Nitrogen-vacancy (NV) centers in diamond are considered sensors for detecting magnetic fields. Pulsed optically detected magnetic resonance (ODMR) is typically used to detect AC magnetic fields; however, this technique can only be implemented after careful calibration that involves aligning an external static magnetic field, measuring continuous-wave (CW) ODMR, determining the Rabi frequency, and setting the microwave phase. In contrast, CW-ODMR can be simply implemented by continuous application of green CW laser and a microwave filed. In this letter, we report a method that uses NV centers and CW-ODMR to detect AC magnetic fields. Unlike conventional methods that use NV centers to detect AC magnetic fields, the proposed method requires neither a pulse sequence nor an externally applied DC magnetic field; this greatly simplifies the procedure and apparatus needed to implement this method. This method provides a sensitivity of 2.5 µT/ Hz at room temperature. Thus, this simple alternative to existing AC magnetic field sensors paves the way for a practical and feasible quantum sensor.

Recently we have demonstrated AC magnetic field sensing scheme using a simple continuous-wave optically detected magnetic resonance of nitrogen-vacancy centers in diamond [Appl. Phys. Lett. 113, 082405 (2018)]. This scheme is based on electronic spin double resonance excited by continuous microwaves and radio-frequency (RF) fields. Here we measured and analyzed the double resonance spectra and magnetic field sensitivity for various frequencies of microwaves and RF fields. As a result, we observed a clear anticrossing of RF-dressed electronic spin states in the spectra and estimated the bandwidth to be approximately 5 MHz at the center frequency of 9.9 MHz.

We demonstrate control of all the three transitions among the ground state sublevels of NV centers by applying magnetic driving fields. To address the states of a specific NV axis among the four axes, we apply a magnetic field orthogonal to the NV axis. We control two transitions by microwave pulses and the remaining transition by radio frequency (RF) pulses. In particular, we investigate the dependence of Rabi oscillations on the frequency and intensity of the RF pulses. In addition, we perform a π pulse by the RF pulses and measured the coherence time between the ground state sublevels. Our results pave the way for control of NV centers for the realization of quantum information processing and quantum sensing.

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