Quantum Information and the PCP Theorem

• Authors: Ran Raz
• Venue: FOCS '05 Proceedings of the 46th Annual IEEE Symposium on Foundations of Computer Science
• Year: 2005

Our main result is that the membership x \in SAT (for x of length n) can be proved by a logarithmic-size quantum state |\Psi \rangle , together with a polynomial-size classical proof consisting of blocks of length polylog(n) bits each, such that after measuring the state |\Psi \rangle the verifier only needs to read one block of the classical proof. This shows that if a short quantum witness is available then a (classical) PCP with only one query is possible. Our second result is that the class QIP/qpoly contains all languages. That is, for any language L (even nonrecursive), the membership x \in L (for x of length n) can be proved by a polynomial-size quantum interactive proof, where the verifier is a polynomial-size quantum circuit with working space initiated with some quantum state |\Psi _{L,n} \rangle(depending only on L and n). Moreover, the interactive proof that we give is of only one round, and the messages communicated are classical. The advice |\Psi _{L,n} \rangle given to the verifier can also be replaced by a classical probabilistic advice, as long as this advice is kept as a secret from the prover. Our result can hence be interpreted as: the class IP/rpoly contains all languages. For the proof of the second result, we introduce the quantum low-degree-extension of a string of bits. The main result requires an additional machinery of quantum lowdegree- test.