Master equation for high-precision spectroscopy

The progress in high-precision spectroscopy requires one to verify the accuracy of theoretical models, such as the master equation describing spontaneous emission of atoms. For this purpose, we apply the coarse-graining method to derive a master equation of an atom interacting with the modes of the electromagnetic field. This master equation naturally includes terms due to quantum interference in the decay channels and fulfills the requirements of the Lindblad theorem without the need for phenomenological assumptions. We then consider the spectroscopy of the 2S-4P line of atomic hydrogen and show that these interference terms, typically neglected, significantly contribute to the photon count signal. These results can be important in understanding spectroscopic measurements performed in recent experiments for testing the validity of quantum electrodynamics.