Application of Quantum Precision Measurement Technology in Paleomagnetic Research
Updated 2023-04-11

Paleomagnetism is an interdisciplinary discipline between geology, physics, and geophysics. Paleomagnetism generally studies the direction and strength of the Earth's magnetic field, planetary launch and its evolution pattern during geological periods by measuring the natural residual magnetization intensity of rocks or ancient artifacts.


Rocks are a combination of natural minerals, and their residual magnetism generally comes from ferromagnetic minerals in rocks, containing primary and secondary remanent magnetism. The so-called primary remanent magnetism refers to the geomagnetic field information recorded when the rocks were formed. In contrast, the residual magnetism obtained after the formation of rocks is called secondary remanence, such as that obtained by rocks under the action of external magnetic fields (e.g., natural lightning strikes, erosion by running water and sand). Since paleomagnetism studies the characteristics of the geomagnetic field at the time of rock formation, accurate measurement of primary remanent magnetism becomes an important research tool.


Currently, rock magnetism is analyzed by measuring the net magnetic moment of large samples of millimeter to centimeter size. Common instruments for scientific analysis include superconducting petrographs and vibrating sample magnetometers. However, at the submicron scale, geological samples are usually inhomogeneous in mineralogy and texture, with only a small fraction of ferromagnetic particles carrying residual magnetization. Therefore, characterizing rock magnetism in this context requires a technique that can image magnetic fields at the nanoscale of space and with high sensitivity. For example, scanning superconductivity microscopy (SQUID), magnetoresistive microscopy, and Hall microscopy, which are being widely used, are examples.



(a) Quantum diamond microscopy at Harvard University (b) Measurement of residual magnetization in geological samples


In 2011, researchers demonstrated that nitrogen-vacancy chromatic cores (NV chromatic cores for short) in diamond can be used for magnetic imaging on the submicron scale.In 2017, R.L. Walsworth et al. at Harvard University used a self-built quantum diamond microscope based on NV chromatic cores to achieve imaging of rock magnetic fields with a metric spatial resolution of 5 um and a field-of-view range of 4 mm.By By reducing the distance between the diamond and the sample (≤10 um), a magnetic moment sensitivity of 10-16 A-m2 was achieved, which is comparable to and even surpasses the mainstream equipment such as SQUID, magnetoresistive microscope, and Hall microscope. In addition, the quantum diamond microscope also has the advantage of optical imaging function and fast imaging speed.   


It can be seen that in the detection and analysis of geological and magnetic meteorites, quantum diamond microscopy shows great potential for application, opening up a new path for weak magnetic imaging. With the continuous human exploration of the Moon, Mars and other deep space fields, quantum diamond microscopy will also be applied in the characterization and analysis of lunar rocks and Martian rocks.


(a) Schematic diagram of the structure of the quantum diamond microscope (b) Diamond placed on the sample


The general structure of the quantum diamond microscope is shown in the figure (a) above. The entire probe is placed in a three-dimensional Helmholtz coil, and its function is to apply an external magnetic field to desimulate the NV color center energy level in the diamond. During the experiment, the diamond is pressed against the surface of the rock sample, as shown in the figure (b) above. The working principle of the quantum diamond microscope is briefly described below.


First, the initialization of the NV color center quantum state within the diamond is achieved by using laser focusing. Then the frequency of the input microwave is scanned and acted on the NV color center by the radiation antenna. When the microwave frequency resonates with the energy level of the NV color center, the ms=±1 state of the NV color center reverses with the Buju number of the ms=0 state and the fluorescence intensity decreases. The fluorescence signal that varies with the microwave frequency in the imaging region is collected by the objective lens and imaged by the collection optical path to the sCMOS camera.



In the experiment, the imaging area is set to M × N pixel points by the sCMOS camera, and each time the picture acquisition is performed is equivalent to M × N pixel points for fluorescence signal detection at the same time. After the data acquisition of all frequency points is completed, the complete optical detection magnetic resonance spectrum (ODMR spectrum for short) is obtained, and the magnetic field intensity of each pixel point is solved by the corresponding data operation. Finally, the position of each pixel point is corresponded to the magnetic field intensity, and the magnetic field intensity of each pixel point is distinguished by color to realize two-dimensional magnetic field imaging.


CIQTEK - Quantum Diamond Microscope



In 2022, the "Quantum Diamond Microscope" independently developed by CIQTEK has been released to the world, which is an advanced commercial quantum precision measurement instrument based on diamond NV color center technology with advantages of high spatial resolution, large field of view and fast imaging speed, and is expected to generate new scientific growth points in the field of paleomagnetism. It also has broad application prospects in many fields such as semiconductors, materials science, cell magnetism, and in vitro diagnostics.