Quantum electrodynamics of source field detection and quantum beats

The quantum electrodynamics of two atoms in free space, separated by many wavelengths and interacting with one another via the coupling of their dipole moments to the electromagnetic field, is studied using the Heisenberg-operator approach. The problem is formulated in such a way that one of the atoms, which has two levels, plays the role of a detector; the other atom, whose levels are arbitrary but do not involve cascadelike transitions, plays the role of the source. Several aspects of the interaction are considered, including: a review of photodetection theory, the theory of an alternative detection method that uses an excited-state detector, and a study of quantum beats. These dynamical studies of emission and detection provide new insights into the observation of quantum beats in the transition rate of a detector. We show that ground-state beats are not observed by a detector atom making an upward transition, but that ground-state beats are, in principle, observable if the detector atom makes a downward transition.