System Integration of High-Performance Continuous-Variable Quantum Key Distribution

Congratulations Dr.Piétri !

Abstract

Quantum Key Distribution (QKD) is the most prominent and the most mature application of quantum communications. It provides a way for two trusted users, usually named Alice and Bob, once they are provided with a public quantum channel and a public but authenticated classical channel, to exchange a secret key with a security based, not on computational assumptions as it is currently the case with classical cryptography, but on the laws of Physics, and hence, protects even against unbounded adversaries. Combined with a perfectly secure encryption scheme, QKD allows for secure message transmission with information-theoretic security.

QKD protocols rely on the no-cloning theorem, and the basic principle that measuring a quantum system inherently modifies its state. These protocols can be mostly divided in two families: Discrete Variable (DV) protocols where the information is encoded on discrete properties of single photons, and Continuous Variable (CV) protocols where the information is encoded on continuous degrees of freedom; and in practice the quadratures of the electromagnetic field. While DV protocols have more maturity, can achieve longer distances, and require less signal processing, their CV counterparts can work at room temperature with high efficiency and at high rate.

This thesis mainly focuses on CV-QKD protocols, and tackles several challenges associated with the integration of CV-QKD systems. It showcases the integration of optical components to create a silicon photonics-based receiver for CV-QKD, and benchmark its performance in a full CV-QKD setup, showing an operation up to 23 km of distance. It also showcases the software integration of our CV-QKD experimental platform, as an open-source suite called QOSST: Quantum Open Software for Secure Transmissions. The software performs hardware control, digital signal processing for Alice and Bob (including clock, frequency and phase synchronisation), classical communications with authentication, parameter estimation and secret key rate computation for CV-QKD operations. It is hardware-agnostic and can run in a number of scenarios. It also provides extensive documentation, in the hope that it can help reduce the barrier to enter the world of CV-QKD research, as well as that it can be expanded and improved by other interested groups. The autonomy of the software allows the finding of crucial relationships between signal processing parameters and performance. Using our setup, we demonstrate positive key rates up to 25 km of fiber distance. Our prototype is then integrated into a deployed network in the Paris area, in particular, showing the feasibility on a 15 km deployed link between two remote nodes in Paris. This quantum communication infrastructure is also used to deploy DV-QKD commercial systems, and perform an experiment with a trusted node efficiently secured with Post-Quantum Cryptography on a 57 km link.

The energetic cost of CV-QKD is also investigated, both with a hardware-dependent approach and a more theoretical approach to give lower bounds on the energetic consumption. While the theoretical approach gives the global scaling, the hardware dependent approach shows what to expect for the first generation of CV-QKD systems, as well as an interesting comparison between the hardware cost and the post-processing cost.

Finally, the detectors used for the CV-QKD setup are considered for another protocol involving the verification of Boson Sampling. Initial simulations and experimental preparation highlight the challenges involved in such an experiment.