We propose a scheme to directly measure the entangled W states with cross-Kerr nonlinearity mediums. We find that a well-designed structure can measure three-photon, four-photon and even N-photon W states in one step. Only PBSs and cross-Kerr nonlinearity mediums are used in this scheme, which is feasible for experiments. We explore the measurement of the three-photon W states and extend it to four-photon and N-photon W states.

中文翻译：

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跨克尔非线性的一步一步直接测量W纠缠态

我们提出了一种使用交叉-Kerr非线性介质直接测量纠缠W态的方案。我们发现，设计良好的结构可以一步测量三光子，四光子甚至N光子的W态。该方案仅使用PBS和Cross-Kerr非线性介质，这对于实验是可行的。我们探索三光子W态的测量，并将其扩展到四光子和N光子W态。