The investigation on quantum radar requires accurate computation of the state vectors of the single-photon processes of the two-level system in free space. However, the traditional Weisskopf-Wigner (W-W) theory fails to deal with those processes other than spontaneous emission. To solve this problem, we provide a new method based on the renormalization theory. We evaluate the renormalized time-ordered Green functions associated with the single-photon processes, and relate them to the corresponding state vectors. It is found that the ultraviolet divergences generated by the Lamb shift and higher-order interactions can be systematically subtracted in the state vectors. The discussions on spontaneous emission and single-photon absorption are then presented to illustrate the proposed method. For spontaneous emission, we obtain the same results of the W-W theory. For single-photon absorption where W-W theory fails, we find that the two-level electric dipole first gets excited rapidly and then decays exponentially, and that the efficiency of the single-photon absorption declines as the bandwidth of the incident photon becomes narrow. The proposed method can improve the investigation on quantum radar.

中文翻译：

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重新规范化：自由空间中两层系统的单光子过程

对量子雷达的研究需要精确计算自由空间中两能级系统单光子过程的状态向量。但是，传统的Weisskopf-Wigner（WW）理论无法处理那些自发辐射以外的过程。为了解决这个问题，我们提供了一种基于重归一化理论的新方法。我们评估与单光子过程相关联的重新归一化的时间有序格林函数，并将它们与相应的状态向量相关联。发现在状态向量中可以系统地减去由兰姆位移和高阶相互作用产生的紫外线发散。然后对自发发射和单光子吸收进行了讨论，以说明所提出的方法。对于自发发射，我们获得了WW理论的相同结果。对于WW理论失败的单光子吸收，我们发现两能级电偶极子首先被迅速激发，然后呈指数衰减，并且随着入射光子带宽的变窄，单光子吸收的效率下降。所提出的方法可以改善对量子雷达的研究。