Quantum light sources emitting triggered single photons or entangled photon pairs have the potential to boost the performance of quantum key distribution (QKD) systems. Proof-of-principle experiments affirmed these prospects, but further efforts are necessary to push this field beyond its current status. In this work, we show that temporal filtering of single-photon pulses enables a performance optimization of QKD systems implemented with realistic quantum light sources, both in experiment and simulations. To this end, we analyze the influence of temporal filtering of sub-Poissonian single-photon pulses on the expected secret key fraction, the quantum bit error ratio, and the tolerable channel losses. For this purpose, we developed a basic QKD testbed comprising a triggered solid-state single-photon source and a receiver module designed for fourstate polarization coding via the BB84 protocol. Furthermore, we demonstrate real-time security monitoring by analyzing the photon statistics, in terms of g (2) (0), inside the quantum channel by correlating the photon flux recorded at the four ports of our receiver. Our findings are useful for the certification of QKD and can be applied and further extended for the optimization of various implementations of quantum communication based on sub-Poissonian quantum light sources, including device-independent schemes of QKD as well as quantum repeaters. Our work represents an important contribution towards the development of QKD-secured communication networks based on quantum light sources.

We present a design study of quantum light sources based on hybrid circular Bragg Gratings (CBGs) for emission wavelengths in the telecom O-band. The evaluated CBG designs show photon extraction efficiencies > 95% and Purcell factors close to 30. Using simulations based on the finite element method, and considering the influence of possible fabrication imperfections, we identify optimized high-performance CBG designs which are robust against structural aberrations. In particular, full 3D simulations reveal that the designs show robustness regarding deviations of the emitter position in the device well within reported positioning accuracies of deterministic fabrication technologies. Furthermore, we investigate the coupling of the evaluated hybrid CBG designs to single-mode optical fibers, which is particularly interesting for the development of practical quantum light sources. We obtain coupling efficiencies of up to 77% for off-the-shelf fibers, and again proof robustness against fabrication imperfections. Our results show prospects for the fabrication of closeto-ideal fiber-coupled quantum light sources for long distance quantum communication.Quantum light sources emitting indistinguishable single-photon or entangled-photon states are key building blocks for future photonic quantum technologies [1-3], with applications ranging from quantum communication [4][5][6] to quantum computing [7][8][9]. In this context, quantum light sources based on epitaxial semiconductor quantum dots (QDs) are of particular interest, due to the availability of deterministic nanofabrication techniques [10][11][12][13] and the possibility to engineer and tailor photonic devices to specific needs [14]. Although the performance of QD-based quantum light sources has rapidly improved during the last decade, it remains challenging to meet all requirements set by applications in photonic quantum technologies in a single device concept. Micropillar cavities [15], for instance, are well suited for achieving large photon extraction efficiencies due to high β-factors and strong Purcell enhancement [16][17][18]. The underlying working principle, however, is intrinsically narrowband, hindering schemes which require the collection of photons from multiple, spectrally separated excitonic states [19,20]. Photonic microlenses deterministically fabricated above pre-selected QDs [21], on the other hand, provide enhanced photon extraction in a broad spectral range. The achievable extraction efficiencies exceed 50% at large numerical apertures, but do not reach the level of micropillar cavities and the microlens geometry does not provide reasonable Purcell enhancement. Devices based on circular Bragg gratings (CBGs) simultaneously offer high extraction efficiencies and Purcell enhancement [22]. Embedded in free-standing semiconductor membranes, however, they are challenging to fabricate.Very recently, results on quantum light sources based on hybrid CBGs [23] with embedded QDs attracted much interest. Using these devices, benchmarks have been reported expe...