光子在量子力学基础问题、量子通信和量子计算等研究领域起着非常重要的作用。可是，光子是什么仍是一个公开的问题。The problem of the photon is best summarized by a 1951 quote from Albert Einstein:“In 50 years of pondering, I still have not come closer to an answer for the question what light quanta are. Today every Tom, Dick and Harry thinks he knows, but he is wrong”. During the Les Houches summer school of 1964, the misuse of the word “photon” annoyed Lamb, who had received the Nobel Prize in 1955 for discovering the shift of the energy levels in a hydrogen atom”. Roy J. Glauber could connect a light quantum in the field with a click in the detector, who had received the Nobel Prize in 2005 for his contribution to the quantum theory of optical coherence, he once jokingly summarized his theory of photo detection by the sentence:“I don’t know anything about photons, but I know one when I see one”.(see: Roy J. Glauber. Quantum theory of optical coherence. WILEY-VCH Verlag, Weinheim, Germany, 2007).
光子一词的使用现在已经十分普遍。光子的波动性质可以由麦克斯韦方程组很好地描述，电磁场与量子力学中的波函数对应，麦克斯韦方程与量子力学中的薛定谔方程或柯莱因-戈登方程对应。在与量子力学对应的经典电磁理论中，光子有时用单色平面电磁波描述。这意味着在整个空间中找到光子的几率是一样的，显然与实验事实矛盾。光子有时用电磁波包描述。电磁波包来自于各种频率平面电磁波的叠加，没有确定的频率，与光子具有确定能量的实验事实矛盾。也就是说，量子力学本身不能给出光子粒子特性的确切描述。电磁场的量子化与通常的二次量子化对应，除了引入产生与湮灭算符以及态矢量的概念外，几乎没有给出关于光子的更多信息。事实上，光子与其它微观粒子一样满足力学运动方程，具有一定的大小，光子的这些粒子特性已经被光线弯曲实验与康普顿试验所证实。一个光子有多大？如何知道一个光子的大小？在《Electriomagnetic fields ,size, and copy of a single photon》（https://arxiv.org/pdf/1604.03869）一文中已经给出了答案。下面是对相关内容的简要介绍。
