  We address the problem of energy-error tradeoff in the discrimination between
two linear passive quantum optical devices with a single use. We provide an
analytical derivation of the optimal strategy for beamsplitters and an
iterative algorithm converging to the optimum in the general case. We then
compare the optimal strategy with a simpler strategy using coherent input
states and homodyne detection. It turns out that the former requires much less
energy in order to achieve the same performances.
