Spin dynamics under the influence of elliptically rotating fields: Extracting the field topology from time-averaged quantities

We focus on quantum systems that can be effectively described as a localized spin-s particle subject to a static magnetic field coplanar to a coexisting elliptically rotating time-periodic field. Depending on the values taken on by the static and rotating components, the total magnetic field shows two regimes with different topological properties. Along the boundary that separates these two regimes, the total magnetic field vanishes periodically in time and the system dynamics becomes highly nonadiabatic. We derive a relation between two time-averaged quantities of the system which is linked to the topology of the applied magnetic field. Based on this finding, we propose a measurable quantity that has the ability to indicate the topology of the total magnetic field without knowing a priori the value of the static component. We also propose a possible implementation of our approach by a trapped-ion quantum system. The results presented here are independent of the initial state of the system. In particular, when the system is initialized in a Floquet state, we find some interesting properties of the quasienergy spectrum which are linked to the topological change of the total magnetic field. Throughout the paper, the theoretical results are illustrated with numerical simulations for the case of a two-level quantum system.