Optimal periodic control of spin systems: Application to the maximization of the signal-to-noise ratio per unit time

We propose an optimal control algorithm for periodic spin dynamics. This nontrivial optimization problem involves the design of a control field maximizing a figure of merit, while finding the initial and final states of the dynamics, which are not known but are subjected to specific periodic conditions. As an illustrative example, we consider the maximization of the signal-to-noise ratio per unit time of spin systems. In the case of a homogeneous spin ensemble and a very short control duration, we show numerically that the optimal field corresponds to the Ernst angle solution. We investigate the optimal control process for longer control durations and their sensitivity to offset inhomogeneities.