Nitrogen-vacancy (NV) centers in diamond can be used as highly sensitive quantum sensors for detecting magnetic fields at room temperature. Pulsed optically detected magnetic resonance (ODMR) is typically used to detect AC magnetic fields, but 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 a green CW laser and a microwave field, and can be used to detect DC or low-frequency (kHz-range) AC magnetic fields. We report a method that uses NV centers and CW-ODMR to detect high-frequency (MHz-range) AC magnetic fields. This method fully utilizes spin-1 properties of electron spins of NV centers. Unlike conventional methods, the proposed method does not require a pulse sequence; this greatly simplifies the procedure and apparatus needed for implementation. A sensitivity of 2.5 μ T / Hz is found for our present experimental apparatus, the sensitivity of which is currently limited by inhomogeneous broadening and low measurement contrast of samples used and by the low collection efficiency of the optical setup, both of which could be improved in the future. Thus, this simple alternative to existing AC magnetic field sensors paves the way for the development of a practical and feasible quantum sensor.
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