manuscript, this is indeed the first step towards vector magnetography at the nanoscale. It remains to be seen whether their technique can actually be applied to real samples, such as molecules or condensed matter systems. "However, he says:" The most important thing 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 configurations. "

While these studies aimed to measure magnetic fields, scientists say that the same basic methodology can be used to measure other properties of molecules, including rotation, pressure, electric fields, and other characteristics.


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