  States supported by chaotic open quantum systems fall into two categories: a
majority showing instantaneous ballistic decay, and a set of quantum resonances
of classically vanishing support in phase space. We present a theory describing
these structures within a unified semiclassical framework. Emphasis is put on
the quantum diffraction mechanism which introduces an element of probability
and is crucial for the formation of resonances. Our main result are boundary
conditions on the semiclassical propagation along system trajectories.
Depending on whether the trajectory propagation time is shorter or longer than
the Ehrenfest time, these conditions describe deterministic escape, or
probabilistic quantum decay.
