“…National Institute of Standards and Technology (NIST) of America [2,3], National Physical Laboratory of UK [4,5], Istituto Elettrotecnico Nazionale of Italy [6], Institut National de Metrologie of France [7], National Institute of Advanced Industrial Science and Technology of Japan [8] and Anhui Institute of Optics and Machinery of Chinese Academy of Sciences [9][10][11]have successively researched calibration methods based on correlated photons. Lasers of different wavelengths have been used to pump KDP, BBO, LiIO3, PPLN, and other nonlinear crystals, and the quantum efficiency of Si single photon detector [12], photomultiplier tube [13], CCD [14], InGaAs APD [8] has been calibrated.…”