推导自发参量下转换双光子态的数学表达式，并给出其几率解释。

Then state function of SPDC biphoton light field was derived and a probability explanation was given.

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相关的自发参量上转换和下转换光子表现出光束亮度高、空间发散小以及在很宽频谱范围内可调谐等特点。

The correlated spontaneous up conversion and down conversion photons exhibits a high beam brightness, small spatial divergence, and broad spectral tunability.

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