  Optical detection of spin state has been widely used for the solid state spin
qubit in the application of quantum information processing. The signal contrast
determines the accuracy of quantum state manipulation, sensitivity of quantum
sensing and resolution of quantum imaging. Here, we demonstrated a time-gated
fluorescence detection method for enhancing the spin state signal contrast of
nitrogen vacancy (NV) center in diamond. By adjusting the delay between time
gate and the excitation laser pulse, we improved both the signal contrast and
signal-to-noise ratio for NV spin detection. An enhancement ratio of 1.86 times
was reached for the signal contrast. Utilizing the time-gated fluorescence
detection, we further demonstrated a high contrast quantum imaging of
nanoparticle's stray magnetic field. Without any additional manipulation of the
quantum state, we expect that this method can be used to improve the
performance of various applications with NV center.
