  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
$\delta(f;\Omega)$ 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
$\Omega\subset \mathbb R^2$ of positive, finite measure. We then show that an
optimal power of the deficit $\delta(f;\Omega)$ controls both the
$L^2$-distance of $f$ to an appropriate class of Gaussians and the distance of
$\Omega$ to a ball, through the Fraenkel asymmetry of $\Omega$. Our proof is
completely quantitative and hence all constants are explicit. We also establish
suitable generalizations of this result in the higher-dimensional context.
