We study the ergodic properties of quantized ergodic maps of the torus. It
is known that these satisfy quantum ergodicity: For almost all eigenstates,
the expectation values of quantum observables converge to the classical ph
ase-space average with respect to Liouville measure of the corresponding cl
assical observable.
The possible existence of any exceptional subsequences of eigenstates is an
important issue, which until now was unresolved in any example. The absenc
e of exceptional subsequences is referred to as quantum unique ergodicity (
QUE). We present the first examples of maps which satisfy QUE: Irrational s
kew translations of the two-torus, the parabolic analogues of Arnold's cat
maps. These maps are classically uniquely ergodic and not mixing. A crucial
step is to find a quantization recipe which respects the quantum-classical
correspondence principle.
In addition to proving QUE for these maps, we also give results on the rate
of convergence to the phase-space average. We give upper bounds which we s
how are optimal. We construct special examples of these maps for which the
rate of convergence is arbitrarily slow.