  A simple, room-temperature, cavity- and vacuum-free interface for a
photon-matter interaction is implemented. In the experiment, a heralded single
photon generated by the process of spontaneous parametric down-conversion is
absorbed by an ensemble of nitrogen-vacancy color centers. The broad absorption
spectrum associated with the phonon sideband solves the mismatch problem of a
narrow absorption bandwidth in a typical atomic medium and broadband spectrum
of quantum light. The heralded single photon source is tunable in the spectral
range 452-575 nm, which overlaps well with the absorption spectrum of
nitrogen-vacancy centers.
