  Heralded single-photon sources with on-demand readout are promising
candidates for quantum repeaters enabling long-distance quantum communication.
The need for scalability of such systems requires simple experimental
solutions, thus favouring room-temperature systems. For quantum repeater
applications, long delays between heralding and single-photon readout are
crucial. Until now, this has been prevented in room-temperature atomic systems
by fast decoherence due to thermal motion. Here we demonstrate efficient
heralding and readout of single collective excitations created in warm caesium
vapour. Using the principle of motional averaging we achieve a collective
excitation lifetime of $0.27\pm 0.04$ ms, two orders of magnitude larger than
previously achieved for single excitations in room-temperature sources. We
experimentally verify non-classicality of the light-matter correlations by
observing a violation of the Cauchy-Schwarz inequality with $R=1.4\pm 0.1>1$.
Through spectral and temporal analysis we identify intrinsic four-wave mixing
noise as the main contribution compromising single-photon operation of the
source.
