Scientists from Tokyo Tech have developed a highly sensitive diamond quantum magnetometer, a significant step towards practical ambient condition magnetoencephalography (MEG). MEG is a biomedical imaging technique used for mapping brain activity. Current MEG requires a magnetically shielded room, but this new magnetometer could enable MEG to work in normal environments, opening up possibilities for daily diagnosis and brain-machine interfaces. The magnetometer uses diamond quantum sensors with nitrogen-vacancy centers, offering better millimeter-scale resolution than conventional MEGs. The team, led by Associate Professor Naota Sekiguchi, achieved a record sensitivity, marking a significant step toward realizing ambient condition MEG.
Diamond Quantum Magnetometer: A Leap Towards Practical Magnetoencephalography
Magnetoencephalography (MEG) is a biomedical imaging technique that maps brain activity by recording magnetic fields produced by electrical currents generated by neurons in the brain. This technique requires highly sensitive magnetometers and currently necessitates a magnetically shielded room for operation. The ultimate goal is to develop MEG that can function in normal environments, thus enabling daily diagnosis, brain-machine interfaces, and fundamental research on brain function.
A team of researchers from Tokyo Tech has made a significant stride towards this goal by developing a highly sensitive diamond quantum magnetometer. This novel magnetometer utilizes nitrogen-vacancy (NV) centers and is based on continuous-wave optically detected magnetic resonance (CW-ODMR), a method that is simpler and easier than other techniques and can achieve millimeter-scale resolution.
Nitrogen-Vacancy Centers and Continuous-Wave Optically Detected Magnetic Resonance
NV centers are defects in the structure of a diamond, where a nitrogen atom substitutes for a carbon atom, next to a vacancy. These centers are promising candidates for realizing ambient condition MEG as they offer significantly better millimeter-scale resolution than conventional centimeter-scale MEGs.
In CW-ODMR, a continuous microwave field manipulates the spin states of the NV centers while they are illuminated by a laser. The intensity of the laser-induced fluorescence changes depending on the external magnetic field. By measuring these changes in fluorescence, the external magnetic field can be detected and measured.
The Novel Diamond Quantum Magnetometer
The team, led by Associate Professor Naota Sekiguchi from the Department of Electrical and Electronic Engineering at Tokyo Institute of Technology, developed a sensitive CW-ODMR-based diamond quantum magnetometer. This magnetometer uses a single crystalline diamond fabricated using a high-pressure high-temperature (HPHT) method. After HPHT synthesis, a piece of crystal was cut out parallel to the (111) crystal plane, and negatively charged NV centers were produced in the crystal using electron beam irradiation followed by annealing at 1,000 ℃.
The NV center ensemble was placed in the sensor head, designed to approach the target to about one millimeter with a sensing volume of 4 x 10-3 mm3. The ensemble was excited by a linearly polarized green laser with a wavelength of 532 nanometers, and a high refractive index hemispherical lens was used to enhance the collection efficiency of the laser-induced fluorescence.
Achievements and Future Directions
By carefully tuning the experimental conditions, the researchers achieved a record sensitivity of 9.4 ± 0.1 pT Hz-1/2 in the frequency range of 5 to 100 Hz, without a magnetic flux concentrator (MFC). Analysis of Allan deviation showed that the magnetometer can measure magnetic fields as low as 0.3 pT and maintain remarkable sensitivity for a long time. Furthermore, its design is suitable for practical applications such as for MEG of a living animal.
These achievements mark a significant step toward realizing ambient condition MEG with millimeter-scale resolution. Looking ahead, Sekiguchi concludes, “In the future, we plan to measure the MEG of animals using the sensors developed in this study and to realize MEG measurements with diamond quantum sensors. Ultimately, we aim to achieve MEG without the need for magnetic shielding.”
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