Interaction of a heralded single photon with nitrogen-vacancy centers in a diamond.

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.