Optimization of the optical pumping and free evolution periods of times t_p and T. (a) Black squares represent the measured amplitude of the Ramsey signal (Amp) for different pumping times, while the red solid line represents the exponential fitting. (b) Black squares represent the measured amplitudes of the Ramsey signal at different free evolution times, where the amplitude decreases with increasing T because of the decoherence of the dark state. The red circles represent Δ ν , which is approximately 1/2T. The blue triangles represent the calculated values of Δ ν / Amp . The lines drawn in panel (b) are simply connections between the dots. The optimal t_p and T values are measured to be 500 μ s and 2 ms, respectively.