Stability of the Faber-Krahn inequality for the Short-time Fourier Transform

We prove a sharp quantitative version of the Faber--Krahn inequality for the short-time Fourier transform (STFT). To do so, we consider a deficit $δ(f;Ω)$ which measures by how much the STFT of a function $f\in L^2(\mathbb R)$ fails to be optimally concentrated on an arbitrary set $Ω\subset \mathbb R^2$ of positive, finite measure. We then show that an optimal power of the deficit $δ(f;Ω)$ controls both the $L^2$-distance of $f$ to an appropriate class of Gaussians and the distance of $Ω$ to a ball, through the Fraenkel asymmetry of $Ω$. Our proof is completely quantitative and hence all constants are explicit. We also establish suitable generalizations of this result in the higher-dimensional context.