Davide Orsucci

We propose a QKD transmitter that is able to encode qubits in two different degrees of freedom, namely polarisation and time-bin/phase, and according to different protocols, including but not limited to BB84, DPS and COW.

Quantum Key Distribution (QKD) can be performed securely only when the Quantum Bit Error Rate (QBER) is below a certain threshold which, due to the unavoidable presence of noise, limits the maximum transmission distance. Advantage Distillation (AD) is a classical post-processing technique that enhances QKD protocols by increasing error tolerance, thus extending the communication range. AD operates by post-selecting blocks of bits and extracting fewer correlated bits between Alice and Bob, which exhibits a reduced QBER, while Eve's mutual information does not significantly increase. This process ultimately lowers the information disclosure in the information reconciliation step, while the relative key shortening during privacy amplification remains largely unaffected. In this study, we present the first comprehensive finite key size analysis of the decoy-state version of the BB84 protocol including AD post-processing. Our results demonstrate a notable improvement in QBER tolerance through AD, with the 1-decoy version outperforming the 2-decoy version of the protocol. This analysis has significant implications for long-distance and satellite-based QKD applications, which are constrained by QBER, as it shows that substantial performance enhancements can be achieved by improved post-processing techniques.

Recent years have seen tremendous progress in increasing distances for distribution of quantum states and quantum entanglement, most notably in quantum key distribution. Even though these advances point towards breaching 1000 km and more in the near future, true global connectivity for secure intercontinental quantum links will likely require the operation of trusted networks based on quantum repeaters. To overcome associated losses in even the best optical fibers on ground, operating repeater nodes in space to utilize low-loss inter-satellite links may prove to be the only viable strategy. Successfully deployed QKD experiments and quantum technology in space, brings this idea closer to realization. Nevertheless, conceptual designs [9, 10] and component development are still in their infancy and it will require extraordinary engineering achievements to materialize robust space-based quantum networks. Here, we present recent efforts at the German Aerospace Center (DLR) to investigate the realization of robust global quantum networks. We are developing a holistic approach which bundles expertise on the necessary components for space-based quantum repeaters, i.e. photon sources, quantum memories, optical links, laser terminals, and orbital simulations. From this, we derive a common set of requirements to push concrete technological implementation. The long-term goal of this project is to develop space-hardened components for successful operation of intercontinental space-based quantum networks.