  Quantum technologies could largely benefit from the control of quantum
emitters in sub-micrometric size crystals. These are naturally prone to the
integration in hybrid devices, including heterostructures and complex photonic
devices. Currently available quantum emitters sculpted in nanocrystals suffer
from spectral instability, preventing their use as single photon sources e.g.,
for most quantum optics operations. In this work we report on unprecedented
performances of single-photon emission from organic nanocrystals (average size
of hundreds \SI{}{\nano\meter}), made of anthracene (Ac) and doped with
dibenzoterrylene (DBT) molecules. The source has hours-long photostability with
respect to frequency and intensity, both at room and at cryogenic temperature.
When cooled down to \SI{3}{K}, the 00-zero phonon line shows linewidth values
(\SI{50}{MHz}) close to the lifetime-limit. Such optical properties in a
nanocrystalline environment make the proposed organic nanocrystals a unique
single-photon source for integrated photonic quantum technologies.
