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News from the Annual Report (in German only)

The Max-Planck-RIKEN-PTB Center for Time, Constants, and Fundamental Symmetries holds a workshop on recent advances on Tuesday March 9th, 2021, with the following speakers:

Ichiro Ushijima (RIKEN): Transportable optical lattice clocks with 18 digit precision
Richard Lange (PTB): Improved Limits for Violations of Local Position Invariance and Local Lorentz Invariance from Atomic Clocks
Peter Micke (PTB): Coherent laser spectroscopy of highly charged ions using quantum logic
Matthew Anders Bohman (MPIK/RIKEN): Sympathetic Cooling of Trapped Protons
Antonia Schneider (MPIK): High-precision measurement of the hyperfine structure of 3He+ in a Penningtrap
Kathrin Kromer (MPIK): Latest results of the high-precision mass measurements with PENTATRAP
Jack Devlin (CERN/RIKEN): Constraining the coupling between axion-like dark matter and photons using an antiproton superconducting detection circuit in a cryogenic Penning trap

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In the finalized project BiRD JRP 16NRM08 (https://www.birdproject.eu), funded by the EU program EURAMET EMPIR Call 2016 – „Metrology Research for Pre‐ and Co‐normative projects“, basic research was carried out on such measurands which determine the appearance of products. These are, amongst color and its angle-dependent distribution also gloss, sparkle, and graininess. The term sparkle describes the specific appearance of surfaces which can be observed under directed illumination by reflection of point-like scattering centers and is similar to the appearance of sparkling stars in the night-sky. This effect is generated e.g. by high-reflective microparticles, which are embedded in the lacquer coat of a surface. Well-known examples are metallic automobile coatings. Graininess describes the appearance of such surfaces when illuminated diffusely [1]. Although the sparkle-effect is widely applied in industrial production, there is no standardized and commonly accepted definition of the related measurand. Few commercial systems generate device-dependent quantities by using non-disclosed evaluation procedures. To improve this situation, within the course of the BiRD-project the technical working group CIE JTC 12 [2] was founded, in which measurands for sparkle and graininess and measurement procedures to determine them were defined. Based on these provisions a comparison between four national metrology institutes was carried out, in which the technical realizations and the appropriateness of the measurand’s definition were tested [3]. Despite some variability in measurement apparatus’ very satisfactory results were gained in the objective determination of the sparkle measurands. The same holds true for sphere-based determinations of graininess. Using results from also conducted psycho-visual experiments on the same samples, it was possible to compare the visual appearance with the objective results [4]. The excellent correlation shows that appropriate measurands were selected. Therefore, for the first time, traceable measurements are presented and corresponding sparkle- and graininess-scales are available, which will serve as a basis for further investigations and standardization work....

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In the project BxDiff JRP 18SIB03 (https://bxdiff.cmi.cz/), which is supported within the EU program EURAMET EMPIR Call 2018 - SI Broader Scope, measurement systems in the status of national standard facilities are created to perform BTDF - (Bidirectional transmittance distribution function) and BSSRDF - (Bidirectional surface scattering distribution function) investigations. PTB funds therein the development of a dedicated BTDF measurement facility in terms of a so-called “Größtgerät” (strategic investment). Main aims of the 17 involved partners stemming from national metrology institutes, research and university institutes, and material processing companies, are to close the measurement-technology gap for the mentioned measurands as well as to expand existing techniques into currently not accessible regimes. For example, it is envisaged to apply BRDF (Bidirectional reflectance distribution function) measurements to small samples with sizes in the (sub)-millimeter range.

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Measuring the total momentum transfer of the absorbed and re-emitted photons from a highly reflective surface (reflection of the laser beam from an optical mirror) as a force provides the possibility of measuring the optical power with direct traceability to SI units.  PTB and the Technical University of Ilmenau (TUI) recently reviewed the viability and precision of the photon-momentum-based optical power measurement method that employs an amplification effect caused by a multi-reflected laser beam trapped in an optical cavity. Trial measurements were performed at two different metrology laboratories: the laboratory for mass/force at the TUI, and the clean room laser radiometry laboratory at PTB, using a portable force measurement setup developed by TUI, consisting of two electromagnetic force compensation balances. We compared the results of the optical power measurements performed with the force measurement setup, via the photon-momentum-based method, with those performed using a calibrated reference standard detector traceable to PTB's primary standard for optical power, the cryogenic radiometer. The comparison was carried out for an optical power range between 1 W and 10 W at a wavelength of 532 nm, which corresponds to a force of approximately 2000 nN at the upper limit, yielding approximately 2.3% relative standard uncertainty in the case of 33 reflections. Thus, conflating the high-precision force metrology technique at μN to nN levels with the optical setup required to achieve specular multi-reflection configuration of the laser beam, where a macroscopic optical cavity with ultra-high reflective mirrors (>99.995%) can adjustably be suspended from the force sensors, depending on required geometry of reflections, we show that the uncertainty of the optical power measurements upon further increase of the nominally applied optical power, the number of laser beam reflections, or the reflectivity coefficient of the mirrors can be markedly reduced.

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Annular subaperture stitching interferometry (ASSI) is a common approach for the measurement of aspherical surfaces. A common obstacle of ASSI is the occurrence of lateral displacement errors when the sensor or specimen is repositioned between the subaperture measurements. Our contribution focuses on modeling of the statistical displacement errors. Avirtual experiment is presented simulating the propagation of the displacement errors through a cumulative and a global stitching algorithm to the retrieved surface form. For the considered experimental setup, the uncertainty in lateral position depends on the positioning uncertainties of the employed motion system and the uncertainty in the absolute distance measurement between the sensor and specimen. The lateral displacement uncertainty is determined experimentally employing a calibratable lateral grating. Thus, it is traceable to the SI unit of the length (meter). The experimental results show that the lateral displacement errors may be modeled by a normal distribution, and the results of the virtual experiment indicate that the statistical lateral displacement errors transfer linear through the stitching procedure and also cause a normal distributed topography error. This enables the assignment of an expanded uncertainty to each individual sample point employing the Zernike polynomial expression of the topography measurement.

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The challenges faced in a comparison of measuring the detection efficiency of free-running InGaAs/InP single-photon avalanche detectors (InGaAs/InP SPAD) were studied by four European National Metrology Institutes (NMIs) meeting at a single laboratory. The main purpose of this study is to develop a trustable measurement technique and to provide a snapshot of the methods used by the four NMIs for measuring such photon-counting detectors at telecom wavelengths in order to establish proper procedures for characterising such devices. The detection efficiency measurements were performed using different experimental setups and reference standards with independent traceability chains at the wavelength of 1550 nm. A dedicated model to correct the dead time and dark count effects on the SPAD’s free-running counting process was developed, allowing the correct value of the photon rate impinging on the detector to be recovered from simple ratemeter measurements. The detection efficiency was measured for mean photon number per pulse between 0.01 and 2.4, corresponding to photon rates between approximately 1100 photon/s and 193,000 photon/s, respectively. We found that the measured values reported by the participants are all consistent within the stated uncertainties, proving the consistency of the measurement approach developed.

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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.

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Multiple Aperture Shear-Interferometry (MArS) is a shape measurement technique that uses multi-spot illumination to overcome the problem of a limited observation aperture of conventional interferometric techniques and thus considerably simplifies the measurement of optical aspheres and freeform surfaces. Using a shear interferometry setup, MArS measures the coherence function in order to obtain wave vector distributions created from multi-spot LED illumination reflected by the specimen. Based on the wave vectors we reconstruct the surface topography of aspheric lenses using an inverse ray tracing approach and prior knowledge about the individual source locations. We present the topographic measurement of two aspheric lenses with different global curvature radii measured with the same identical reflection setup. In addition, we examine the achievable accuracy of the wave vector measurement using a single light source to find physical limits of MArS.

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Each mobile telecommunications network requires synchronization between its components. Deutsche Telekom Technik AG has decided to test a method developed by the AGH University of Science and Technology in Krakow, Poland, for the future monitoring of selected points in its network. It is based on optical time transmission (OTT), which has been tested in cooperation with PTB, among others on glass fiber links of Deutsche Telekom AG (DTAG). The results exceed by far the requirements of the International Telecommunication Union (ITU-T) for 5G networks.

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For the analysis of measured data in spectroscopic Mueller matrix ellipsometry (MME) there exist some commonly used optimization techniques to calculate for example layer thicknesses of samples under test. Concentrating on metrological aspects of MME we identified a non-optimal treatment of depolarization in all these techniques. Therefore, we recently developed an improved optimization method to take depolarization in MME adequately into account. In a further step we also included statistical measurement noise and derived a likelihood function, which enabled us to apply both the maximum likelihood method and Bayesian statistics as well as the Bayesian information criterion for the data evaluation. In this paper we concentrate on the application of this new method for measurements of SiO2-layer thicknesses on silicon. With a state-of-the-art SENTECH SENresearch 4.0 Mueller ellipsometer we measured different SiO2-layer thickness standard samples with calibrated thicknesses between about 6 nm and 1000 nm. The MME results are compared to the calibration data. For all samples an SiO2-SiO double-layer-model turned out to be optimal. The measured total oxid layer thicknesses match excellently with the calibration values within the estimated uncertainties. All results are presented here. It is a first comparison with traceable reference measurements demonstrating the validity of our novel MME analysis method.

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