Geometric driving of two-level quantum systems

Abstract

We investigate a class of cyclic evolutions for driven two-level quantum systems (effective spin 12) with a particular focus on the geometric characteristics of the driving and their specific imprints on the quantum dynamics. By introducing the concept of geometric driving curvature for any field trajectory in the parameter space, we are able to unveil underlying patterns in the overall quantum behavior: The knowledge of the driving curvature provides a nonstandard and fresh access to the interrelation between field and spin trajectories, and the corresponding quantum phases acquired in nonadiabatic cyclic evolutions. In this context, we single out setups in which the driving field curvature can be employed to demonstrate a pure geometric control of the quantum phases. Furthermore, the driving field curvature can be naturally exploited to introduce the geometrical torque and derive a general expression for the total quantum phase acquired in a cycle. Remarkably, such relation allows to access the mechanisms controlling the changeover of the quantum phase across a topological transition and to disentangle the role of the spin and field topological windings. As for implementations, we discuss a series of physical systems and platforms to demonstrate how the geometric control of the quantum phases can be realized for pendular field drivings. This includes setups based on superconducting islands coupled to a Josephson junction and inversion-asymmetric nanochannels with suitably tailored geometric shapes.