Philippe Grangier

Quantum key distribution (QKD) is a cryptographic primitive allowing two distant parties, Alice and Bob, to establish a secret key in an untrusted environment controlled by some eavesdropper, Eve. We introduce a new continuous-variable QKD protocol using a continuous but non-Gaussian modulation, allowing for an efficient reconciliation scheme and thus for improved performances. The modulation we consider crucially uses algebraic properties of the octonions, which can be seen as points on the unit sphere in R^8. We establish the security of this protocol against collective attacks, provided that the quantum channel is linear.

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.