Our bodies generate extremely weak magnetic fields as electric currents flow through the heart, brain and other tissues. These signals are used in magnetocardiography and magnetoencephalography to assess heart function and brain activity, respectively. These fields can be detected at room temperature using diamond sensors containing nitrogen-vacancy (NV) centers, in which a carbon atom is replaced by a nitrogen atom adjacent to an empty lattice site.
However, conventional NV-center sensors typically require watt-level lasers to detect the extremely weak biomagnetic fields, which are usually below the picotesla level. These high-power lasers generate significant heat, limiting how close the sensor can be placed to biological tissue. Since biomagnetic fields rapidly weaken with distance, overcoming thermal and close-proximity challenges is essential for practical biomagnetic sensing.
A research team led by Professor Takayuki Iwasaki from the Department of Electrical and Electronic Engineering, School of Engineering, Institute of Science Tokyo, Japan, has developed a diamond quantum magnetometer using a low-power laser of just 210 mW, a light-trapping diamond waveguide and a compact microwave antenna. The new sensor limits its temperature rise to only 13 K while allowing it to be placed just 2 mm (0.08 inches) from the sample, enabling close-proximity biomagnetic measurements without compromising thermal safety.
The paper was published in Applied Physics Letters on July 22, 2026, and was selected as a Featured Article.
"This achievement represents an important step toward practical biomagnetic measurements using diamond quantum sensors by simultaneously realizing room-temperature operation, low heat generation and short-range sensing," Iwasaki says.
Making weak laser light count
The sensor consists of a diamond containing a large number of negatively charged NV centers. It operates using Ramsey interferometry, in which the quantum spin state of the NV centers evolves between two microwave pulses. Laser pulses initialize and read out the spin state, allowing the magnetic field to be determined.
To maintain high sensitivity with a low-power laser, the researchers needed to use the excitation light much more efficiently. To achieve this, they designed a light-trapping diamond waveguide, which guides the laser light through the diamond by repeated total internal reflection. This allows the laser light to interact with many more NV centers, producing much stronger fluorescence without increasing the laser power.
As a result, the system collected approximately 20 mW of fluorescence from an incident laser power of 210 mW, corresponding to an optical power conversion efficiency of 9.5%. This efficiency allows the sensor to operate with laser powers about 10 times lower than those typically used for biomagnetic measurements.
Close-range sensing without overheating
The researchers also designed a compact printed-circuit-board microwave antenna, allowing the sensor to be positioned just 2 mm (0.08 inches) from the sample. Together, these design improvements enabled the sensor to achieve a magnetic field sensitivity of approximately 3 pT/√Hz in the 100–400 Hz frequency range while maintaining the diamond temperature at approximately 38°C (100°F), safely below the 42°C (108°F) biological heat-pain threshold.
When tested with a dry phantom that simulates brain magnetic signals, the sensor successfully detected a magnetic field of 77.7 pT at a sensor-to-sample distance of approximately 2.5 mm (0.1 inches).
The proposed Ramsey-based sensor overcomes the thermal limitations that have traditionally restricted pulsed quantum sensing, demonstrating that highly sensitive biomagnetic measurements can be performed safely at close range and making it a promising platform for future biomagnetic sensing applications.
"We anticipate this work will serve as a crucial stepping stone for the quantum sensing community, proving that practical, high-sensitivity pulsed measurements are viable for biological applications," Iwasaki says.
Publication details
Yuta Araki et al, A highly sensitive diamond NV magnetometer using Ramsey interferometry with a short sensor-to-sample distance, Applied Physics Letters (2026). DOI: 10.1063/5.0334709
Who's behind this story?
BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021. Full profile →
Bachelor's in mathematical biology, Master's in creative writing. Well-traveled with unique perspectives on science and language. Full profile →
Citation: Detecting the body's magnetic fields with a low-power Ramsey-based magnetometer (2026, July 22) retrieved 22 July 2026 from https://phys.org/news/2026-07-body-magnetic-fields-power-ramsey.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.