  We demonstrate the efficient heralded generation of high purity
narrow-bandwidth single photons from a transient collective spin excitation in
a hot atomic vapour cell. Employing optical homodyne tomography, we fully
reconstruct the density matrix of the generated photon and observe a Wigner
function reaching the zero value without correcting for any inefficiencies. The
narrow bandwidth of the photon produced is accompanied by a high generation
rate yielding a high spectral brightness. The source is therefore compatible
with atomic-based quantum memories as well as other applications in light-atom
interfacing. This work paves the way to preparing and measuring arbitrary
superposition states of collective atomic excitations.
