  We study the correlation properties of single photons carrying orbital
angular momentum (OAM) in a Hanbury Brown and Twiss (HBT) type experiment. We
have characterized single photon sources obtained by pumping a nonlinear
crystal with a laser beam carrying different OAM under same experimental
conditions. For heralded twisted single photons carrying OAM, we calculate
$g^{(2)}(0)$, a measurable parameter characterizing the quality of a single
photon source, and observe an increment with the OAM of the single photon.
