Trivial Low Energy States for Commuting Hamiltonians, and the Quantum   PCP Conjecture
  We consider whether or not Hamiltonians which are sums of commuting projectors have "trivial" ground states which can be constructed by a local quantum circuit of bounded depth and range acting on a product state. While the toric code only has nontrivial ground states, commuting projector Hamiltonians which are sums of two-body interactions have trivial ground states.   We define an "interaction complex" for a Hamiltonian, generalizing the interaction graph, and we show that if this complex can be continuously mapped to a 1-complex using a map with bounded diameter of pre-images then the Hamiltonian has a trivial ground state assuming one technical condition on the Hamiltonian (this condition holds for all stabilizer Hamiltonians, and we also prove the result for all Hamiltonians under an assumption on the 1-complex). While this includes cases considered by Ref., it also includes other Hamiltonians whose interaction complexes cannot be coarse-grained into the case of Ref.   One motivation for this is the quantum PCP conjecture. Many commonly studied interaction complexes can be mapped to a 1-complex after removing a small fraction of sites. For commuting projector Hamiltonians on such complexes, a trivial ground state for the Hamiltonian with those sites removed is a low energy trivial state for the original Hamiltonian. Such states can act as a classical witness to the existence of a low energy state. While this result applies only to commuting Hamiltonians, it suggests that to prove a quantum PCP conjecture one should consider interaction complexes which cannot be mapped to 1-complexes after removing a small fraction of cells. We define this more precisely below; in a sense this generalizes the idea of an expander graph. Surprisingly, such complexes do exist as will be shown elsewhere, and have useful properties in quantum coding theory. 
M. B. Hastings
quant-ph