Optimal Rydberg State Choice for Rydberg Atom Microwave Sensing*

Rydberg electromagnetically induced transparency offers great possibilities for the development of atom-based SI-traceable microwave (MW) sensing and communication devices. It utilizes the Autler-Townes (AT) splitting resulting from the coupling of two Rydberg states by a MW field. To create reliable and accurate devices, sources of systematic uncertainty must be carefully quantified. In particular, knowing the conditions under which the AT splitting is linear with the MW field is of great importance. In this work, using cesium atoms in a vapour cell, we investigate non-linearities originating from multi-photon couplings between neighbouring Rydberg states. By studying four different Rydberg transitions in the same frequency range we show that those couplings can break the linearity and symmetry of the observed AT splitting.  We present a model which accurately predicts the behaviour of the AT splitting for any Rydberg transition accounting for multi-photon transitions. We also show that these couplings are strongly dependent on polarisation and use our model to advantageously determine the polarisation purity of the MW field.