Noise cancellation in transient coherent population trapping by differential detection

We report a damped coherent population trapping (CPT) process, which has the very same mechanism as the coherent population beating (CPB). A differential CPB scheme is proposed based on the optical path delay, through which the left- and right-hand circularly polarized lights are modulated with a phase difference of π/2. The CPB signals oscillate with the frequency in a radio frequency (RF) range, which makes the detuning frequency equal to the splitting frequency between the hyperfine energy level of the ground state of cesium. The differential CPB signal with non-zero amplitude is obtained after the two signals being subtracted from each other, thereby improving the signal-to-noise ratio of the CPB. The measurements are explained well with a simple, four-level model and are interpreted as a simple harmonic oscillator interaction with four light fields. The Allan variance of the CPB atomic clock is measured in the conventional and the differential detection configurations. The 1-s stability of the differential CPB scheme is , which is better than that of under the conventional configuration. The results confirm that the differential CPB scheme can improve the signal-to-noise ratio and hence the short-term stability.