Romain Alléaume

We have shown how to conduct QKD vulnerability assessment in practice, based on a sound methodology inherited from Common Criteria. Taking a running CV-QKD system as a reference platform, we have experimentally tested and rated two different attack paths exploiting a common threat: detector saturation. Our results illustrate the importance of rating attacks in order to prioritize the implementation of countermeasures and to steer the design and engineering of practical QKD systems towards the highest possible security standards, paving the way to their security certification.

Continous-Variable Quantum Key Distribution (CV-QKD) relies on accurate noise calibration at the receiver to ensure the security of quantum communication. Traditional calibration methods often oversimplify noise characteristics, neglecting the impact of local oscillator (LO) noise and the critical role of noise dynamics, which can lead to imprecise Shot Noise Calibration (SNC). Our contributions are threefold: 1) we propose an operational framework for calibration, relying on the notion of stationarity 2) in this framework, we give a method allowing to derive the optimal calibration time 3) leveraging our knowledge of noise dynamics, we introduce a novel SNC method. We demonstrate that our improved calibration techniques offer higher performance and higher tolerance to receiver defects, which can enhance the performance and cost-effectiveness of CV-QKD systems.

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.

We introduce an explicit construction for a key distribution protocol in the Quantum Computational Timelock (QCT) security model, where one assumes that computationally secure encryption may only be broken after a time much longer than the coherence time of available quantum memories. Taking advantage of the QCT assumptions, we build a key distribution protocol called HM-QCT from the Hidden Matching problem for which there exists an exponential gap in one-way communication complexity between classical and quantum strategies.

Quantum key distribution (QKD) contrasts with classical cryptographic methods because it provides information-theoretical security on the distilled key. In some security demanding contexts, the perfect confidentiality that can be obtained with QKD combined with One-Time-Pad, may be insufficient. This will be in particular the case if the mere existence of communication can divulge crucial information. Performing QKD covertly solves this by insuring low probability of detection for the QKD signal states. We study here for the first time Covert continuous variable (CV) QKD. We establish, in the general case of a thermal noise channel, that the covertness conditions impose drastic limits to the performance of such protocols. We then propose an original solution to overcome this limitation by performing a computationally-secure coherent block encoding, analogous to spread spectrum, to the signal pulses of a Gaussian modulated coherent state CV-QKD protocol. The resulting protocol provides covertness, under computational assumptions while preserving the information-theoretical security on the final QKD key. We show that our method enables QKD over realistic WDM environments such as a 30 km optical backbone populated by 25 standard channels.

Extending the functionality and overcoming the performance limitation under which QKD can operate requires either quantum repeaters or new security models. Investigating the latter option, we introduce the Quantum Computational Hybrid (QCH) security model, where we assume that computationally secure encryption may only be broken after a time much longer than the coherence time of available quantum memories. We propose an explicit d-dimensional key distribution protocol, that we call MUB-Quantum Computational Timelock (MUB-QCT) where one bit is encoded on a qudit state chosen among d + 1 mutually unbiased bases (MUBs). Short-term-secure encryption is used to share the basis information with legitimate users while keeping it unknown from Eve until after her quantum memory decoheres. This allows to reduce Eve’s optimal attack to an immediate measurement followed by post-measurement decoding. We demonstrate that MUB-QCT enables everlasting secure key distribution with input states containing up to O(\sqrt{d})photons. This leads to a series of important improvements when compared to QKD: on the functional side, the ability to operate securely between one sender and many receivers, whose implementation can moreover be untrusted ; significant performance increase, characterized by a O(\sqrt{d}) multiplication of key rates and an extension by 25km x log(d) of the attainable distance over fiber. Implementable with a large number of modes with current or near-term multimode photonics technologies, the MUB-QCT construction has the potential to provide a radical shift to the performance and practicality of quantum key distribution.