Invariant measures and arithmetic quantum unique ergodicity

We classify measures on the locally homogeneous space Γ\ SL(2, ℝ) × L which are invariant and have positive entropy under the diagonal subgroup of SL(2, ℝ) and recurrent under L. This classification can be used to show arithmetic quantum unique ergodicity for compact arithmetic surfaces, and a similar but slightly weaker result for the finite volume case. Other applications are also presented. In the appendix, joint with D. Rudolph, we present a maximal ergodic theorem, related to a theorem of Hurewicz, which is used in theproof of the main result.