A number of quantum-based methods for computing and cryptography rely on the control and measurement of single photons. In current research applications, the focus is on characterizing the fundamental quantum nature of single photon sources and absolute counting accuracy is not important. This changes when these applications cross the threshold to commercial applications. In this case, the counting efficiency of single photon detectors becomes an important performance metric.
At DFM we have developed a light source for calibration of single photon detectors at 1550 nm based on an attenuated laser. This source can deliver controlled power in a range from nanowatt into the femtowatt power regime, where the mean photon flux is on the order of <10.000 photons/s. The calibration has a relative uncertainty below 5% (k=2).
The output power is traceable to the primary reference for optical power at DFM to ensure accurate calibration.
The output of the source is fiber coupled for reproducible coupling to detectors. Several different types of fiber outputs can be used to accommodate a wide range of detector types.
The current version of our calibration facility targets users in the telecom range. Single photon detectors are however also relevant at other part of the light spectrum. Visible light applications where small light signals have to be measured accurately i.e. in astronomy or biological analysis can also benefit from traceable calibration of detector sensitivity. For this purpose, we are developing new versions of the light source to address the need for calibration in these areas.