经典与量子电动力学中电磁波的自旋角动量与轨道角动量
Spin and orbital angular momenta of electromagnetic waves in classical and quantum electrodynamics
AI总结：
本文对比经典与量子电动力学框架，分析平面波及有限波包的自旋与轨道角动量计算难点，并阐明多模结构带来的量子推广问题。
摘要：
在自由空间中传播的平面单色电磁波可以具有一定量的自旋角动量，但不能拥有任何轨道角动量。即使是平面波的自旋角动量，若不借助某些数学极限论证，也难以进行计算。对于持续时间有限且横截面积有限的波包，可以使用经典电动力学的标准方法计算自旋角动量和轨道角动量。将这些结果推广到量子电动力学中的有限波包，需要结合波包的多模结构进行微妙论证。本文介绍了用于分析电磁波自旋角动量和轨道角动量的经典方法与量子光学方法中的若干细微之处。
英文摘要：
A plane, monochromatic electromagnetic wave propagating in free space can have a certain amount of spin angular momentum but cannot possess any orbital angular momentum. Even the spin angular momentum of the plane-wave is difficult to evaluate without resort to certain mathematical limit arguments. Both spin and orbital angular momenta can be computed for a wavepacket of finite duration and finite cross-sectional area using standard methods of classical electrodynamics. Extending these results to finite wavepackets in quantum electrodynamics requires subtle arguments in conjunction with the multimodal structure of the wavepacket. This paper presents some of the nuances of classical as well as quantum-optical methods for analyzing the spin and orbital angular momenta of electromagnetic waves.
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