Philipp Sohr

Quantum communication over global distances relies on the faithful distribution of entanglement, which can be achieved either via satellites or through fibre-based quantum networks with quantum memories. While satellite links are fundamentally constrained in coverage and by cost, memory-based networks offer a scalable alternative—provided their core components reach sufficient technological maturity. In this work, we investigate the loading process of spin-based quantum memories, focusing on the noise introduced when attempts to store incoming photonic qubits fail. While decoherence and readout errors have been the subject of substantial prior research, loading-induced noise—in particular the effect of lost heralding photons after spin interaction—remains largely unexplored in repeater protocol design. We close this gap by providing a detailed analysis of this specific source of noise, which has so far received limited attention. To mitigate the loading-induced noise, we propose a reinitialisation strategy that resets the memory after a maximal cutoff time if loading fails. We model the associated noise channel, quantify the trade-offs between rate and fidelity, and optimise the strategy across a range of link distances for memory-assisted MDI-QKD. The results demonstrate that protocol-level control of memory loading substantially improves entanglement distribution performance, paving the way for more reliable near-term quantum networks.

High-quality, distributed quantum entanglement is the distinctive resource for quantum communication and forms the foundation for the unequalled level of security that can be assured in quantum key distribution. While the entanglement provider does not need to be trusted, the secure key rate drops to zero if the entanglement used is too noisy. In this work, we show experimentally that QPA is able to increase the secure key rate achievable with QKD by improving the quality of distributed entanglement, thus increasing the quantum advantage in QKD. Beyond that, we show that QPA enables key generation at noise levels that previously prevented key generation. We provide a detailed characterisation of the gain in secure key rate achieved in our proof-of-principle experiment at different noise levels. The use of hyperentanglement in the field-tested polarisation and energy-time degrees of freedom enhances the efficiency of our scheme, making it an attractive option for deployment in high-loss regimes.

Satellite-based implementations are essential to realise QKD systems with global reach. Our current work aims to develop a consistent protocol that specifies the individual procedural steps of Decoy-State BB84 for space applications, accompanied by a rigorous security analysis. To this end, we are bringing together the results of decades of fundamental research and patching gaps where necessary to make it ready for application in accredited systems. On the poster, we will present interim results as well as the main challenges we are facing. For a more detailed abstract, please see the submitted pdf file above.