Frequency-Tunable AC Magnetometry by Using Electronic Spin Triple-Resonance of Nitrogen-Vacancy Center in Diamond

Nitrogen-vacancy (NV) center in diamond is expected to be a practical quantum sensor with high sensitivity and high spatial resolution due to the room-temperature long spin coherence time and atomic-scale size^[1]. Quantum sensing using NV centers is implemented by measuring spin-dependent photoluminescence (PL) and manipulating spin state by microwave (MW). Pulsed-optically detected magnetic resonance (pulsed-ODMR) is typically used for AC magnetometry^[2,3]. Although achieving high sensitivity, it can be disadvantage to require sophisticated setup, rapid control and measurements, strong MW pulse irradiations and its calibration. Recently, our group proposed and demonstrated MHz-range AC magnetic field sensing using continuous-wave ODMR (CW-ODMR) which is simpler than pulsed-ODMR ^[4,5]. We utilized radio-frequency (RF)-dressed states of NV electronic spins by applying MW and RF magnetic fields (double resonance). However, it is difficult to change the frequency of detectable AC magnetic fields because of an intrinsic strain field in a diamond.<br/>In this study, to avoid such potential drawback, we propose frequency-tunable AC magnetic field sensing by CW-ODMR based on electronic spin triple resonance. In this scheme, the MHz-range magnetic field (target RF) is applied to resonant transition between RF-dressed states generated by another RF field (control RF). Consequently, RF-doubly dressed states is generated and observed by CW-ODMR measurements to estimate the amplitude of target RF field. The target frequency is expected to be varied by controlling the RF-dressed states with changing the control RF field amplitude.<br/>The NV center has spin-1 system described by eigenstates B, D, 0 under applying DC magnetic field perpendicular to the specific NV axis of it. When MW induces the transition from 0 to B(D), the PL intensity decrease (CW-ODMR). By measuring CW-ODMR spectrum with control RF field resonant to B-D transition (7.5 MHz), we observe four dips due to the splitting of B and D levels (RF-dressed states). As increasing the amplitude of control RF field, the splitting energy became larger. Furthermore, we applied the target RF field resonant to the lower transition between the RF-dressed states (5.4 MHz). Consequently, the energy levels of RF-dressed states were split into eight levels. This result demonstrates that RF-doubly dressed states were generated. By sweeping the frequency of target RF field, the anti-crossing structure was observed near the resonant frequency between the RF-dressed states. We found that the splitting energy of the anti-crossing spectra linearly increased as increasing the target RF amplitude. This result demonstrate that the amplitude of the target RF field can be estimated by measuring the splitting energy. Moreover, it is found that the detectable frequency of the target RF field can be tuned by changing the amplitude of the control RF. The obtained signal was larger than our previous work utilizing the RF-dressed states because the coherence time was extended by applying two different driving fields. This result shows that the sensitivity can be improved compared with that for the sensor based on the double resonance.<br/>Acknowledgement<br/>This work was supported by MEXT Q-LEAP (No. JPMXS0118067395), MEXT KAKENHI (No. 18H01502, 20H05661, 22H01558), and CSRN, Keio University. This work was supported by Leading Initiative for Excellent Young Researchers MEXT Japan and JST presto (Grant No. JPMJPR1919) Japan and Kanazawa University SAKIGAKE Project 2020. YM acknowledges support of Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (20H05661).<br/>Reference<br/>[1] L. Rondin, <i>et al.</i>, Rep. Prog. Phys. <b>77</b>, 056503 (2014).<br/>[2] J. R. Maze, <i>et</i> <i>al</i>., Nature <b>455</b>, 644 (2008).<br/>[3] L. M. Pham, <i>et</i> <i>al</i>., Phys. Rev. B <b>86</b>, 045214 (2012).<br/>[4] S. Saijo, <i>et al</i>., Appl. Phys. Lett. <b>113</b>, 082405 (2018).<br/>[5] T. Yamaguchi, <i>et</i> <i>al</i>., Jpn. J. Appl. Phys. <b>58</b>, 100901 (2019).