  We give a quantum interactive proof system for the local Hamiltonian problem
on n qubits in which (i) the verifier has a single round of interaction with
five entangled provers, (ii) the verifier sends a classical message on O(log n)
bits to each prover, who reply with a constant number of qubits, and (iii)
completeness and soundness are separated by an inverse polynomial in n. As the
same class of proof systems, without entanglement between the provers, is
included in QCMA, our result provides the first indication that quantum
multiprover interactive proof systems with entangled provers may be strictly
more powerful than unentangled-prover interactive proof systems. A
distinguishing feature of our protocol is that the completeness property
requires honest provers to share a large entangled state, obtained as the
encoding of the ground state of the local Hamiltonian via an error-correcting
code. Our result can be interpreted as a first step towards a multiprover
variant of the quantum PCP conjecture.
