  The problem of quantum state filtering consists of determining whether an
unknown quantum state, which is chosen from a known set of states, is either a
particular, specified state, or not. We consider this problem for the case that
the filtering procedure is required to be unambiguous, which necessitates
admitting inconclusive answers when the given states are not orthogonal. As an
application, we propose an efficient, probabilistic procedure for
distinguishing between sets of Boolean functions, which is an extension of the
Deutsch-Jozsa algorithm.
