Title: Determining Acceptance Possibility for a Quantum Computation is Hard for PH

Author: Stephen Fenner, University of Southern Maine
        Frederic Green, Clark University
        Steven Homer, Boston University
        Randall Pruim, Boston University

Date: April 2, 1998

Abstract:

It is shown that determining whether a quantum computation has a
non-zero probability of accepting is at least as hard as the
polynomial time hierarchy.  This hardness result also applies to
determining in general whether a given quantum basis state appears
with nonzero amplitude in a superposition, or whether a given quantum
bit has positive expectation value at the end of a quantum
computation.