  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.
