Quantum optical coherence theory based on Feynman's path integral

The first, second and higher-order interference of classical and nonclassical light is interpreted in quantum optical coherence theory based on Feynman{}s path integral. Compared to the existed classical optical coherence theory based on Maxwell{}s electromagnetic theory and Glauber{}s quantum optical coherence theory based on wave mechanics formulation of quantum mechanics, quantum optical coherence theory based on Feynman{}s path integral provides a new tool to study optical coherence and has the advantage of understanding the connection between the physics and mathematical calculations. For instance, based on the results of transient first-order interference of two independent light beams, it is predicted that the well-accepted electric field model for thermal light introduced by many optical textbooks may not be accurate. The physics of two-photon bunching of thermal light and Hong-Ou-Mandel dip of entangled photon pairs is the same, which can be interpreted by constructive and destructive two-photon interference, respectively. The second-order interference of two independent light beams are introduced in detail. Interpreting the interference of light in quantum optical coherence theory based on Feynman{}s path integral is helpful to understand the physics of light, and may eventually lead to the answer of the question puzzling us for a long time: what is photon?