Heralded single-photon absorption by a single atom

Emission and absorption of single photons by single atoms is a fundamental limit of matter-light interaction, manifesting its quantum mechanical nature. As a controlled process, it is also a key tool in quantum optical information technology(1-3). Controlled single-photon emission is well advanced(4-14); for controlled single-photon absorption by a single atom, proposals exist but only preliminary experimental steps have been taken(15-19). Here we report the absorption of single photons by a single trapped ion: employing a photon pair source, detection of the quantum-correlated partner photon heralds the presence of the resonant photon at the atom. We find clear correlations between the detection of the herald and the absorption process in the atom; we also demonstrate polarization control of this process. Our experiment evidences previously unexplored interaction between a single absorber and a quantum light source; with improved control over the coupling, it will open up new avenues in quantum technology.