Abstract A sample fabrication approach based on organic molecular beam deposition creating a new molecule-based emitter structure and its optical analysis is presented. The polycyclic aromatic hydrocarbon dibenzoterrylene (DBT, C38H20) was deposited between layers of C60-fullerenes, which serve as a protective matrix. It was found that this structure maintains the DBT’s optical properties [1, 2], but a potentially quenching impact cannot be excluded. Raman spectra of the composite were collected proving the molecules’ preserved integrity after being evaporated at high temperatures. Finally, the results of confocal laser scanning microscopy show fluorescent molecular clusters.

Abstract We present the metrological characterization of a single-photon source based on a germanium-vacancy center in diamond under a solid immersion lens in a confocal microscope setup at room temperature. It was characterized in terms of the emission’s spectral distribution, single-photon purity, temporal stability and the emitter’s excited state lifetime and saturation behavior. An Allan deviation analysis was performed on the emission of the single-photon source to determine the optimal averaging time of the photon flux. The single-photon source was used for the relative calibration of the detection efficiency of two single-photon avalanche diode detectors. The results were compared with measurements using attenuated laser light for the calibration of the detectors.

We report on the metrological characterization of the emission from a germanium-vacancy center in diamond under a microfabricated solid immersion lens in a confocal laser-scanning microscope setup. Ge ions were implanted into a synthetic diamond at 3 MeV, and germanium-vacancy centers were then formed by subsequent annealing. Afterward, solid immersion lenses were fabricated in a focused ion beam scanning electron microscope. The photoluminescence was investigated at room temperature in terms of the spectral distribution, the excited state lifetime, the second-order correlation function, and the saturation behavior, proving simultaneous high single-photon purity and high brightness. Two methods were exploited to minimize the residual multi-photon probability: spectral filtering and temporal filtering. According to these results, we assume that Raman scattered photons and emission from neighboring color centers play an important role in the residual multi-photon emission probability. The system efficiency of the single-photon source was investigated and found to be in accordance with the value calculated from all sources of loss in the setup. The branching ratio of the germanium-vacancy center for the decay into the ground state and into metastable state was calculated. The results enable the usage of the single-photon source in future quantum radiometric experiments.

We report on the characterization of the angular-dependent emission of two different single-photon emitters based on nitrogen-vacancy centers in nanodiamond and on core-shell CdSe/CdS quantum dot nanoparticles. The emitters were characterized in a confocal microscope setup by spectroscopy and Hanbury-Brown and Twiss interferometry. The angular-dependent emission is measured using a back focal plane imaging technique. A theoretical model of the angular emission patterns of the 2D dipoles of the emitters is developed to determine their orientation. Experiment and model agree well with each other.

Single-photon detectors are a pivotal component in photonic quantum technologies. A precise and comprehensive calibration of the intrinsic detection efficiency is of utmost importance to ensure the proper evaluation of the performance in view of the specific technological application of interest, such as the protection against security breaches in quantum cryptographic solutions. Here we report on a systematic study on and comprehensive analysis of the estimation of the intrinsic detection efficiency of two commercial single-photon detectors based on single-photon avalanche diodes (SPADs) for various mean photon numbers and at high laser pulse repetition rates using different techniques. We observed an unexpected and signifucant drop in intrinsic detection efficiency at detection rates of 10 % and higher relative to the maximum detection rate. It is demonstrated that for data analysis a statistical model for the detection rate conveniently can be used if no timestamped data are available. We conclude that the full characterization of single-photon detectors used in critical applications should include the sensitivity of their intrinsic detection efficiency to high event rates.

Single-photon sources have a variety of applications. One of these is quantum radiometry, which is reported on in this paper in the form of an overview, specifically of the current state of the art in the application of deterministic single photon sources to the calibration of single photon detectors. To optimize single-photon sources for this purpose, extensive research is currently carried out at the European National Metrology Institutes (NMIs), in collaboration with partners from universities. Single-photon sources of different types are currently under investigation, including sources based on defect centres in (nano-)diamonds, on molecules and on semiconductor quantum dots. We will present, summarise, and compare the current results obtained at European NMIs for single-photon sources in terms of photon flux, single-photon purity, and spectral power distribution as well as the results of single-photon detector calibrations carried out with this type of light sources.

a narrow-bandwidth quantum emitter Hristina Georgieva, Marco López, Helmuth Hofer, Beatrice Rodiek, Justus Christinck, Peter Schnauber, Arsenty Kaganskiy, Tobias Heindel, Sven Rodt, Stephan Reitzenstein, and Stefan Kück Physikalisch-Technische Bundesanstalt, Braunschweig, Germany, Institut für Festkörperphysik, Technische Universität Berlin, Berlin, Germany Corresponding e-mail address: hristina.georgieva@ptb.de

The characterization of a core-shell CdSe/CdS quantum dot emitter at room temperature will be presented. For the characterization, the photon flux, the spectral distribution and the singlephoton purity are determined. The central wavelength of the quantum dot is 642 nm, the bandwidth (FWHM) is about 18 nm. Furthermore, the relative detector calibration of two singlephoton detectors (SPADs) will be shown using this quantum emitter.

Quantum metrology requires a stable single-photon emission and a high single-photon purity. Since nitrogen-vacancy (NV-) centers provide both features at room temperature, they are promising candidates for the application in this field [1, 2]. The knowledge about a suitable sample preparation technique is crucial, because the quality of the single-photon emission strongly depends on the sample purity and on the spatial resolvability of the emitters. This work presents the comparison and optimization of various sample fabrication techniques of nitrogen vacancy center doped nanodiamonds on standard cover glasses. The preparation is followed by a comparative characterization of the centers of the various samples. The sample fabrication includes the removal of contaminants on the cover glass surface. This was carried out by using peroxymonosulfuric acid (piranha solution, H2SO5) in comparison to the commercially available lye Hellmanex III (by Hellma Analytics). After cleaning the cover glasses, volumes of various nanodiamond dilutions were applied via spin coating. In subsequent steps, the nanodiamonds themselves were cleaned with peroxymonosulfuric acid, too, to remove contaminants resulting from the manufacturing process, e.g. graphite. The samples were analyzed by using a confocal laser scanning microscope with an oil immersion objective. Single-photon purity was determined by measuring the second order correlation function with a Hanbury Brown and Twiss setup. Spectral analysis revealed the presence of NV−- and NV°-centers. It was shown that a suitable cleansing method has an immense impact on single-photon emission, as was proven by a comparative characterization of differently manufactured nanodiamonds.

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 which 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.

We apply an InGaAs quantum dot based single-photon source for the absolute detection efficiency calibration of a silicon single-photon avalanche diode operating in Geiger mode. The single-photon source delivers up to (2.55 ± 0.02) × 106 photons per second inside a multimode fiber at the wavelength of 929.8 nm for above-band pulsed excitation with a repetition rate of 80 MHz. The purity of the single-photon emission, expressed by the value of the 2nd order correlation function g(2)(τ = 0), is between 0.14 and 0.24 depending on the excitation power applied to the quantum dot. The single-photon flux is sufficient to be measured with an analog low-noise reference detector, which is traceable to the national standard for optical radiant flux. The measured detection efficiency using the single-photon source remains constant within the measurement uncertainty for different photon fluxes. The corresponding weighted mean thus amounts to 0.3263 with a standard uncertainty of 0.0022.

We report on the characterization of the angular-dependent emission of single-photon emitters based on single nitrogen-vacancy (NV-) centers in nanodiamond at room temperature. A theoretical model for the calculation of the angular emission patterns of such an NV-center at a dielectric interface will be presented. For the first time, the orientation of the NV-centers in nanodiamond was determined from back focal plane images of NV-centers and by comparison of the theoretical and experimental angular emission pattern. Furthermore, the orientation of the NV-centers was also obtained from measurements of the fluorescence intensity in dependence on the polarization angle of the linearly polarized excitation laser. The results of these measurements are in good agreement. Moreover, the collection efficiency in this setup was calculated to be higher than 80% using the model of the angular emission of the NV-centers.

The traceability of measurements of the parameters characterizing single-photon sources, such as photon flux and optical power, paves the way towards their reliable comparison and quantitative evaluation. In this paper, we present an absolute measurement of the optical power of a single-photon source based on an InGaAs quantum dot under pulsed excitation with a calibrated single-photon avalanche diode (SPAD) detector. For this purpose, a single excitonic line of the quantum dot emission with a bandwidth below 0.1 nm was spectrally filtered by using two tilted interference filters. Since high count rates are essential for many metrological applications, we optimized the setup efficiency by combining high overall transmission of the optical components with a geometrical enhancement of the extraction efficiency of a single quantum dot by a monolithic microlens to reach photon fluxes up to 3.7 ⋅ 10 5 photons per second at the SPADs. A relative calibration of two SPAD detectors with a relative standard uncertainty of 0.7% was carried out and verified by the standard calibration method using an attenuated laser. Finally, an Allan deviation analysis was performed giving an optimal averaging time of 92 s for the photon flux.

In this paper we present the traceable calibration of the detection efficiency of Si and InGaAs/InP single-photon avalanche detectors (SPADs). We describe the calibration facilities and the reference standard detectors used, as well as the traceability chain of the measurements to the primary standard for optical power (Cryogenic Radiometer) of PTB. As an example, the detection efficiency of a Si and InGaAs/InP SPAD detector measured at wavelengths of 850 nm and 1550 nm, respectively, for different photon flux rates is presented. The standard uncertainty of the measurements is ≤ 1 %.

Recently, a single-photon source based on the nitrogen vacancy center in a nanodiamond with a traceable spectral photon flux was realized [1]. In this article, we report on the determination of the measurement uncertainty of the spectral photon flux (about 4 %) and discuss its different components. Furthermore, the angular distribution of the emission of an NV center in a nanodiamond located in the vicinity of a dielectric boundary is calculated based on the model of Lukosz and Kunz. The agreement between the angular distribution calculation and the first-ever measurement of the angle-dependent emission in nanodiamond NV centers is satisfactory.

Quantum metrology requires high efficient single-photon detectors and single-photon sources for its proper operation. To achieve that, they also need an appropriate metrological support that guarantees the traceability of their measurements to primary standards. The calibration of single-photon detectors with respect to its detection efficiency is frequently carried out by the double attenuator technique, described in detail in [1]. Alternatively, the detection efficiency can be determined by a direct comparison against a traceable low optical flux detector (LOFD) [2]. Both measurement procedures were recently compared for determining the Si-SPAD quantum detection efficiency [3]. The comparison was carried out at a wavelength of 770 nm and at optical power levels from 90 fW to 1300 fW, with mean relative deviations < 1 %. Besides the calibration of single-photon detectors, pure and reliable single-photon sources that provide high single photon fluxes and a negligible background are needed for instance in quantum computing and cryptography as well as for radiometric applications, e.g. single-photon detector calibration [4]. The source under investigation here is based on a nitrogen-vacancy (NV) center in a nanodiamond. It was absolutely characterized in terms of its absolute optical radiant flux and spectral power distribution [5]. The photon flux is adjustable between 190 000 photons per second and 260 000 photons per second (corresponding to 55 fW and 75 fW, respectively. The purity of the single-photon emission, the g-value, is between 0.10 and 0.23. This result is considered being the first step towards the realization of a standard single-photon source. Details of the setups and the calibration procedures as well as recent progress will be discussed at the conference. References: [1] M. López, H. Hofer, S. Kück, “Detection efficiency calibration of single-photon silicon avalanche photodiodes traceable using double attenuator technique, Journal of Modern Optics 62, S21 – S27 (2015). [2] J. Mountford et al. “Development of a switched integrator amplifier for high-accuracy optical measurements”, Applied Optics 47, 31, 5821-5828 (2008). [3] G. Porrovevecchio et al., Comparison at the sub-100 fW optical power level of calibrating a single-photon detector using a high sensitive, low-noise silicon photodiode and the double attenuator technique, Metrologia 53, 11151122, 2016. [4] W. Schmunk et al., Photon number statistics of NV centre emission, Metrologia 49, 156-160, 2012. [5] B. Rodiek et al., Experimental realization of an absolute single-photon source based on a single nitrogen vacancy center in a nanodiamond, Optica 4 (1), 71-76,2017.

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. (C) 2017 Optical Society of America

A comparison down to sub-100-fW optical power level was carried out between a low-noise Silicon photodiode and a low optical flux measurement facility based on a double attenuator technique. The comparison was carried out via a silicon single-photon avalanche diode (Si-SPAD), which acted as transfer standard. The measurements were performed at a wavelength of 770 nm using an attenuated laser as a radiation source at optical power levels between approximately 86 fW and approximately 1325 fW, corresponding to approximately 330 000 photons s(-1) and approximately 5.2 x 10(6) photons s(-1), respectively. The mean relative deviation of the detection efficiencies of the Si-SPAD, determined by the Si-photodiode and the low optical flux measurement facility, i.e. between two completely independent traceability routes, was < 0.2%, thus well within the combined standard uncertainty of the two measurements. To our knowledge, this is the first comparison for the detection efficiency of a single photon detector using a direct optical flux measurement by a conventional Si-photodiode at such low power levels.

We present the calibration results of a Si-SPAD detector using a low optical flux detector (LOFD) and the double attenuator technique. The calibration consists in determining the detection efficiency of the detector via these two approaches with independent traceability chains. The calibration was carried out at a wavelength of 770 nm and at different optical power levels from 90 fW to 1300 fW. The mean relative deviation of the detection efficiency determined using the LOFD and the double attenuator technique was < 0.2 %, thus within the combined standard uncertainty of the two measurements.

Silicon single-photon avalanche diodes (Si-SPADs) are the most used devices for measuring ultra-weak optical radiant fluxes in many quantum technology fields, such as quantum optics, quantum communication, quantum computing, etc. In all these fields, the detection efficiency is the main parameter, which has to be accurately known for achieving reliable measurements. In this paper we present the improvements performed on the setup described in López et al. (J Mod Opt 62:S21–S27, 2015) for determining the detection efficiency of Si-SPAD detectors with a low measurement uncertainty. The improvement arises from the precise alignment of the Si-SPAD detector and the low deviation reached between the total calculated filter transmission and the individual filter transmission measurements (≤0.05%) performed with an integrating sphere with attached Si-photodiode as standard detector.