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

中文翻译：

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两量子位线性光量子滤波器的优化实现

我们设计了最佳干涉测量方案，以在计算基础上实现对角线的双量子位线性光学量子滤波器。滤波是通过在多端口线性光学干涉仪中编码量子位的两个光子的干涉来实现的，然后在滤波器的每个输出端口中调节单个光子的存在。因此，滤波器在符合基础上运行，类似于许多线性光学单一量子门。就降低过滤的总体成功概率而言，实现具有线性光学器件的滤波器可能需要额外的开销，并且最佳滤波器是使总体成功概率最大化的那些。我们详细讨论了对称实滤波器的情况，并将我们的分析扩展到非对称和复杂滤波器。