Industrial technologies based on the production, manipulation, and detection of single and entangled photons are emerging. Quantum key distribution is one of the most commercially advanced, and among the first to directly harness the peculiar laws of quantum physics. To assist the development of this quantum industry, the National Physical Laboratory is using traditional, and quantum, methods to develop traceable metrology for the devices used in these technologies. We report on the instrumentation we have developed to characterize such systems.

The dark count probability as defined in the paragraph immediately following equation (3) should be P dark not P true .The ordinate axis in figure 7(a) should be 'Relative detection probability' not 'Relative detection efficiency'.