We discuss the problem of implementing generalized measurements (POVMs) withlinear optics, either based upon a static linear array or including conditionaldynamics. In our approach, a given POVM shall be identified as a solution to anoptimization problem for a chosen cost function. We formulate a generalprinciple: the implementation is only possible if a linear-optics circuitexists for which the quantum mechanical optimum (minimum) is still attainableafter dephasing the corresponding quantum states. The general principle enablesus, for instance, to derive a set of necessary conditions for the linear-opticsimplementation of the POVM that realizes the quantum mechanically optimalunambiguous discrimination of two pure nonorthogonal states. This extends ourprevious results on projection measurements and the exact discrimination oforthogonal states.
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