2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/209480Partially rectangular domains are compact two-dimensional Riemannian manifolds $X$, either closed or with boundary, that contain a flat rectangle or cylinder. In this paper we are interested in partially rectangular domains with ergodic billiard flow; examples are the Bunimovich stadium, the Sinai billiard or Donnelly surfaces. We consider a one-parameter family $X_t$ of such domains parametrized by the aspect ratio $t$ of their rectangular part. There is convincing theoretical and numerical evidence that the Laplacian on $X_t$ with Dirichlet or Neumann boundary conditions is not quantum unique ergodic (QUE). We prove that this is true for all $t \in [1,2]$ excluding, possibly, a set of Lebesgue measure zero. This yields the first examples of ergodic billiard systems proven to be non-QUE.11 pages, 1 figure. The paper, authored by Andrew Hassell, now includes an appendix by Andrew Hassell and Luc Hillairet, extending the result to all partially rectangular billiardsAnalysis of PDEsMathematical Physics35P20, 58J50text