It is a folklore result in arithmetic quantum chaos that quantum
unique ergodicity on the modular surface with an effective rate of
convergence follows from subconvex bounds for certain triple product
-functions.
The physical space manifestation of this result, namely the equidistribution of mass
of Hecke–Maaß cusp forms, was proven to follow from subconvexity by Watson,
whereas the phase space manifestation of quantum unique ergodicity has
only previously appeared in the literature for Eisenstein series via work
of Jakobson. We detail the analogous phase space result for Hecke–Maaß
cusp forms. The proof relies on the Watson–Ichino triple product formula
together with a careful analysis of certain archimedean integrals of Whittaker
functions.