  Consider the task of verifying that a given quantum device, designed to
produce a particular entangled state, does indeed produce that state. One
natural approach would be to characterise the output state by quantum state
tomography; or alternatively to perform some kind of Bell test, tailored to the
state of interest. We show here that neither approach is optimal amongst local
verification strategies for two qubit states. We find the optimal strategy in
this case and show that quadratically fewer total measurements are needed to
verify to within a given fidelity than in published results for quantum state
tomography, Bell test, or fidelity estimation protocols. We also give efficient
verification protocols for any stabilizer state. Additionally, we show that
requiring that the strategy be constructed from local, non-adaptive and
non-collective measurements only incurs a constant-factor penalty over a
strategy without these restrictions.
