MIT researchers have developed a new method to measure precisely atomic-scale magnetic fields. This approach could be applied in characterizing magnetic materials, mapping the electrical impulses inside a firing neuron, and probing exotic quantum physical phenomena.
This method is developed by graduate student Yi-Xiang Liu, former graduate student Ashok Ajoy, and professor of nuclear science and engineering Paola Cappellaro and published in the journal Physical Review Letters.
The method revolves around an existing platform that probes magnetic fields with high precision that use nitrogen-vacancy (NV) atoms or tiny defects in diamonds. Two adjacent places comprise these defects in the orderly lattice of carbon atoms in the diamond where there are missing carbon atoms. Nitrogen atom replaces one of them while the other is left empty.
Single NV centers are used in previous studies in detecting magnetic fields but have the capacity to measure variations in a single dimension, aligned with the axis of the sensor. However, it would be helpful to measure the sideways component of the magnetic field for some applications that include mapping out the connections between neurons by measuring the exact direction of each firing impulse.
The new method improves the first one through the use of a secondary oscillator provided by the nitrogen atom's nuclear spin. The orientation of the secondary oscillator is moved the sideways component of the field. "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," according to Sci Tech Daily.
The precision of the method in the second dimension is the same as that of the first dimension even with the use of a single sensor. An optical confocal microscope is used to read out the results. A red glow or fluorescence results when exposed to green light. These NV centers act as qubit or the equivalent of quantum computing in ordinary computing.
"We can tell the spin state from the fluorescence," Liu explains. "If it's dark," producing less fluorescence, "that's a 'one' state, and if it's bright, that's a 'zero' state," she says. "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 is determined by th needle of a simple magnetic compass. However, information about the strength of the magnetic field is not provided. On the contrary, there are devices that do the opposite. This new detector system can provide this directional information.
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.
