  The problem of simulating sparse Hamiltonians on quantum computers is well
studied. The evolution of a sparse N x N Hamiltonian H for time t can be
simulated using O(||Ht||poly(log N)) operations, which is essentially optimal
due to a no--fast-forwarding theorem. Here, we consider non-sparse Hamiltonians
and show significant limitations on their simulation. We generalize the
no--fast-forwarding theorem to dense Hamiltonians, ruling out generic
simulations taking time o(||Ht||), even though ||H|| is not a unique measure of
the size of a dense Hamiltonian $H$. We also present a stronger limitation
ruling out the possibility of generic simulations taking time poly(||Ht||,log
N), showing that known simulations based on discrete-time quantum walk cannot
be dramatically improved in general. On the positive side, we show that some
non-sparse Hamiltonians can be simulated efficiently, such as those with graphs
of small arboricity.
