Semiclassical Measures for Higher-Dimensional Quantum Cat Maps

Consider a quantum cat map M associated with a matrix A ? Sp(2n, Z), which is a common toy model in quantum chaos. We show that the mass of eigenfunctions of M on any nonempty open set in the position-frequency space satisfies a lower bound which is uniform in the semiclassical limit, under two assumptions: (1) there is a unique simple eigenvalue of A of largest absolute value and (2) the characteristic poly-nomial of A is irreducible over the rationals. This is similar to previous work (Dyatlov and Jin in Acta Math 220(2):297-339, 2018; Dyatlov et al. in J Am Math Soc 35(2):361-465, 2022) on negatively curved surfaces and (Schwartz in The full delocalization of eigenstates for the quantized cat map, 2021) on quantum cat maps with n = 1, but this paper gives the first results of this type which apply in any dimension. When condition (2) fails we provide a weaker version of the result and discuss relations to existing counterexamples. We also obtain corresponding statements regarding semiclassical measures and damped quantum cat maps.