Compact quantum sensor achieves picotesla sensitivity

The magnetic fields measured in magnetoencephalography (MEG) are extremely weak, typically on the order of picoteslas or less. Detecting these signals requires the development of highly sensitive magnetic sensors that can operate at room temperature and be positioned close to biological tissues. A research group including Graduate Student Yuta Araki, Researcher Takeharu Sekiguchi, Specially Appointed Professor Mutsuko Hatano, and Professor Takayuki Iwasaki at the School of Engineering, Institute of Science Tokyo; Group Leader Tokuyuki Teraji of the Semiconductor Defect Design Group, Research Center for Electronic and Optical Materials, National Institute for Materials Science; Director Takeshi Ohshima of the Quantum Materials and Applications Research Center, Takasaki Institute for Advanced Quantum Science, National Institutes for Quantum Science and Technology; and Senior Manager Takayuki Shibata at Advanced Research and Innovation Center, DENSO CORPORATION, developed a highly sensitive quantum magnetic sensor based on Ramsey interferometry using nitrogen-vacancy (NV) centers in diamond. The study was published in Applied Physics Letters and selected as a Featured Article.

To date, continuous-wave optically detected magnetic resonance (cw-ODMR) has been the most widely used method for magnetic sensing with nitrogen-vacancy (NV) centers. However, transitioning to Ramsey interferometry, a pulsed measurement technique, is considered essential for achieving higher sensitivity. Conventional Ramsey-type diamond magnetometers, however, have required high-power laser excitation on the order of several watts to attain enhanced sensitivity, resulting in significant sensor heating. In addition, the microwave components used in these systems have typically been relatively large, preventing the sensor from being placed close to the sample. This has been a major limitation for the detection of biomagnetic fields, which decrease rapidly in strength with increasing distance from their source.

The research group developed a new Ramsey-type diamond quantum magnetic sensor that combines low heat generation with short-distance measurement capability. To improve the efficiency of excitation light utilization, the researchers adopted light-trapping diamond waveguide technology. By guiding excitation light through total internal reflection within the diamond, they enhanced the interaction between the light and the NV centers, enabling sufficient fluorescence signals to be obtained with much lower laser power.

As a result, the team achieved high fluorescence detection efficiency using only 210 milliwatts of laser power, compared with previous studies that required up to approximately 4 watts. They also developed a compact printed-circuit-board-based microwave antenna, reducing the minimum distance between the sensor and the sample to 2.0 mm. This represents more than a threefold reduction compared with previously reported Ramsey-type sensors. Because weak magnetic fields, including biomagnetic signals, attenuate rapidly with distance from their source, this reduction significantly improves effective measurement sensitivity.

For signal detection, the researchers implemented a system combining a double-quantum four-Ramsey sequence with lock-in detection. This approach reduced the influence of noise while achieving a sensitivity of 2.93 pT/√Hz in the 100-400 Hz frequency range. The temperature increase during operation was limited to approximately 13℃, and the sensor's operating temperature remained below 42℃, a commonly used benchmark for biological safety.

Using a dry phantom designed to simulate brain magnetic field distributions, the team successfully detected a weak magnetic field of 77.7 picoteslas with a high signal-to-noise ratio (>4.3) and without signal averaging at a distance of approximately 2.5 mm from the sensor.

Although additional signal integration will be required to detect sub-picotesla biomagnetic signals such as those generated by the human brain, the successful phantom experiments conducted in a configuration resembling biological measurements demonstrate the technology's strong potential for practical applications.

In the future, the technology could be applied to medical diagnostic devices such as magnetoencephalography (MEG) and magnetocardiography (MCG) systems. It may also contribute to basic life science research and drug discovery through non-invasive measurements of neural and muscular activity in small animals. Compact, highly sensitive quantum sensors that operate at room temperature have the potential to transform biomagnetic sensing, which has traditionally depended on large and costly facilities.

Looking ahead, the research group aims to further improve sensitivity by extending coherence times through enhancements in diamond crystal quality, advancing control of NV-center charge states, and reducing noise associated with reference light. The team also plans to develop a measurement system capable of long-term stable operation by improving resistance to resonance-frequency shifts caused by temperature fluctuations.

Journal Information
Publication: Applied Physics Letters
Title: A highly sensitive diamond NV magnetometer using Ramsey interferometry with a short sensor-to-sample distance
DOI: 10.1063/5.0334709