We report on the experimental realization of an absolute single-photon source based on a single nitrogen vacancy (NV) center in a nanodiamond at room temperature and on the calculation of its absolute spectral photon flux from experimental data. The single-photon source was calibrated with respect to its photon flux and its spectral photon rate density. The photon flux was measured with a low-noise silicon photodiode traceable to the primary standard for optical flux, taking into account the absolute spectral power distribution using a calibrated spectroradiometer. The optical radiant flux is adjustable from 55 fW, which is almost the lowest detection limit for the silicon photodiode, and 75 fW, which is the saturation power of the NV center. These fluxes correspond to total photon flux rates between 190,000 photons per second and 260,000 photons per second, respectively. The single-photon emission purity is indicated by a $g^{(2)}(0)$ value, which is between 0.10 and 0.23, depending on the excitation power. To our knowledge, this is the first single-photon source absolutely calibrated with respect to its absolute optical radiant flux and spectral power distribution, traceable to the corresponding national standards via an unbroken traceability chain. The prospects for its application, e.g., for the detection efficiency calibration of single-photon detectors as well as for use as a standard photon source in the low photon flux regime, are promising.

A highly accurate method for the determination of the detection efficiency of a silicon single-photon avalanche diode (Si-SPAD) is presented. This method is based on the comparison of the detected count rate of the Si-SPAD compared to the photon rate determined from a calibrated silicon diode using a modified attenuator technique, in which the total attenuation is measured in two attenuation steps. Furthermore, a validation of this two-step method is performed using attenuators of higher transmittance. The setup is a tabletop one, laser-based, and fully automated. The measurement uncertainty components are determined and analyzed in detail. The obtained standard measurement uncertainty is < 0.5%. Main contributions are the transmission of the neutral density filters used as attenuators and the spectral responsivity of the calibrated analog silicon diode. Furthermore, the dependence of the detection efficiency of the Si-SPAD on the mean photon number of the impinging laser radiation with Poissonian statistics is investigated.

We report on the spectroscopic investigation of ${\mathrm{Pr}}^{3+}$-doped binary, ternary, and quaternary fluoride compounds exhibiting photon cascade emission. Under vacuum-ultraviolet synchrotron excitation at wavelengths below 200 nm in the $4f5d$ bands of the ${\mathrm{Pr}}^{3+}$ ion the emission spectra show intraconfigurational $4f\text{−}4f$ transitions originating from the ${}^{1}S_0$ and ${}^{3}P_0$ multiplets. In the ternary and quaternary compounds investigated, i.e., Ca-, Sr-, Ba-, and Rb-fluorides, broadband emission due to transitions from the $4f5d$ level of ${\mathrm{Pr}}^{3+}$ are also observed. The different types of emission transitions observed in the compounds investigated were analyzed in detail. The internal quantum efficiency for the visible spectral range (390 nm–750 nm), ${\eta }_{\mathrm{vis}}$, is determined with the help of the Judd-Ofelt theory and the branching ratios of the ${}^{1}S_0$ emission transitions. The dependence of this quantum efficiency ${\eta }_{\mathrm{vis}}$ from the ionic radius of the second cation $\left(R_{2\mathrm{nd}}\right)$, the effective ligand distance, and the coordination number is investigated.

In quantum communication systems, the precise estimation of the detector´s response to the incoming light is necessary to avoid security breaches. The typical working regime uses a free-running single-photon avalanche diode in combination with attenuated laser pulses at telecom wavelength for encoding information. We demonstrate the validity of an analytical model for this regime that considers the effects of dark counts and dead time on the measured count rate. For the purpose of gaining a better understanding of these effects, the photon detections were separated from the dark counts via a software-induced gating mechanism. The model was verified by experimental data for mean photon numbers covering three orders of magnitude as well as for laser repetition frequencies below and above the inverse dead time. Consequently, our model would be of interest for predicting the detector response not only in the field of quantum communications, but also in any other quantum physics experiment where high detection rates are needed.