  We used quantum process tomography to investigate and identify the function
of a nonideal two-qubit quantum-state filters subject to various degree of
decoherence. We present a simple decoherence model that explains the
experimental results and point out that a beamsplitter followed by a
post-selection process is not, as commonly believed, a singlet-state filter. In
the ideal case it is a triplet-state filter.
