  We discuss and implement experimentally a method for characterizing quantum
gates operating on superpositions of coherent states. The peculiarity of this
encoding of qubits is to work with a non-orthogonal basis, and therefore some
technical limitations prevent us from using standard methods, such as process
tomography. We adopt a different technique, that relies on some a-priori
knowledge about the physics underlying the functioning of the device. A
parameter characterizing the global quality of the quantum gate is obtained by
\virtually" processing an entangled state.
