Atomic phase delay in a rubidium atomic clock

The phase delay that exists between the oscillating atomic system and the probing microwave field in a rubidium atomic clock is examined. This phase delay must be accounted for in the feedback control circuitry of the clock in order to maximize the device´s short-term stability. It is found both experimentally and theoretically that the atomic phase delay can vary by more than 100°, and that in the regime of normal clock operation it is a rapidly changing function of microwave power. Defining P₀ as the normal operating power entering the clock cavity, it is shown that at P₀ the phase-delay sensitivity to microwave power variations is maximized and on the order of ten degrees per dB. It is unadvisable to try to operate the clock at some different microwave power where this sensitivity is smaller, since the amplitude of the first harmonic signal is also maximized at P₀