Optimal implementation of two-qubit linear-optical quantum filters

We design optimal interferometric schemes for implementation of two-qubit linear optical quantum filters diagonal in the computational basis. The filtering is realized by interference two photons encoding qubits a multiport interferometer, followed conditioning on presence single photon each output port filter. filter thus operates coincidence basis, similarly to many unitary gates. Implementation with optics may require an additional overhead terms reduced overall success probability and are those that maximize probability. discuss detail case symmetric real extend our analysis also asymmetric complex filters.