Single-photon spectroscopy of a single molecule
  Exploring the interaction of light and matter at the ultimate limit of single photons and single emitters is of great interest both from a fundamental point of view and for emerging applications in quantum engineering. However, the difficulty of generating single photons with specific wavelengths, bandwidths and brightness as well as the weak interaction probability of a single photon with an optical emitter pose a formidable challenge toward this goal. Here, we demonstrate a general approach based on the creation of single photons from a single emitter and their use for performing spectroscopy on a second emitter situated at a distance. Although we used organic molecules as emitters, our strategy is readily extendable to other material systems such as quantum dots and color centers. Our work ushers in a new line of experiments that provide access to the coherent and nonlinear couplings of few emitters and few propagating photons. 
Y. L. A. Rezus, S. G. Walt, R. Lettow, A. Renn, G. Zumofen, S.   Goetzinger, and V. Sandoghdar
quant-ph physics.optics