  We demonstrate the robustness of BosonSampling to imperfections in the linear
optical network that cause a small deviation in the matrix it implements. We
show that applying a noisy matrix $\tilde{U}$ that is within $\epsilon$ of the
desired matrix $U$ in operator norm leads to an output distribution that is
within $\epsilon n$ of the desired distribution in variation distance, where
$n$ is the number of photons. This lets us derive a sufficient tolerance each
beamsplitters and phaseshifters in the network.
  This result considers only errors that result from the network encoding a
different unitary than desired, and not other sources of noise such as photon
loss and partial distinguishability.
