  We observe the interaction of a single trapped calcium ion with single
photons produced by a narrow-band, resonant down-conversion source [A. Haase et
al., Opt. Lett. 34, 55 (2009)], employing a quantum jump scheme. Using the
temperature dependence of the down-conversion spectrum and the tunability of
the narrow source, absorption of the down-conversion photons is quantitatively
characterized.
