Experimental investigation of a nonideal two-qubit quantum-state filter by quantum process tomography

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