Graph states as ground states of many-body spin- 1∕2 Hamiltonians

We consider the problem of whether graph states can be ground states of local interaction Hamiltonians. For Hamiltonians acting on n qubits that involve at most two-body interactions, we show that no n-qubit graph state can be the exact, nondegenerate ground state. We determine for any graph state the minimal d such that it is the nondegenerate ground state of a d-body interaction Hamiltonian, while we show for d^′-body Hamiltonians H with d^′<d that the resulting ground state can only be close to the graph state at the cost of H having a small energy gap relative to the total energy. When allowing for ancilla particles, we show how to utilize a gadget construction introduced in the context of the k-local Hamiltonian problem, to obtain n-qubit graph states as nondegenerate (quasi)ground states of a two-body Hamiltonian acting on n^′>n spins.