  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 {\mu}T/Hz$^{1/2}$ at room temperature. Thus, this
simple alternative to existing AC magnetic field sensors paves the way for a
practical and feasible quantum sensor.
