Spectroscopy of a single molecule using single photons emitted by another molecule

Any study of the light-matter interaction involves photons and matter particles such as atoms. However, the great majority of such experiments have been performed using large ensembles of matter and light particles. Recent advances in quantum optics and nano-optics have made it possible to detect, examine, and manipulate single atoms, ions, or molecules using laser light. Furthermore, single atoms, ions and molecules have become established as sources of single photons by many groups. Nevertheless, experiments on the interaction of single photons with single atoms have been few and restricted to the strong coupling regime in high-finesse cavities. The fundamental challenge in such experiments is to confine light efficiently enough so that the electric field associated with a single photon can lead to atomic excitation. Our recent theoretical and experimental works have indicated that this should be possible using a tightly-focused laser beam [1, 2]. In this presentation, we report for the first time on the spectroscopy of single molecules using a freely-propagating stream of single photons produced by another single molecule placed at a large distance, as sketched in Figure 1a [3].