  We study the observable properties of quantum systems which involve a quantum
continuum as a subpart. We show in a very general way that in any system, which
consists of at least two isolated states coupled to a continuum, the spectral
function of one of the states exhibits an isolated zero at the energy of the
other state. Several examples of quantum systems exhibiting such isolated zeros
are discussed. Although very general, this phenomenon can be particularly
useful as an indirect detection tool for the continuum spectrum in the lab
realizations of quantum critical behavior.
