Experimental Investigation of 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 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. 
Yoshihiro Nambu and Kazuo Nakamura