Quantum unique ergodicity for Maass forms on the modular surface

Let $\mathfrak{X}:=\mathcal{H}/SL(2,\mathbb{Z})$ be the modular surface, let $\Delta$ be its Laplacian, and let $\phi_i$ be a sequence of $L^2$-normalized eigenfunctions of $\Delta$ on $\mathfrak{X}$ with associated eigenvalues $\lambda_i\to\infty$. Define the probability measures $d_i z:=|\phi_i|^2\,dz$, where $dz$ is hyperbolic area measure. Quantum unique ergodicity for Maass forms. The measures $d_i z$ converge in the weak-* sense to the normalized measure $(3/\pi)\,dz$. The Maass-form case on the modular surface was settled by Soundararajan, building on earlier work of Lindenstrauss. The paper discusses this theorem as background while proving the analogous result for holomorphic integral-weight modular forms.