Control for Schrödinger equations on 2-tori: rough potentials

For the Schrödinger equation, $(i \partial_t + Δ) u = 0$ on a torus, an arbitrary non-empty open set $Ω$ provides control and observability of the solution: $ \| u |_{t = 0} \|_{L^2 (\T^2)} \leq K_T \| u \|_{L^2 ([0,T] \times Ω)} $. We show that the same result remains true for $(i \partial_t + Δ- V) u = 0$ where $ V \in L^2 (\T^2) $, and $ \T^2 $ is a (rational or irrational) torus. That extends the results of \cite{AM}, and \cite{BZ4} where the observability was proved for $ V \in C (\T^2) $ and conjectured for $ V \in L^\infty (\T^2) $. The higher dimensional generalization remains open for $ V \in L^\infty (\T^n) $.

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