Single atoms interacting with single photons
Prof. Dr. Jürgen Eschner (Universität des Saarlandes)

<p> The interaction of single atoms with single photons is the fundamental quantum limit of matter-light coupling. Experiments with single cold atoms and single photons have provided seminal demonstrations of quantum phenomena such as quantum measurement, quantum jumps, photon graininess, and entanglement. Through continuous innovation, such phenomena are now turning into tools which allow controlling the interaction of single atoms and single photons at the quantum level; one of the most promising applications of such quantum optical tools is quantum information technology. Emphasizing the relation between basic quantum phenomena and possible future applications, I will present several exemplary experiments in which single or few trapped, laser-cooled atomic ions are manipulated with coherent light or with individual photons, while their light emission is recorded at the single-photon level. <p>

<p> In one experiment, the mechanical action of single photons on the atom is revealed. The information from the scattered photons is then used to monitor the position of the ion in the trap with a resolution close to the quantum limit and to control the ion's motion by electronic feedback. Furthermore, interference from single photons scattered by two distant ions (1 m apart) is observed, which allows creating entanglement between them. Currently, we investigate the interaction of an ion with single photons from a resonant, narrow-band spontaneous parametric down-conversion source, in order to test schemes of photon-to-atom entanglement transfer, as possible building blocks in future quantum networks. <p>