Wave packets of radiation can act as “flying qubits” and transmit quantum states and quantum gate operations in quantum optics and quantum information technologies. A theoretical description of how a wave packet of quantum radiation interacts with a local material quantum system is, hence, crucial and necessary, but no textbook provides a general description of this elementary process!

In the talk, I will show how to cast the problem in a form that permits a (simple) density matrix theory for the excitation of a general quantum system by an incident quantum pulse of radiation. This theory differs significantly from the treatment of interactions between, e.g., an atom and a single optical mode in cavity QED. In particular, the theory acknowledges the multi-mode character of the final state of the field and it permits evaluation of the quantum state of any specified outgoing wave packet mode. We present applications of the theory to recent experiments with atomic and superconducting systems that interact with pulses of optical, microwave and acoustic radiation.