We show that the NP-complete problems max cut and independent set can be formulated as the 2-local Hamiltonian problem as defined by Kitaev. The 5-local Hamiltonian problem was the first problem to be shown to be complete for the quantum complexity class QMA — the quantum analog of NP. Subsequently, it was shown that 3-locality is already sufficient for QMA-completeness. It is still not known whether the 2-local Hamiltonian problem is QMA-complete. Therefore it is interesting to determine what problems can be reduced to the 2-local Hamiltonian problem. Kitaev showed that 3-SAT can be formulated as a 3-local Hamiltonian problem. We extend his result by showing that 2-locality is sufficient in order to encompass NP.
The commuting local-Hamiltonian (CLH) problem is a restriction of the local-Hamiltonian problem, in which the terms of the Hamiltonian are required to pairwise commute. A long line of work has shown that the problem lies in
NP for certain families of commuting local Hamiltonians. Nevertheless, there has been no formal evidence against the possibility that the general CLH problem is
QMA-complete.
CLH is complete for the complexity class
QIMA, defined through quantum verifiers whose local gates commute. Therefore, CLH is
QMA-complete if and only if
QIMA=QMA. In this work, we introduce a classical-oracle analogue
QIMAO and construct a classical oracle
O such that
BQPO⊆QIMAO.
Since
BQPO⊆QMAO for any classical oracle
O, this implies
QIMAO=QMAO for our constructed oracle. Thus, our result provides relativized evidence against the possibility that the general commuting local-Hamiltonian problem is
BQP-hard, and hence also against the possibility that it is
QMA-complete.