  In quantum computation with continous-variable systems, quantum advantage can
only be achieved if some non-Gaussian resource is available. Yet, non-Gaussian
unitary evolutions and measurements suited for computation are challenging to
realize in the lab. We propose and analyze two methods to apply a polynomial
approximation of any unitary operator diagonal in the amplitude quadrature
representation, including non-Gaussian operators, to an unknown input state.
Our protocols use as a primary non-Gaussian resource a single-photon counter.
We use the fidelity of the transformation with the target one on Fock and
coherent states to assess the quality of the approximate gate.
