QUROPE Quantum Information Processing and Communication in Europe

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, 2, 3. Controlled single-photon emission is well advanced 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14; for controlled single-photon absorption by a single atom, proposals exist but only preliminary experimental steps have been taken 15, 16, 17, 18, 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.