  The nitrogen-vacancy (NV) center is a promising candidate to realize
practical quantum sensors with high sensitivity and high spatial resolution,
even at room temperature and atmospheric pressure. In conventional
high-frequency AC magnetometry with NV centers, the setup requires a pulse
sequence with an appropriate time synchronization and strong microwave power.
To avoid these practical difficulties, AC magnetic field sensing using
continuous-wave opticallydetected magnetic resonance (CW-ODMR) was recently
demonstrated. That previous study utilized radio frequency (RF) dressed states
generated by the coherent interaction between the electron spin of the NV
center and the RF wave. However, the drawback of this method is that the
detectable frequency of the AC magnetic fields is fixed. Here, we propose and
demonstrate frequency-tunable magnetic field sensing based on CW-ODMR. In the
new sensing scheme, we obtain RF double-dressed states by irradiation with a RF
field at two different frequencies. One creates the RF dressed states and
changes the frequency of the target AC field. The other is a target AC field
that induces a change in the CW-ODMR spectrum by generating the RF
double-dressed states through coherent interaction with the RF dressed states.
The sensitivity of our method is estimated to be comparable to or even higher
than that of the conventional method based on a RF field with a single
frequency. The estimated bandwidth is 7.45 MHz, higher than that of the
conventional method using the RF dressed states. Our frequency-tunable magnetic
field sensor based on CW-ODMR paves the way for new applications in diamond
devices.
