Stefan Kück

We present a disseminable single-photon source based on an InGaAs quantum dot in a micro-mesa. This source achieves a maximum photon flux of (2.74 ± 0.03) × 106 photons/s at a wavelength of (929.7 ± 0.1) nm, with a multiphoton suppression of g(2)(0) = 0.22 ± 0.02. The semiconductor chip can be installed in any cryogenic system and the compact size of all optical components facilitates easy dissemination for interlaboratory comparisons. The consistency in results regarding single-photon purity and the similarity of the measured photon fluxes across two distinct laboratory environments underscores the robustness and versatility of this single-photon source, which shows its potential to be used as a standard source for interlaboratory comparisons in quantum radiometry.

In the realm of fundamental quantum science and technologies, non‐classical states of light, such as single‐photon Fock states, are widely studied. However, current standards and metrological procedures are not optimized for low light levels. Progress in this crucial scientific domain depends on innovative metrology approaches, utilizing reliable devices based on quantum effects. A new generation of molecule‐based single‐photon sources is presented, combining their integration in a polymeric micro‐lens with pulsed excitation schemes, thereby realizing suitable resources in quantum radiometry. The strategy enhances the efficiency of generated single photon pulses and improves stability, providing a portable source at 784.7 nm that maintains consistent performance even through a cooling and heating cycle. The calibration of a single‐photon avalanche detector is demonstrated using light sources with different photon statistics, and the advantages of the single‐molecule device are discussed. A relative uncertainty on the intrinsic detection efficiency well below 1% is attained, representing a new benchmark in the field.

Current gravitational wave (GW) observatories rely on photon calibrators that use laser radiation pressure to generate displacement fiducials used to calibrate detector output signals. Reducing calibration uncertainty enables optimal extraction of astrophysical information such as source distance and sky position from detected signals. For the ongoing O4 observing run that started on 24 May 2023, the global GW detector network is employing a new calibration scheme with transfer standards calibrated at both the National Institute of Standards and Technology (NIST) and the Physikalisch-Technische Bundesanstalt (PTB). These transfer standards will circulate between the observatories and the metrology institutes to provide laser power calibration traceable to the International System of Units (SI) and enable assessment and reduction of relative calibration errors for the observatory network. The Laser Interferometer Gravitational-Wave Observatory (LIGO) project and the Virgo project are currently participating in the new calibration scheme. The Large-scale Cryogenic Gravitational-wave Telescope project (KAGRA) is expected to join in 2024, with the LIGO Aundha Observatory in India joining later. Before implementing this new scheme, a NIST-PTB bilateral comparison was conducted. It validated the scale representation by both laboratories, with a degree of equivalence of −0.2% and an associated expanded uncertainty of 0.32% (k = 2) which is significantly lower than previous studies. We describe the transfer of power sensor calibration, including detailed uncertainty estimates, from the transfer standards calibrated by NIST and PTB to the sensors operating continuously at the interferometer end stations. Finally, we discuss the ongoing calibration of Pcal-induced displacement fiducials for the O4 observing run. Achieved combined standard uncertainty levels as low as 0.3% facilitate calibrating the interferometer output signals with sub-percent accuracy.

We present a comprehensive metrological characterization of a commercial single-photon source with high photon flux emission for use in radiometry. The source is based on an InGaAs quantum dot in a micropillar. A comparative analysis of two excitation schemes—phonon-assisted excitation and two-photon excitation—explores differences in excitation power dependence, temporal stability and single-photon purity. The commercial source exhibits excellent properties for the field of quantum radiometry, achieving simultaneously a photon flux of (17.19 ± 0.09) million photons/s for a pulse repetition rate of 79.4 MHz, and a single-photon purity of 98%. Its optical power of (3.68 ± 0.02) pW is directly determined with a traceably calibrated low-noise photodiode. The ability to directly compare the photocurrent in a low-noise photodiode with the count rate at a single-photon avalanche detector allows for a seamless transition between the classical and quantum realizations of optical power. Therefore, we were able to build another bridge between classical and quantum radiometry by using a deterministic single-photon source.

We present here the first comparison between National Metrology Institutes of high accuracy continuous wave optical power measurements in the kilowatt regime. The National Institute of Standards and Technology (NIST) performed measurements with a power meter relying on photon momentum. The Physikalisch-Technische Bundesanstalt (PTB) performed measurements with a modified off-the-shelf thermal power meter. The non-absorbing photon momentum measurement approach permits the two power meters to measure the same laser beam optical path simultaneously, resulting in a direct comparison of the meters supported by an optical system to accommodate differences in instrument settling times. The results show agreement within the expanded uncertainties for each instrument. NIST and PTB illustrate a degree of equivalence of 0.49% with an expanded uncertainty of 1.37% (k = 2) for an average result across all power levels.

This article provides a report of the recent workshop on “The metrology of quantities which can be counted” organised jointly by the International Committee for Weights and Measures’ Consultative Committees for Amount of Substance (CCQM) and for Units (CCU). The workshop aimed to trigger a discussion on counting and number quantities across the metrological community so that a common understanding of counting and a common nomenclature could be achieved and there was clarity on the differences between these increasingly important concepts. This article details the background to the workshop, provides a summary of the presentations given and the discussions on the topics raised. It also reports the conclusions, agreed actions and next steps resulting from the workshop.

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.

Single-photon detectors are a pivotal component in photonic quantum technologies. A precise and comprehensive calibration of the inherent 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 and comprehensive analysis of the estimation of the inherent detection efficiency of two commercial single-photon detectors based on single-photon avalanche diodes for various mean photon numbers and at high laser pulse repetition rates using different techniques. We observe an unexpected and significant drop in the inherent 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 time-stamped data are available. We conclude that the full characterization of single-photon detectors used in critical applications should include the sensitivity of their inherent 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.