In this study, we developed a diamond quantum magnetometer based on Ramsey interferometry with a short sensor-to-sample distance. Conventional biomagnetic sensors with ensemble nitrogen-vacancy centers using continuous-wave optically detected magnetic resonance and Ramsey methods typically rely on watt-level lasers to achieve high sensitivity, resulting in thermal issues. In contrast, by employing the light-trapping diamond waveguide technique in a high-pressure and high-temperature diamond sample treated with electron beam irradiation, we obtained a high photon conversion efficiency of 9.5%, enabling us to simultaneously achieve a high sensitivity of 2.93(7) pT/Hz^1/2 in the 100-400 Hz frequency range and a minimal temperature increase of only approximately 13 K at a low laser power of 210 mW. Using a dry phantom designed to mimic magnetoencephalography signals, we measured a weak magnetic field of 77.7(2) pT without signal averaging at a sensor-to-sample distance of 2.5 mm. This short-distance measurement prevents severe spatial signal attenuation, yielding a high signal-to-noise ratio. The development here is crucial for practical biomagnetic applications based on Ramsey interferometry.
@article{arxiv.2603.13754,
title = {A Highly Sensitive Diamond NV Magnetometer Using Ramsey Interferometry with a Short Sensor-to-Sample Distance},
author = {Yuta Araki and Takeharu Sekiguchi and Yuji Hatano and Naota Sekiguchi and Chikara Shinei and Masashi Miyakawa and Takashi Taniguchi and Tokuyuki Teraji and Hiroshi Abe and Shinobu Onoda and Takeshi Ohshima and Takayuki Shibata and Mutsuko Hatano and Takayuki Iwasaki},
journal= {arXiv preprint arXiv:2603.13754},
year = {2026}
}