We analyze single particle coherence and interference in the presence of
particle loss and derive an inequality that relates the preservation of
coherence, the creation of superposition with the vacuum, and the degree of
particle loss. We find that loss channels constructed using linear optics form
a special subclass. We suggests a generalized spectroscopy where, in analogy
with the absorption spectrum, we measure a "coherence loss spectrum" and a
"superposition creation spectrum". The theory is illustrated with examples
