  In this note we address the question of the $\theta$ dependence in non
abelian gauge theories from a pure quantum information point of view. The main
result is that the topological susceptibility is the quantum Fisher information
of the ground state and that the maximally efficient quantum estimator of
$\theta$ can be identified with the physical axion. In this setup the low
energy dynamics of the axion is fully determined by quantum estimation theory.
