Collapses in quantum-classical probabilistically checkable proofs and the quantum polynomial hierarchy

Abstract: We investigate structural properties of quantum proof systems by establishing collapse results that uncover simplifications in their complexity landscape. We extend classical results such as the Karp-Lipton theorem to quantum polynomial hierarchy with quantum proofs and establish uniqueness preservation for quantum-classical probabilistically checkable proof systems. Our main contributions are threefold. First, we prove that restricting quantum-classical PCP systems to uniqueness does not reduce computational power: $\mathsf{UniqueQCPCP} = \mathsf{QCPCP}$ under $\mathsf{BQ}$-operator and randomized reductions, demonstrating robustness similar to the $\mathsf{UniqueQCMA} = \mathsf{QCMA}$ result. Second, we establish a non-uniform quantum analogue of the Karp-Lipton theorem, showing that if $\mathsf{QMA} \subseteq \mathsf{BQP}/\mathsf{qpoly}$, then $\mathsf{QPH} \subseteq \mathsf{Q\Sigma}_2/\mathsf{qpoly}$, extending the classical collapse theorem to quantum complexity with quantum advice. Third, we introduce a consistent variant of the quantum polynomial hierarchy ($\mathsf{CQPH}$) with consistency constraints across interaction rounds while maintaining product-state proofs, proving its unconditional collapse $\mathsf{CQPH} = \mathsf{CQ\Sigma}_2$. This contrasts with prior work on quantum-entangled polynomial hierarchy, showing that consistency rather than entanglement drives the collapse. These results contribute to understanding structural boundaries in quantum complexity theory and the interplay between constraint types in quantum proof systems.