“…The evolution of the wave packet can effectively be described by three so-called semi-classical variables: the semi-classical position r(t), momentum k(t), and composition (generalized polarization) η(t) of the wave packet in the two-fold degenerate subspace [30,31]. The evolution of those variables is described by semi-classical equations of motion [16,30,31] (see also SI), originating in electronic solid-state physics, but commonly applied to other waves such as light [32,33] or acoustic waves [34,35].…”