Abstract: If very frequent measurements are
performed on a quantum system,
in order to ascertain whether it is still in its initial state,
transitions to other states are hindered and the quantum Zeno
effect takes place. This phenomenon stems from
general features of the Schroedinger equation that yield
quadratic behavior of the survival probability at short times.
However, the quantum Zeno effect (QZE) does not
necessarily freeze everything. On the contrary, for frequent
projections onto a multi-dimensional subspace, the system can
evolve away from its initial state, although it remains in the
subspace defined by the measurement. This continuing time
evolution within the projected subspace is named "quantum Zeno
dynamics" and has interesting features. In particular, it is
reversible for a wide class of systems.
We give an *elementary introduction* to these issues and then
recast the quantum Zeno effect in terms of an adiabatic
theorem, and view it as a consequence of the dynamical
coupling to another quantum system: in the strong coupling
limit the system is forced to evolve in a set of orthogonal
subspaces of the total Hilbert space and a dynamical superselection
rule arises . Some significant examples will be proposed and
their practical relevance discussed.
We focus on decoherence, irreversibility and decoherence-free