Florian Moll

The CubeSat mission QUBE aims to evaluate novel, miniaturized Quantum Key Distribution (QKD) building blocks in space including an optical downlink to an optical ground station. The mission will be launched in July 2024 and will provide important insights into new technology which potentially could form the backbone of a cost-effective satellite based QKD system on a global scale.

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.

To establish a reliable Quantum Key Distribution (QKD) link from a satellite in low-earth orbit (LEO) to a ground station, a series of ground-to-ground tests are essential. These tests, varying in distance and objectives, are for fine-tuning both the sender and receiver systems to address the unique challenges of satellite communications. We detail the methods and outcomes of progressively complex QKD tests, for the different components of the satellite and ground station. The experiments clearly show the performance of the quantum optics payload as well as the capability of the QKD receiver and the optical ground station (OGS).

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.