Single-atom-single-photon interaction

A single atom is undoubtedly one of the best objects to study fundamental questions of quantum physics and to demonstrate quantum mechanics in a most elegant and vivid way. Single trapped ions have thus set historic milestones in the study of well-controlled quantum systems, and this has led to their application for frequency metrology and optical clocks, and to their great potential for being used in quantum information processing. We propose to advance the study of a single ion interacting over a large distance with its mirror image. This is a case of fundamental interest under several perspectives. The self-interaction, when photons emitted from the ion are returned and can stimulate further absorption or emission, approaches the limit of a controlled single-atom- single-photon-coupling, i.e. an interaction between the individual constituents of both matter and light. On the other hand, the presence of a mirror changes the electromagnetic boundary conditions of the ion and therefore modifies its interaction with the zero-point fluctuations of the electromagnetic field, the vacuum field. The experiments done so far raise questions about the interpretation of the nature of this self-interaction, in particular about the time that it takes for the ion to "notice" the presence of the mirror. This is one issue which will be addressed in the proposed investigations. Another important element in the development of sophisticated ion experiments is the control of the ion`s motion; in fact, for many experiments it is necessary to "cool" the ion to the quantum-mechanical ground state of the harmonic oscillator potential in which it is trapped. By recording the fluorescence light of an ion coupled to a distant mirror one can observe its motion in real-time, with an accuracy that is much better than the wavelength of the fluorescence light itself. The signal from such an observation can then be processed and electronically fed back to the ion in such a way that cooling of the ion motion occurs. The improvement and characterization of this novel cooling scheme is another part of the proposed work. Finally, we propose to lay the experimental foundation for transferring the information about an ion`s internal state to another one by light. Such a technique could be a key component in a future distributed quantum processor where quantum information has to be transported efficiently over long distances.