Wave meets particle…

At present, the question as to how two different “metrological universes” can be connected is a challenge for the quantitative measurement of electromagnetic radiation: on the one hand, conventional radiometry at high radiant powers, as is necessary, for instance, to measure solar irradiation on Earth, and on the other hand, optical measurement in the quantum universe, as is needed, e. g., to investigate single atoms or molecules.

At the electron storage ring Metrology Light Source (MLS), PTB's primary radiation source standard for calculable synchrotron radiation in radiometry, this connection has now been successfully established. When passing deflection magnets, the electrons stored inside the ring emit radiation incoherently, i. e. without influencing each other. Thus, the total radiation power can be calculated by multiplying the computable radiation for one electron within the scope of the conventional theory of electrodynamics by the number of all the electrons stored inside the ring. This simple relation between the ring current and the emitted radiation
power, which can, however, strictly be applied over numerous orders of magnitude, allows, on principle, single-photon detectors to be calibrated by means of computed synchrotron radiation. Due to the optical arrangement required for focussing and for a spectral filtering of the broadband synchrotron radiation, this can, however, not be achieved with sufficient accuracy.

This problem has now been solved by tracing the calibration of single-photon detectors back to a primary detector standard, a cryogenic electric substitution radiometer (cryogenic radiometer) – the most accurate primary standard available in radiometry. Hereby, the MLS was used as a scalable radiation source in which the number of stored electrons can be varied from 1 to 10¹¹ and, thus, the radiation power can be correspondingly adjusted. Behind a spectral filter for a selected optical wavelength, the number of photons emitted per electron was first determined at approx. 10¹⁰ stored electrons using a photodiode which had been calibrated against the cryogenic radiometer beforehand. Then, the single-photon detector was calibrated at only a few hundred stored electrons by measuring its count rate per electron. After applying several corrections, it was then possible to determine the quantum efficiency of the singlephoton detector, tracing it back to the cryogenic radiometer, and to attain a combined measurement uncertainty of < 0.2 %, taking the photon statistics into account.

This measurement procedure is used in an enhanced form within the scope of the EMRP project “Metrology for Industrial Quantum Communication Technologies” to calibrate fibre-coupled single-photon detectors.