Matteo Schiavon

Space-based quantum key distribution would allow, in the near future, secure communications be- tween parties over continental distances, complementing short-range fiber-based quantum networks. However, further demonstrations of daylight operations over free-space channels and the full compatibility with the telecom-based fiber infrastructure are still necessary. Here we present the prototype for daylight QKD at 1550 nm we developed as a demonstrator for application of QKD both on ground and in Space. Our QKD source, exploiting integrated silicon photonics technology, allows to reach a QBER of 1% during the field-test performed over a 145 m link, and represents a promising resource to design quantum optical payloads to be implemented in future satellite missions.

Quantum Key Distribution (QKD) is a field with a potentially major impact on cybersecurity and telecommunications. QKD protocols allow two distant parties to share a secret key regardless of the capacities of an eavesdropper. Thanks to quantum physics laws, an attempt to measure the signal on a quantum channel will necessarily introduce a disturbance, thus an eavesdropper cannot go unnoticed. The first protocols studied used Discrete Variables (DV), but their implementation requires specific technology such as single-photon detectors. Protocols using so-called continuous variables (CV) allow for the use of standard telecommunication components. Recent studies show that high key rates can be achieved using CV-QKD. Exail coordinates the QKISS project, as part of the development of the European Quantum Communication Infrastructure (EuroQCI), together with Thales SIX, LIP6 (CNRS/Sorbonne Université) and Institut d'Optique (CNRS), with the aim of industrializing CV-QKD systems.We have explored different hardware and software configurations to reduce the excess noise and increase the secret key rate of our prototype system. Based on these studies, we have built a demonstrator, used to realize field tests. We are also inlvolved in different European projects, to concretize the European Quantum Communication Infrastructure.

In this study, we constructed a full end-to-end atmospheric channel model for a GEO quantum exchange. This model allowed to assess the performances of two MDI-QKD protocols in such conditions : MP-QKD and TF-QKD, thus setting the limits of these protocols with current and future technology on detection, emission and optic tools. We demonstrated that an untrusted GEO link between two independent parties can be achieved when using reasonable size of telescope diameter. The key rates predicted would allow transmitting up to 260 bit/s for TF-QKD and up to 180 bit/s for MP-QKD for a telescope pupil diameter of 1m at 2.5 GHz of repetition rate.

Quantum Key Distribution (QKD) is arguably the most mature application of principles of quantum mechanics to cryptography, and several lab and field demonstrations have been realized. However the realization of QKD in deployed networks, with high distances and/or complex network architecture is still a challenge. Trusted nodes is a known solution to these issues, but requires the delegation of trust to third parties. Here, we propose a trusted node protocol where the requirements of trust delegation are lowered, with no overhead in the consumption of the key exchanged with QKD, allowing to keep the same secret key rate. This protocol is then applied to 2 links in the Parisian Quantum Network, composed of dark dedicated fibers between 8 nodes in the Parisian region, for a total fiber distance of 57 km. Our results show the overall key exchange with no degradation of the key rate.