  We first show that quantum resonant states observe particle number
conservation and hence are consistent with the probabilistic interpretation of
quantum mechanics. We then present for a class of quantum open systems, a
resonant-state expansion of the sum of the retarded and advanced Green's
functions. The expansion is given purely in terms of all discrete eigenstates
and does not contain any background integrals. Using the expansion, we argue
that the Fano asymmetry of resonance peaks is interpreted as interference
between discrete eigenstates. We microscopically derive the Fano parameters for
several cases.
