  The technologies of quantum information and quantum control are rapidly
improving, but full exploitation of their capabilities requires complete
characterization and assessment of processes that occur within quantum devices.
We present a method for characterizing, with arbitrarily high accuracy, any
quantum optical process. Our protocol recovers complete knowledge of the
process by studying, via homodyne tomography, its effect on a set of coherent
states, i.e. classical fields produced by common laser sources. We demonstrate
the capability of our protocol by evaluating and experimentally verifying the
effect of a test process on squeezed vacuum.
