  We discuss the problem of implementing generalized measurements (POVMs) with
linear optics, either based upon a static linear array or including conditional
dynamics. In our approach, a given POVM shall be identified as a solution to an
optimization problem for a chosen cost function. We formulate a general
principle: the implementation is only possible if a linear-optics circuit
exists for which the quantum mechanical optimum (minimum) is still attainable
after dephasing the corresponding quantum states. The general principle enables
us, for instance, to derive a set of necessary conditions for the linear-optics
implementation of the POVM that realizes the quantum mechanically optimal
unambiguous discrimination of two pure nonorthogonal states. This extends our
previous results on projection measurements and the exact discrimination of
orthogonal states.
