  Using a Hong-Ou-Mandel interferometer, we apply the techniques of quantum
process tomography to characterize errors and decoherence in a prototypical
two-photon operation, a singlet-state filter. The quantum process tomography
results indicate a large asymmetry in the process and also the required
operation to correct for this asymmetry. Finally, we quantify errors and
decoherence of the filtering operation after this modification.
