Doubly heralded single-photon absorption by a single atom

We report on a single-photon-to-single-atom interface, where a single photon generated by spontaneous parametric down-conversion (SPDC) is absorbed by a single trapped ion. The photon is heralded by its time-correlated partner generated in the SPDC process, while the absorption event is heralded by a single photon emitted in its course. Coincidence detection marks doubly heralded absorption, enabling photon-to-atom quantum state transfer (Kurz et al. in Nat Commun 5:5527, 2014; Muller and Eschner in Appl Phys B 114:303, 2013). Background in the coincidence measurement is strongly suppressed by a new method that discriminates real absorption events from dark-count-induced coincidences.