Single-photon sources as new quantum standards: recent developments.

Single-photon sources play an important role in many of the emerging quantum technologies. Beside their implementation in quantum computing and quantum key distribution [1, 2], they are also interesting for the radiometry [1]. Ideally, single-photon sources have neither multi-photon emission nor background emission. Such a source has the potential to become a standard source in radiometry [2]. Moreover, a single-photon source is ideal for calibrating single-photon detectors, because of the omitted influence of the photon statistics on the calibration results [1]. For that reason, a single-photon source was developed at PTB, which photon flux is absolutely known and traceable to primary standards.  
The source, based on nitrogen-vacancy (NV-) center in nanodiamond, is absolutely calibrated to the national standards via an unbroken traceability chain in terms of its absolute spectral photon flux per wavelength and absolute spectral radiant flux per wavelength at room temperature. The absolute photon flux is measured with a low noise silicon photodiode traceable to the cryogenic radiometer and the spectral distribution was determined by using a calibrated spectrometer. The single-photon emission is tunable in the range from 55 fW to 75 fW, which corresponds to 190,000 photons per second and 260,000 photons per second, respectively. The purity of its single-photon emission is determined by the 2nd order autocorrelation function which is as low as 0.10 for a single-photon emission of 190,000 photons per second [1]. Furthermore, the standard measurement uncertainty of the absolute spectral photon flux was determined to be about 5 %, the largest part arises from the determination of the spectral power distribution of the emission [3].
In order to get a better understanding of the emission characteristics of such a single-photon source, back focal plane imaging is used to investigate the angular emission behavior of a NV-center located in the vicinity of a dielectric. The measured spatial emission distribution was compared to the calculated angular emission based on the model of Lukosz [4].

Rodiek, Beatrice; 4.5, Angewandte Radiometrie, PTB-Braunschweig
Christinck, Justus; 4.5, Angewandte Radiometrie, PTB-Braunschweig
López, Marco; 4.5, Angewandte Radiometrie, PTB-Braunschweig
Hofer, Helmuth; 4.5, Angewandte Radiometrie, PTB-Braunschweig
Georgieva, Hristina; 4.5, Angewandte Radiometrie, PTB-Braunschweig
Kück, Stefan; 4, Optik, PTB-Braunschweig