Klaus Mølmer

Protocols for scalable quantum information processing employ photon or phonon wave packets that can communicate quantum states and gate operations between distant material quantum systems in a larger network. The precise description of how travelling pulses of quantum radiation interact with a local material quantum system would seem a crucial and well established theory component in quantum optics and quantum information technologies. I shall discuss why and how a fundamental theory for such processes must differ from the treatment of interactions between, e.g., an atom and standing wave modes in cavity QED. In fact, no textbook provides a formal description of this elementary interaction process. We have recently been able 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. Our theory also provides the output quantum state of the radiation, restricted to any choice of a single or a few wave packet modes. I shall show multiple applications of this theory of relevance to recent experiments with atomic and superconducting qubits interacting with pulses of optical, microwave and acoustic radiation.