We investigate in this work the numerical resolution of a quantum control problem; the specificity of the approach is that, instead of searching directly for the optimal laser intensity that drives the system toward its target, we consider here as main variable the evolution semigroup i.e. the set of propagators indexed with time. The precise form of the generator of the semigroup (e.g. dipolar) is then enforced as a constraint. We present both an algorithm and associated numerical results.