Absolute detector quantum-efficiency measurements using correlated photons

An experimental system using correlated photons for radiometric purposes has been set up at the National Institute of Standards and Technology (NIST). We use pairs of correlated photons to produce spatial maps of the absolute efficiency of a photomultiplier photocathode at four wavelengths. We also compare this technique with measurements carried out by conventional means tied to existing radiometric standards. These initial comparisons show an average agreement within similar to 0,6% between the two methods. The results show that correlated photons can be a useful tool in radiometry and that the method holds enough promise to warrant further studies.