  In this paper we consider quantum interactive proof systems, i.e.,
interactive proof systems in which the prover and verifier may perform quantum
computations and exchange quantum messages. It is proved that every language in
PSPACE has a quantum interactive proof system that requires only two rounds of
communication between the prover and verifier, while having exponentially small
(one-sided) probability of error. It follows that quantum interactive proof
systems are strictly more powerful than classical interactive proof systems in
the constant-round case unless the polynomial time hierarchy collapses to the
second level.
