  We present a new approach to simulating Hamiltonian dynamics based on
implementing linear combinations of unitary operations rather than products of
unitary operations. The resulting algorithm has superior performance to
existing simulation algorithms based on product formulas and, most notably,
scales better with the simulation error than any known Hamiltonian simulation
technique. Our main tool is a general method to nearly deterministically
implement linear combinations of nearby unitary operations, which we show is
optimal among a large class of methods.
