Singular constant control trajectories and transition degeneracies in a closed 4-level quantum system

We analyze in this paper controllability aspects for a 4-dimensional quantum system. The “strong regularity” (cf. [1]) has been proven to be sufficient condition for global controllability. A conjecture in the literature asks whether this condition (up to the introduction of an additive constant in the control field) is also necessary. We prove here a negative result. The result also applies to the study of singularity of trajectories associated to constant control fields. The theoretical result is supported by numerical simulations.