  This work reports on the pilot study, performed by INRIM, NPL and PTB, on the
measurement of the g^(2)(0) parameter in the visible spectral range of a test
single-photon source based on a colour centre in diamond. The development of
single-photon sources is of great interest to the metrology community as well
as the burgeoning quantum technologies industry. Measurement of the g^(2)(0)
parameter plays a vital role in characterising and understanding single-photon
emission. This comparison has been conducted by each partner individually using
its own equipment at INRIM laboratories, which were responsible for the
operation of the source
