Investigations on Microwave Power Shift in Compact Vapor-Cell Atomic Clock

We employ a three-level Bloch vector model to simulate Ramsey fringes of our Rb vapor-cell atomic clock. The residual inhomogeneity of the microwave magnetic field across the buffer-gas cell is a key ingredient for correctly modelling the behavior at high microwave powers (pulse area > π/2. We add to this model different position-dependent frequency shifts that arise from external fields, and we identify inhomogeneous light-shifts as the dominant contribution for reproducing the measured clock frequency shifts as a function of microwave power. Other frequency shifts give negligible contributions.