Interacting boson problems can be QMA-hard

Computing the ground-state energy of interacting electron (fermion) problems has recently been shown to be hard for QMA, a quantum analogue of the complexity class NP. Fermionic problems are usually hard, a phenomenon widely attributed to the so-called sign problem occurring in Quantum Monte Carlo simulations. The corresponding bosonic problems are, according to conventional wisdom, tractable. Here, we discuss the complexity of interacting boson problems and show that they can also be QMA-hard. In addition, we show that the bosonic version of the so-called N-representability problem is QMA-complete, as hard as its fermionic version. As a consequence, these problems are unlikely to have efficient quantum algorithms.