@misc{oai:repo.qst.go.jp:00076040,
 author = {Masuyama, Yuta and Iwasaki, Takayuki and Hatano, Mutsuko and Ohshima, Takeshi and Masuyama, Yuta and Hatano, Mutsuko and Ohshima, Takeshi},
 month = {Jun},
 note = {One of the promising solid-state quantum sensors is nitrogen-vacancy (NV) centers in diamond which can maintain quantum state at room temperature and under atmospheric pressure condition. Further improvement of sensitivity of the sensor is needed to detect the magnetic field from the brain. Noise reduction and simplified system are key techniques especially for the practical use of highly sensitive DC magnetic sensor. In this study, we devised a digital filtering detection method without synchronization between detector and modulator which is a similar noise reduction effect as a lock-in detection technique. We implemented the detection method for the quantum diamond sensor system with the large sensor volume [1].
The Lock-in detection technique is a typical high sensitivity technique for DC magnetic measurement. There are several different implementations of lock-in detection technique for the quantum diamond sensor including microwave modulation, magnetic field modulation, and laser modulation. However, it is a challenging task to obtain high magnetic sensitivity with a compact sensor module because these lock-in detection techniques require a lock-in amplifier or synchronization system. We devised a digital filtering detection method which is an alternative method to the lock-in detection technique. Fluorescence from the NV center is modulated by frequency modulated microwave during optical detected magnetic resonance (ODMR) measurement. The fluorescence is analyzed by digital signal processing which achieves a similar noise reduction effect as a lock-in detection technique. Our method relies on a fact that ODMR converts frequency modulation to amplitude modulation. Thus, our method is applicable not only to the NV center system but also another color center system.
This work was supported by MEXT Q-LEAP, CAO PRISM, Japan.

[1] Y. Masuyama et al., Rev. Sci. Instrum. 89, 125007 (2018)., Gordon Research Conference "Quantum Sensing Applications in Metrology and Imaging"},
 title = {Implementation of digital filtering detection method  for portable highly sensitive quantum diamond magnetometer},
 year = {2019}
}