An obstruction to small-time local controllability for a bilinear Schr\'odinger equation

We consider the small-time local controllability in the vicinity of the ground state of a bilinear Schr\"odinger equation with Neumann boundary conditions. We prove that, when the linearized system is not controllable, the nonlinear system is not controllable, due to a quadratic obstruction involving the squared norm of the control's primitive. This obstruction has been known since 1983 for ODEs and observed for some PDEs since 2006. However, our situation is more intricate since the kernel describing the quadratic expansion of the solution is not twice differentiable. We thus follow a Fourier-based approach, closer to the one used for quadratic obstructions of fractional Sobolev regularity. In this Fourier-based approach, a challenge is to formulate a necessary and sufficient condition on the convolution kernel, for the quadratic form to be coercive. In previous studies, the coercivity was ensured by a signed asymptotic equivalent for the Fourier transform of the convolution kernel of the form $\widehat{K}(\omega) \sim \omega^{-2}$ as $|\omega| \to \infty$. In our case, $\widehat{K}$ is a distribution which has singularities and changes sign up to infinity. We still prove coercivity because one of the signs appears too infrequently.