  We demonstrate a fiber in-line single photon source based on a hybrid system
of colloidal single quantum dots deposited on an optical nanofiber and cooled
down to cryogenic temperature (3.7 K). We show that a charged state (trion) of
the single quantum dot exhibits a photo-stable emission of single photons with
high quantum efficiency, narrow linewidth (3 meV FWHM) and fast decay time
($10.0\pm0.5$ ns). The single photons are efficiently coupled to the guided
modes of the nanofiber and eventually to a single mode optical fiber. The
brightness (efficiency) of the single photon source is estimated to be
$16\pm2\%$ with a maximum photon count rate of $1.6\pm0.2$ MHz and a high
single photon purity ($g^2(0)=0.11\pm0.02$). The device can be easily
integrated to the fiber networks paving the way for potential applications in
quantum networks.
